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// Copyright 2022 The Dawn & Tint Authors
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice, this
// list of conditions and the following disclaimer.
//
// 2. Redistributions in binary form must reproduce the above copyright notice,
// this list of conditions and the following disclaimer in the documentation
// and/or other materials provided with the distribution.
//
// 3. Neither the name of the copyright holder nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
// SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "src/tint/lang/wgsl/resolver/uniformity.h"
#include <limits>
#include <string>
#include <utility>
#include <vector>
#include "src/tint/lang/core/builtin_value.h"
#include "src/tint/lang/wgsl/program/program_builder.h"
#include "src/tint/lang/wgsl/resolver/dependency_graph.h"
#include "src/tint/lang/wgsl/sem/block_statement.h"
#include "src/tint/lang/wgsl/sem/builtin_fn.h"
#include "src/tint/lang/wgsl/sem/for_loop_statement.h"
#include "src/tint/lang/wgsl/sem/function.h"
#include "src/tint/lang/wgsl/sem/if_statement.h"
#include "src/tint/lang/wgsl/sem/info.h"
#include "src/tint/lang/wgsl/sem/load.h"
#include "src/tint/lang/wgsl/sem/loop_statement.h"
#include "src/tint/lang/wgsl/sem/statement.h"
#include "src/tint/lang/wgsl/sem/switch_statement.h"
#include "src/tint/lang/wgsl/sem/value_constructor.h"
#include "src/tint/lang/wgsl/sem/value_conversion.h"
#include "src/tint/lang/wgsl/sem/variable.h"
#include "src/tint/lang/wgsl/sem/while_statement.h"
#include "src/tint/utils/containers/map.h"
#include "src/tint/utils/containers/scope_stack.h"
#include "src/tint/utils/containers/unique_vector.h"
#include "src/tint/utils/macros/defer.h"
#include "src/tint/utils/memory/block_allocator.h"
#include "src/tint/utils/rtti/switch.h"
#include "src/tint/utils/text/string_stream.h"
// Set to `1` to dump the uniformity graph for each function in graphviz format.
#define TINT_DUMP_UNIFORMITY_GRAPH 0
#if TINT_DUMP_UNIFORMITY_GRAPH
#include <iostream>
#endif
namespace tint::resolver {
namespace {
/// Unwraps `u->expr`'s chain of indirect (*) and address-of (&) expressions, returning the first
/// expression that is neither of these.
/// E.g. If `u` is `*(&(*(&p)))`, returns `p`.
const ast::Expression* UnwrapIndirectAndAddressOfChain(const ast::UnaryOpExpression* u) {
auto* e = u->expr;
while (true) {
auto* unary = e->As<ast::UnaryOpExpression>();
if (unary &&
(unary->op == core::UnaryOp::kIndirection || unary->op == core::UnaryOp::kAddressOf)) {
e = unary->expr;
} else {
break;
}
}
return e;
}
/// CallSiteTag describes the uniformity requirements on the call sites of a function.
struct CallSiteTag {
enum {
CallSiteRequiredToBeUniform,
CallSiteNoRestriction,
} tag;
wgsl::DiagnosticSeverity severity = wgsl::DiagnosticSeverity::kUndefined;
};
/// FunctionTag describes a functions effects on uniformity.
enum FunctionTag {
ReturnValueMayBeNonUniform,
NoRestriction,
};
/// ParameterTag describes the uniformity requirements of values passed to a function parameter.
struct ParameterTag {
enum {
ParameterValueRequiredToBeUniform,
ParameterContentsRequiredToBeUniform,
ParameterNoRestriction,
} tag;
wgsl::DiagnosticSeverity severity = wgsl::DiagnosticSeverity::kUndefined;
};
/// Node represents a node in the graph of control flow and value nodes within the analysis of a
/// single function.
struct Node {
/// Constructor
/// @param a the corresponding AST node
explicit Node(const ast::Node* a) : ast(a) {}
#if TINT_DUMP_UNIFORMITY_GRAPH
/// The node tag.
std::string tag;
#endif
/// Type describes the type of the node, which is used to determine additional diagnostic
/// information.
enum Type {
kRegular,
kFunctionCallArgumentValue,
kFunctionCallArgumentContents,
kFunctionCallPointerArgumentResult,
kFunctionCallReturnValue,
kFunctionPointerParameterContents,
};
/// The type of the node.
Type type = kRegular;
/// `true` if this node represents a potential control flow change.
bool affects_control_flow = false;
/// The corresponding AST node, or nullptr.
const ast::Node* ast = nullptr;
/// The function call argument index, if applicable.
uint32_t arg_index = 0xffffffffu;
/// The set of edges from this node to other nodes in the graph.
UniqueVector<Node*, 4> edges;
/// The node that this node was visited from, or nullptr if not visited.
Node* visited_from = nullptr;
/// Add an edge to the `to` node.
/// @param to the destination node
void AddEdge(Node* to) {
TINT_ASSERT(to != nullptr);
edges.Add(to);
}
};
/// ParameterInfo holds information about the uniformity requirements and effects for a particular
/// function parameter.
struct ParameterInfo {
/// The semantic node in corresponds to this parameter.
const sem::Parameter* sem;
/// The parameter's direct uniformity requirements.
ParameterTag tag_direct = {ParameterTag::ParameterNoRestriction};
/// The parameter's uniformity requirements that affect the function return value.
ParameterTag tag_retval = {ParameterTag::ParameterNoRestriction};
/// Will be `true` if this function may cause the contents of this pointer parameter to become
/// non-uniform.
bool pointer_may_become_non_uniform = false;
/// The parameters that are required to be uniform for the contents of this pointer parameter to
/// be uniform at function exit.
Vector<const sem::Parameter*, 8> ptr_output_source_param_values;
/// The pointer parameters whose contents are required to be uniform for the contents of this
/// pointer parameter to be uniform at function exit.
Vector<const sem::Parameter*, 8> ptr_output_source_param_contents;
/// The node in the graph that corresponds to this parameter's (immutable) value.
Node* value;
/// The node in the graph that corresponds to this pointer parameter's initial contents.
Node* ptr_input_contents = nullptr;
/// The node in the graph that corresponds to this pointer parameter's contents on return.
Node* ptr_output_contents = nullptr;
};
/// FunctionInfo holds information about the uniformity requirements and effects for a particular
/// function, as well as the control flow graph.
struct FunctionInfo {
/// Constructor
/// @param func the AST function
/// @param b the program builder
FunctionInfo(const ast::Function* func, const ProgramBuilder& b) {
name = func->name->symbol.Name();
callsite_tag = {CallSiteTag::CallSiteNoRestriction};
function_tag = NoRestriction;
// Create special nodes.
required_to_be_uniform_error = CreateNode({"RequiredToBeUniform_Error"});
required_to_be_uniform_warning = CreateNode({"RequiredToBeUniform_Warning"});
required_to_be_uniform_info = CreateNode({"RequiredToBeUniform_Info"});
may_be_non_uniform = CreateNode({"MayBeNonUniform"});
cf_start = CreateNode({"CF_start"});
if (func->return_type) {
value_return = CreateNode({"Value_return"});
}
// Create nodes for parameters.
parameters.Resize(func->params.Length());
for (size_t i = 0; i < func->params.Length(); i++) {
auto* param = func->params[i];
auto param_name = param->name->symbol.Name();
auto* sem = b.Sem().Get(param);
parameters[i].sem = sem;
parameters[i].value = CreateNode({"param_", param_name});
parameters[i].value->ast = param;
if (sem->Type()->Is<core::type::Pointer>()) {
// Create extra nodes for a pointer parameter's initial contents and its contents
// when the function returns.
parameters[i].ptr_input_contents =
CreateNode({"ptrparam_", param_name, "_input_contents"}, param);
parameters[i].ptr_output_contents =
CreateNode({"ptrparam_", param_name, "_output_contents"});
parameters[i].ptr_input_contents->type = Node::kFunctionPointerParameterContents;
variables.Set(sem, parameters[i].ptr_input_contents);
local_var_decls.Add(sem);
} else {
variables.Set(sem, parameters[i].value);
}
}
}
/// The name of the function.
std::string name;
/// The call site uniformity requirements.
CallSiteTag callsite_tag;
/// The function's uniformity effects.
FunctionTag function_tag;
/// The uniformity requirements of the function's parameters.
Vector<ParameterInfo, 8> parameters;
/// The control flow graph.
BlockAllocator<Node> nodes;
/// Special `RequiredToBeUniform` nodes.
Node* required_to_be_uniform_error = nullptr;
Node* required_to_be_uniform_warning = nullptr;
Node* required_to_be_uniform_info = nullptr;
/// Special `MayBeNonUniform` node.
Node* may_be_non_uniform = nullptr;
/// Special `CF_start` node.
Node* cf_start = nullptr;
/// Special `Value_return` node.
Node* value_return = nullptr;
/// Map from variables to their value nodes in the graph, scoped with respect to control flow.
ScopeStack<const sem::Variable*, Node*> variables;
/// The set of mutable variables declared in the function that are in scope at any given point
/// in the analysis. This includes the contents of parameters to the function that are pointers.
/// This is used by the analysis for if statements and loops to know which variables need extra
/// nodes to capture their state when entering/exiting those constructs.
Hashset<const sem::Variable*, 8> local_var_decls;
/// The set of partial pointer variables - pointers that point to a subobject (into an array or
/// struct).
Hashset<const sem::Variable*, 4> partial_ptrs;
/// LoopSwitchInfo tracks information about the value of variables for a control flow construct.
struct LoopSwitchInfo {
/// The type of this control flow construct.
std::string type;
/// The input values for local variables at the start of this construct.
Hashmap<const sem::Variable*, Node*, 4> var_in_nodes;
/// The exit values for local variables at the end of this construct.
Hashmap<const sem::Variable*, Node*, 4> var_exit_nodes;
};
/// @returns the RequiredToBeUniform node that corresponds to `severity`
Node* RequiredToBeUniform(wgsl::DiagnosticSeverity severity) {
switch (severity) {
case wgsl::DiagnosticSeverity::kError:
return required_to_be_uniform_error;
case wgsl::DiagnosticSeverity::kWarning:
return required_to_be_uniform_warning;
case wgsl::DiagnosticSeverity::kInfo:
return required_to_be_uniform_info;
default:
TINT_UNREACHABLE() << "unhandled severity";
}
}
/// @returns a LoopSwitchInfo for the given statement, allocating the LoopSwitchInfo if this is
/// the first call with the given statement.
LoopSwitchInfo& LoopSwitchInfoFor(const sem::Statement* stmt) {
return *loop_switch_infos.GetOrAdd(stmt,
[&] { return loop_switch_info_allocator.Create(); });
}
/// Disassociates the LoopSwitchInfo for the given statement.
void RemoveLoopSwitchInfoFor(const sem::Statement* stmt) { loop_switch_infos.Remove(stmt); }
/// Create a new node.
/// @param tag_list a string list that will be used to identify the node for debugging purposes
/// @param ast the optional AST node that this node corresponds to
/// @returns the new node
Node* CreateNode([[maybe_unused]] std::initializer_list<std::string_view> tag_list,
const ast::Node* ast = nullptr) {
auto* node = nodes.Create(ast);
#if TINT_DUMP_UNIFORMITY_GRAPH
// Make the tag unique and set it.
// This only matters if we're dumping the graph.
std::string tag = "";
for (auto& t : tag_list) {
tag += t;
}
std::string unique_tag = tag;
int suffix = 0;
while (tags_.Contains(unique_tag)) {
unique_tag = tag + "_$" + std::to_string(++suffix);
}
tags_.Add(unique_tag);
node->tag = name + "." + unique_tag;
#endif
return node;
}
/// Reset the visited status of every node in the graph.
void ResetVisited() {
for (auto* node : nodes.Objects()) {
node->visited_from = nullptr;
}
}
private:
/// A list of tags that have already been used within the current function.
Hashset<std::string, 8> tags_;
/// Map from control flow statements to the corresponding LoopSwitchInfo structure.
Hashmap<const sem::Statement*, LoopSwitchInfo*, 8> loop_switch_infos;
/// Allocator of LoopSwitchInfos
BlockAllocator<LoopSwitchInfo> loop_switch_info_allocator;
};
/// UniformityGraph is used to analyze the uniformity requirements and effects of functions in a
/// module.
class UniformityGraph {
public:
/// Constructor.
/// @param builder the program to analyze
explicit UniformityGraph(ProgramBuilder& builder)
: b(builder), sem_(b.Sem()), diagnostics_(builder.Diagnostics()) {}
/// Destructor.
~UniformityGraph() {}
/// Build and analyze the graph to determine whether the program satisfies the uniformity
/// constraints of WGSL.
/// @param dependency_graph the dependency-ordered module-scope declarations
/// @returns true if all uniformity constraints are satisfied, otherise false
bool Build(const DependencyGraph& dependency_graph) {
#if TINT_DUMP_UNIFORMITY_GRAPH
std::cout << "digraph G {\n";
std::cout << "rankdir=BT\n";
#endif
// Process all functions in the module.
bool success = true;
for (auto* decl : dependency_graph.ordered_globals) {
if (auto* func = decl->As<ast::Function>()) {
if (!ProcessFunction(func)) {
success = false;
break;
}
}
}
#if TINT_DUMP_UNIFORMITY_GRAPH
std::cout << "\n}\n";
#endif
return success;
}
private:
const ProgramBuilder& b;
const sem::Info& sem_;
diag::List& diagnostics_;
/// Map of analyzed function results.
Hashmap<const ast::Function*, FunctionInfo, 8> functions_;
/// The function currently being analyzed.
FunctionInfo* current_function_;
/// Create a new node.
/// @param tag_list a string list that will be used to identify the node for debugging purposes
/// @param ast the optional AST node that this node corresponds to
/// @returns the new node
inline Node* CreateNode(std::initializer_list<std::string_view> tag_list,
const ast::Node* ast = nullptr) {
return current_function_->CreateNode(std::move(tag_list), ast);
}
/// Get the symbol name of an AST expression.
/// @param expr the expression to get the symbol name of
/// @returns the symbol name
inline std::string NameFor(const ast::IdentifierExpression* expr) {
return expr->identifier->symbol.Name();
}
/// @param var the variable to get the name of
/// @returns the name of the variable @p var
inline std::string NameFor(const ast::Variable* var) { return var->name->symbol.Name(); }
/// @param var the variable to get the name of
/// @returns the name of the variable @p var
inline std::string NameFor(const sem::Variable* var) { return NameFor(var->Declaration()); }
/// @param fn the function to get the name of
/// @returns the name of the function @p fn
inline std::string NameFor(const sem::Function* fn) {
return fn->Declaration()->name->symbol.Name();
}
/// Process a function.
/// @param func the function to process
/// @returns true if there are no uniformity issues, false otherwise
bool ProcessFunction(const ast::Function* func) {
current_function_ = &functions_.Add(func, FunctionInfo(func, b)).value;
// Process function body.
if (func->body) {
ProcessStatement(current_function_->cf_start, func->body);
}
#if TINT_DUMP_UNIFORMITY_GRAPH
// Dump the graph for this function as a subgraph.
std::cout << "\nsubgraph cluster_" << current_function_->name << " {\n";
std::cout << " label=" << current_function_->name << ";";
for (auto* node : current_function_->nodes.Objects()) {
std::cout << "\n \"" << node->tag << "\";";
for (auto* edge : node->edges) {
std::cout << "\n \"" << node->tag << "\" -> \"" << edge->tag << "\";";
}
}
std::cout << "\n}\n";
#endif
/// Helper to generate a tag for the uniformity requirements of the parameter at `index`.
auto get_param_tag = [&](UniqueVector<Node*, 4>& reachable, size_t index) {
auto* param = sem_.Get(func->params[index]);
auto& param_info = current_function_->parameters[index];
if (param->Type()->Is<core::type::Pointer>()) {
// For pointers, we distinguish between requiring uniformity of the contents versus
// the pointer itself.
if (reachable.Contains(param_info.ptr_input_contents)) {
return ParameterTag::ParameterContentsRequiredToBeUniform;
} else if (reachable.Contains(param_info.value)) {
return ParameterTag::ParameterValueRequiredToBeUniform;
}
} else if (reachable.Contains(current_function_->variables.Get(param))) {
// For non-pointers, the requirement is always on the value.
return ParameterTag::ParameterValueRequiredToBeUniform;
}
return ParameterTag::ParameterNoRestriction;
};
// Look at which nodes are reachable from "RequiredToBeUniform".
{
UniqueVector<Node*, 4> reachable;
auto traverse = [&](wgsl::DiagnosticSeverity severity) {
Traverse(current_function_->RequiredToBeUniform(severity), &reachable);
if (reachable.Contains(current_function_->may_be_non_uniform)) {
MakeError(*current_function_, current_function_->may_be_non_uniform, severity);
return false;
}
if (reachable.Contains(current_function_->cf_start)) {
if (current_function_->callsite_tag.tag == CallSiteTag::CallSiteNoRestriction) {
current_function_->callsite_tag = {CallSiteTag::CallSiteRequiredToBeUniform,
severity};
}
}
// Set the tags to capture the direct uniformity requirements of each parameter.
for (size_t i = 0; i < func->params.Length(); i++) {
if (current_function_->parameters[i].tag_direct.tag ==
ParameterTag::ParameterNoRestriction) {
current_function_->parameters[i].tag_direct = {get_param_tag(reachable, i),
severity};
}
}
return true;
};
if (!traverse(wgsl::DiagnosticSeverity::kError)) {
return false;
} else {
if (traverse(wgsl::DiagnosticSeverity::kWarning)) {
traverse(wgsl::DiagnosticSeverity::kInfo);
}
}
}
// If "Value_return" exists, look at which nodes are reachable from it.
if (current_function_->value_return) {
current_function_->ResetVisited();
UniqueVector<Node*, 4> reachable;
Traverse(current_function_->value_return, &reachable);
if (reachable.Contains(current_function_->may_be_non_uniform)) {
current_function_->function_tag = ReturnValueMayBeNonUniform;
}
// Set the tags to capture the uniformity requirements of each parameter with respect to
// the function return value.
for (size_t i = 0; i < func->params.Length(); i++) {
current_function_->parameters[i].tag_retval = {get_param_tag(reachable, i)};
}
}
// Traverse the graph for each pointer parameter.
for (size_t i = 0; i < func->params.Length(); i++) {
auto& param_info = current_function_->parameters[i];
if (param_info.ptr_output_contents == nullptr) {
continue;
}
// Reset "visited" state for all nodes.
current_function_->ResetVisited();
UniqueVector<Node*, 4> reachable;
Traverse(param_info.ptr_output_contents, &reachable);
if (reachable.Contains(current_function_->may_be_non_uniform)) {
param_info.pointer_may_become_non_uniform = true;
}
// Check every parameter to see if it feeds into this parameter's output value.
// This includes checking this parameter (as it may feed into its own output value), so
// we do not skip the `i==j` case.
for (size_t j = 0; j < func->params.Length(); j++) {
auto tag = get_param_tag(reachable, j);
auto* source_param = sem_.Get(func->params[j]);
if (tag == ParameterTag::ParameterContentsRequiredToBeUniform) {
param_info.ptr_output_source_param_contents.Push(source_param);
} else if (tag == ParameterTag::ParameterValueRequiredToBeUniform) {
param_info.ptr_output_source_param_values.Push(source_param);
}
}
}
return true;
}
/// Process a statement, returning the new control flow node.
/// @param cf the input control flow node
/// @param stmt the statement to process d
/// @returns the new control flow node
Node* ProcessStatement(Node* cf, const ast::Statement* stmt) {
return Switch(
stmt,
[&](const ast::AssignmentStatement* a) {
if (a->lhs->Is<ast::PhonyExpression>()) {
auto [cf_r, _] = ProcessExpression(cf, a->rhs);
return cf_r;
}
auto [cf_l, v_l, ident] = ProcessLValueExpression(cf, a->lhs);
auto [cf_r, v_r] = ProcessExpression(cf_l, a->rhs);
v_l->AddEdge(v_r);
// Update the variable node for the LHS variable.
current_function_->variables.Set(ident, v_l);
return cf_r;
},
[&](const ast::BlockStatement* block) {
Hashmap<const sem::Variable*, Node*, 4> scoped_assignments;
auto* sem = sem_.Get(block);
{
// Push a new scope for variable assignments in the block.
current_function_->variables.Push();
TINT_DEFER(current_function_->variables.Pop());
for (auto* s : block->statements) {
cf = ProcessStatement(cf, s);
if (!sem_.Get(s)->Behaviors().Contains(sem::Behavior::kNext)) {
break;
}
}
auto* parent = sem->Parent();
auto* loop = parent ? parent->As<sem::LoopStatement>() : nullptr;
if (loop) {
// We've reached the end of a loop body. If there is a continuing block,
// process it before ending the block so that any variables declared in the
// loop body are visible to the continuing block.
if (auto* continuing =
loop->Declaration()->As<ast::LoopStatement>()->continuing) {
auto& loop_body_behavior = sem->Behaviors();
if (loop_body_behavior.Contains(sem::Behavior::kNext) ||
loop_body_behavior.Contains(sem::Behavior::kContinue)) {
cf = ProcessStatement(cf, continuing);
}
}
}
if (sem_.Get<sem::FunctionBlockStatement>(block)) {
// We've reached the end of the function body.
// Add edges from pointer parameter outputs to their current value.
for (auto& param : current_function_->parameters) {
if (param.ptr_output_contents) {
param.ptr_output_contents->AddEdge(
current_function_->variables.Get(param.sem));
}
}
}
scoped_assignments = std::move(current_function_->variables.Top());
}
// Propagate all variables assignments to the containing scope if the behavior is
// 'Next'.
auto& behaviors = sem->Behaviors();
if (behaviors.Contains(sem::Behavior::kNext)) {
for (auto& var : scoped_assignments) {
current_function_->variables.Set(var.key, var.value);
}
}
// Remove any variables declared in this scope from the set of in-scope variables.
for (auto decl : sem->Decls()) {
current_function_->local_var_decls.Remove(decl.value.variable);
}
return cf;
},
[&](const ast::BreakStatement* brk) {
// Find the loop or switch statement that we are in.
auto* parent = sem_.Get(brk)
->FindFirstParent<sem::SwitchStatement, sem::LoopStatement,
sem::ForLoopStatement, sem::WhileStatement>();
auto& info = current_function_->LoopSwitchInfoFor(parent);
// Propagate variable values to the loop/switch exit nodes.
for (auto& var : current_function_->local_var_decls) {
// Skip variables that were declared inside this loop/switch.
if (auto* lv = var->As<sem::LocalVariable>();
lv &&
lv->Statement()->FindFirstParent([&](auto* s) { return s == parent; })) {
continue;
}
// Add an edge from the variable exit node to its value at this point.
auto* exit_node = info.var_exit_nodes.GetOrAdd(var, [&] {
auto name = NameFor(var);
return CreateNode({name, "_value_", info.type, "_exit"});
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
return cf;
},
[&](const ast::BreakIfStatement* brk) {
// This works very similar to the IfStatement uniformity below, execpt instead of
// processing the body, we directly inline the BreakStatement uniformity from
// above.
auto* sem = sem_.Get(brk);
auto [_, v_cond] = ProcessExpression(cf, brk->condition);
// Add a diagnostic node to capture the control flow change.
auto* v = CreateNode({"break_if_stmt"}, brk);
v->affects_control_flow = true;
v->AddEdge(v_cond);
{
auto* parent = sem->FindFirstParent<sem::LoopStatement>();
auto& info = current_function_->LoopSwitchInfoFor(parent);
// Propagate variable values to the loop exit nodes.
for (auto& var : current_function_->local_var_decls) {
// Skip variables that were declared inside this loop.
if (auto* lv = var->As<sem::LocalVariable>();
lv && lv->Statement()->FindFirstParent(
[&](auto* s) { return s == parent; })) {
continue;
}
// Add an edge from the variable exit node to its value at this point.
auto* exit_node = info.var_exit_nodes.GetOrAdd(var, [&] {
auto name = NameFor(var);
return CreateNode({name, "_value_", info.type, "_exit"});
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
}
if (sem->Behaviors() != sem::Behaviors{sem::Behavior::kNext}) {
auto* cf_end = CreateNode({"break_if_CFend"});
cf_end->AddEdge(v);
return cf_end;
}
return cf;
},
[&](const ast::CallStatement* c) {
auto [cf1, _] = ProcessCall(cf, c->expr);
return cf1;
},
[&](const ast::CompoundAssignmentStatement* c) {
// The compound assignment statement `a += b` is equivalent to:
// let p = &a;
// *p = *p + b;
// Evaluate the LHS.
auto [cf1, l1, ident] = ProcessLValueExpression(cf, c->lhs);
// Get the current value loaded from the LHS reference before evaluating the RHS.
auto* lhs_load = current_function_->variables.Get(ident);
// Evaluate the RHS.
auto [cf2, v2] = ProcessExpression(cf1, c->rhs);
// Create a node for the resulting value.
auto* result = CreateNode({"binary_expr_result"});
result->AddEdge(v2);
if (lhs_load) {
result->AddEdge(lhs_load);
}
// Update the variable node for the LHS variable.
l1->AddEdge(result);
current_function_->variables.Set(ident, l1);
return cf2;
},
[&](const ast::ContinueStatement* c) {
// Find the loop statement that we are in.
auto* parent = sem_.Get(c)
->FindFirstParent<sem::LoopStatement, sem::ForLoopStatement,
sem::WhileStatement>();
auto& info = current_function_->LoopSwitchInfoFor(parent);
// Propagate assignments to the loop input nodes.
for (auto v : info.var_in_nodes) {
auto* in_node = v.value;
auto* out_node = current_function_->variables.Get(v.key);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
return cf;
},
[&](const ast::DiscardStatement*) { return cf; },
[&](const ast::ForLoopStatement* f) {
auto* sem_loop = sem_.Get(f);
auto* cfx = CreateNode({"loop_start"});
// Insert the initializer before the loop.
auto* cf_init = cf;
if (f->initializer) {
cf_init = ProcessStatement(cf, f->initializer);
}
auto* cf_start = cf_init;
auto& info = current_function_->LoopSwitchInfoFor(sem_loop);
info.type = "forloop";
// Create input nodes for any variables declared before this loop.
for (auto& v : current_function_->local_var_decls) {
auto* in_node = CreateNode({NameFor(v), "_value_forloop_in"});
in_node->AddEdge(current_function_->variables.Get(v));
info.var_in_nodes.Replace(v, in_node);
current_function_->variables.Set(v, in_node);
}
// Insert the condition at the start of the loop body.
if (f->condition) {
auto [cf_cond, v] = ProcessExpression(cfx, f->condition);
auto* cf_condition_end = CreateNode({"for_condition_CFend"}, f);
cf_condition_end->affects_control_flow = true;
cf_condition_end->AddEdge(v);
cf_start = cf_condition_end;
// Propagate assignments to the loop exit nodes.
for (auto& var : current_function_->local_var_decls) {
auto* exit_node = info.var_exit_nodes.GetOrAdd(var, [&] {
auto name = NameFor(var);
return CreateNode({name, "_value_", info.type, "_exit"});
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
}
auto* cf1 = ProcessStatement(cf_start, f->body);
// Insert the continuing statement at the end of the loop body.
if (f->continuing) {
auto* cf2 = ProcessStatement(cf1, f->continuing);
cfx->AddEdge(cf2);
} else {
cfx->AddEdge(cf1);
}
cfx->AddEdge(cf);
// Add edges from variable loop input nodes to their values at the end of the loop.
for (auto& v : info.var_in_nodes) {
auto* in_node = v.value;
auto* out_node = current_function_->variables.Get(v.key);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
// Set each variable's exit node as its value in the outer scope.
for (auto& v : info.var_exit_nodes) {
current_function_->variables.Set(v.key, v.value);
}
if (f->initializer) {
// Remove variables declared in the for-loop initializer from the current scope.
if (auto* decl = f->initializer->As<ast::VariableDeclStatement>()) {
current_function_->local_var_decls.Remove(sem_.Get(decl->variable));
}
}
current_function_->RemoveLoopSwitchInfoFor(sem_loop);
if (sem_loop->Behaviors() == sem::Behaviors{sem::Behavior::kNext}) {
return cf;
} else {
return cfx;
}
},
[&](const ast::WhileStatement* w) {
auto* sem_loop = sem_.Get(w);
auto* cfx = CreateNode({"loop_start"});
auto* cf_start = cf;
auto& info = current_function_->LoopSwitchInfoFor(sem_loop);
info.type = "whileloop";
// Create input nodes for any variables declared before this loop.
for (auto& v : current_function_->local_var_decls) {
auto* in_node = CreateNode({NameFor(v), "_value_forloop_in"});
in_node->AddEdge(current_function_->variables.Get(v));
info.var_in_nodes.Replace(v, in_node);
current_function_->variables.Set(v, in_node);
}
// Insert the condition at the start of the loop body.
{
auto [cf_cond, v] = ProcessExpression(cfx, w->condition);
auto* cf_condition_end = CreateNode({"while_condition_CFend"}, w);
cf_condition_end->affects_control_flow = true;
cf_condition_end->AddEdge(v);
cf_start = cf_condition_end;
}
// Propagate assignments to the loop exit nodes.
for (auto& var : current_function_->local_var_decls) {
auto* exit_node = info.var_exit_nodes.GetOrAdd(var, [&] {
auto name = NameFor(var);
return CreateNode({name, "_value_", info.type, "_exit"});
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
auto* cf1 = ProcessStatement(cf_start, w->body);
cfx->AddEdge(cf1);
cfx->AddEdge(cf);
// Add edges from variable loop input nodes to their values at the end of the loop.
for (auto v : info.var_in_nodes) {
auto* in_node = v.value;
auto* out_node = current_function_->variables.Get(v.key);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
// Set each variable's exit node as its value in the outer scope.
for (auto v : info.var_exit_nodes) {
current_function_->variables.Set(v.key, v.value);
}
current_function_->RemoveLoopSwitchInfoFor(sem_loop);
if (sem_loop->Behaviors() == sem::Behaviors{sem::Behavior::kNext}) {
return cf;
} else {
return cfx;
}
},
[&](const ast::IfStatement* i) {
auto* sem_if = sem_.Get(i);
auto [_, v_cond] = ProcessExpression(cf, i->condition);
// Add a diagnostic node to capture the control flow change.
auto* v = CreateNode({"if_stmt"}, i);
v->affects_control_flow = true;
v->AddEdge(v_cond);
Hashmap<const sem::Variable*, Node*, 4> true_vars;
Hashmap<const sem::Variable*, Node*, 4> false_vars;
// Helper to process a statement with a new scope for variable assignments.
// Populates `assigned_vars` with new nodes for any variables that are assigned in
// this statement.
auto process_in_scope =
[&](Node* cf_in, const ast::Statement* s,
Hashmap<const sem::Variable*, Node*, 4>& assigned_vars) {
// Push a new scope for variable assignments.
current_function_->variables.Push();
// Process the statement.
auto* cf_out = ProcessStatement(cf_in, s);
assigned_vars = current_function_->variables.Top();
// Pop the scope and return.
current_function_->variables.Pop();
return cf_out;
};
auto* cf1 = process_in_scope(v, i->body, true_vars);
bool true_has_next = sem_.Get(i->body)->Behaviors().Contains(sem::Behavior::kNext);
bool false_has_next = true;
Node* cf2 = nullptr;
if (i->else_statement) {
cf2 = process_in_scope(v, i->else_statement, false_vars);
false_has_next =
sem_.Get(i->else_statement)->Behaviors().Contains(sem::Behavior::kNext);
}
// Update values for any variables assigned in the if or else blocks.
for (auto& var : current_function_->local_var_decls) {
// Skip variables not assigned in either block.
if (!true_vars.Contains(var) && !false_vars.Contains(var)) {
continue;
}
// Create an exit node for the variable.
auto* out_node = CreateNode({NameFor(var), "_value_if_exit"});
// Add edges to the assigned value or the initial value.
// Only add edges if the behavior for that block contains 'Next'.
if (true_has_next) {
if (true_vars.Contains(var)) {
out_node->AddEdge(*true_vars.Get(var));
} else {
out_node->AddEdge(current_function_->variables.Get(var));
}
}
if (false_has_next) {
if (false_vars.Contains(var)) {
out_node->AddEdge(*false_vars.Get(var));
} else {
out_node->AddEdge(current_function_->variables.Get(var));
}
}
current_function_->variables.Set(var, out_node);
}
if (sem_if->Behaviors() != sem::Behaviors{sem::Behavior::kNext}) {
auto* cf_end = CreateNode({"if_CFend"});
cf_end->AddEdge(cf1);
if (cf2) {
cf_end->AddEdge(cf2);
}
return cf_end;
}
return cf;
},
[&](const ast::IncrementDecrementStatement* i) {
// The increment/decrement statement `i++` is equivalent to `i = i + 1`.
// Evaluate the LHS.
auto [cf1, l1, ident] = ProcessLValueExpression(cf, i->lhs);
// Get the current value loaded from the LHS reference.
auto* lhs_load = current_function_->variables.Get(ident);
// Create a node for the resulting value.
auto* result = CreateNode({"incdec_result"});
result->AddEdge(cf1);
if (lhs_load) {
result->AddEdge(lhs_load);
}
// Update the variable node for the LHS variable.
l1->AddEdge(result);
current_function_->variables.Set(ident, l1);
return cf1;
},
[&](const ast::LoopStatement* l) {
auto* sem_loop = sem_.Get(l);
auto* cfx = CreateNode({"loop_start"});
auto& info = current_function_->LoopSwitchInfoFor(sem_loop);
info.type = "loop";
// Create input nodes for any variables declared before this loop.
for (auto& v : current_function_->local_var_decls) {
auto name = NameFor(v);
auto* in_node = CreateNode({name, "_value_loop_in"}, v->Declaration());
in_node->AddEdge(current_function_->variables.Get(v));
info.var_in_nodes.Replace(v, in_node);
current_function_->variables.Set(v, in_node);
}
// Note: The continuing block is processed as a special case at the end of
// processing the loop body BlockStatement. This is so that variable declarations
// inside the loop body are visible to the continuing statement.
auto* cf1 = ProcessStatement(cfx, l->body);
cfx->AddEdge(cf1);
cfx->AddEdge(cf);
// Add edges from variable loop input nodes to their values at the end of the loop.
for (auto v : info.var_in_nodes) {
auto* in_node = v.value;
auto* out_node = current_function_->variables.Get(v.key);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
// Set each variable's exit node as its value in the outer scope.
for (auto v : info.var_exit_nodes) {
current_function_->variables.Set(v.key, v.value);
}
current_function_->RemoveLoopSwitchInfoFor(sem_loop);
if (sem_loop->Behaviors() == sem::Behaviors{sem::Behavior::kNext}) {
return cf;
} else {
return cfx;
}
},
[&](const ast::ReturnStatement* r) {
Node* cf_ret;
if (r->value) {
auto [cf1, v] = ProcessExpression(cf, r->value);
current_function_->value_return->AddEdge(v);
cf_ret = cf1;
} else {
TINT_ASSERT(cf != nullptr);
cf_ret = cf;
}
// Add edges from each pointer parameter output to its current value.
for (auto& param : current_function_->parameters) {
if (param.ptr_output_contents) {
param.ptr_output_contents->AddEdge(
current_function_->variables.Get(param.sem));
}
}
return cf_ret;
},
[&](const ast::SwitchStatement* s) {
auto* sem_switch = sem_.Get(s);
auto [cfx, v_cond] = ProcessExpression(cf, s->condition);
// Add a diagnostic node to capture the control flow change.
auto* v = CreateNode({"switch_stmt"}, s);
v->affects_control_flow = true;
v->AddEdge(v_cond);
Node* cf_end = nullptr;
if (sem_switch->Behaviors() != sem::Behaviors{sem::Behavior::kNext}) {
cf_end = CreateNode({"switch_CFend"});
}
auto& info = current_function_->LoopSwitchInfoFor(sem_switch);
info.type = "switch";
auto* cf_n = v;
for (auto* c : s->body) {
auto* sem_case = sem_.Get(c);
current_function_->variables.Push();
cf_n = ProcessStatement(v, c->body);
if (cf_end) {
cf_end->AddEdge(cf_n);
}
if (sem_case->Behaviors().Contains(sem::Behavior::kNext)) {
// Propagate variable values to the switch exit nodes.
for (auto& var : current_function_->local_var_decls) {
// Skip variables that were declared inside the switch.
if (auto* lv = var->As<sem::LocalVariable>();
lv && lv->Statement()->FindFirstParent(
[&](auto* st) { return st == sem_switch; })) {
continue;
}
// Add an edge from the variable exit node to its new value.
auto* exit_node = info.var_exit_nodes.GetOrAdd(var, [&] {
auto name = NameFor(var);
return CreateNode({name, "_value_", info.type, "_exit"});
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
}
current_function_->variables.Pop();
}
// Update nodes for any variables assigned in the switch statement.
for (auto var : info.var_exit_nodes) {
current_function_->variables.Set(var.key, var.value);
}
return cf_end ? cf_end : cf;
},
[&](const ast::VariableDeclStatement* decl) {
Node* node;
auto* sem_var = sem_.Get(decl->variable);
if (decl->variable->initializer) {
auto [cf1, v] = ProcessExpression(cf, decl->variable->initializer);
cf = cf1;
node = v;
// Store if lhs is a partial pointer
if (sem_var->Type()->Is<core::type::Pointer>()) {
auto* init = sem_.Get(decl->variable->initializer);
if (auto* unary_init = init->Declaration()->As<ast::UnaryOpExpression>()) {
auto* e = UnwrapIndirectAndAddressOfChain(unary_init);
if (e->Is<ast::AccessorExpression>()) {
current_function_->partial_ptrs.Add(sem_var);
}
}
}
} else {
node = cf;
}
current_function_->variables.Set(sem_var, node);
if (decl->variable->Is<ast::Var>()) {
current_function_->local_var_decls.Add(
sem_.Get<sem::LocalVariable>(decl->variable));
}
return cf;
},
[&](const ast::ConstAssert*) {
return cf; // No impact on uniformity
},
TINT_ICE_ON_NO_MATCH);
}
/// Process an identifier expression.
/// @param cf the input control flow node
/// @param ident the identifier expression to process
/// @param load_rule true if the load rule is being invoked on this identifier
/// @returns a pair of (control flow node, value node)
std::pair<Node*, Node*> ProcessIdentExpression(Node* cf,
const ast::IdentifierExpression* ident,
bool load_rule = false) {
// Helper to check if the entry point attribute of `obj` indicates non-uniformity.
auto has_nonuniform_entry_point_attribute = [&](auto* obj, auto* entry_point) {
// Only the num_workgroups and workgroup_id builtins, and subgroup_size builtin used in
// compute stage are uniform.
if (auto* builtin_attr = ast::GetAttribute<ast::BuiltinAttribute>(obj->attributes)) {
auto builtin = builtin_attr->builtin;
if (builtin == core::BuiltinValue::kNumWorkgroups ||
builtin == core::BuiltinValue::kWorkgroupId) {
return false;
}
if (builtin == core::BuiltinValue::kSubgroupSize) {
if (entry_point->PipelineStage() == ast::PipelineStage::kCompute) {
// Subgroup size is uniform in compute.
return false;
} else {
// Currently the only other allowed usage for subgroup_size is in fragment.
TINT_ASSERT(entry_point->PipelineStage() == ast::PipelineStage::kFragment);
// Subgroup size is considered to be varying for fragment.
return true;
}
}
}
return true;
};
auto* node = CreateNode({NameFor(ident), "_ident_expr"}, ident);
auto* sem_ident = sem_.GetVal(ident);
TINT_ASSERT(sem_ident);
auto* var_user = sem_ident->Unwrap()->As<sem::VariableUser>();
auto* sem = var_user->Variable();
return Switch(
sem,
[&](const sem::Parameter* param) {
auto* user_func = param->Owner()->As<sem::Function>();
if (user_func && user_func->Declaration()->IsEntryPoint()) {
if (auto* str = param->Type()->As<sem::Struct>()) {
// We consider the whole struct to be non-uniform if any one of its members
// is non-uniform.
bool uniform = true;
for (auto* member : str->Members()) {
if (has_nonuniform_entry_point_attribute(member->Declaration(),
user_func->Declaration())) {
uniform = false;
}
}
node->AddEdge(uniform ? cf : current_function_->may_be_non_uniform);
return std::make_pair(cf, node);
} else {
if (has_nonuniform_entry_point_attribute(param->Declaration(),
user_func->Declaration())) {
node->AddEdge(current_function_->may_be_non_uniform);
} else {
node->AddEdge(cf);
}
return std::make_pair(cf, node);
}
} else {
node->AddEdge(cf);
auto* current_value = current_function_->variables.Get(param);
if (auto* ptr = param->Type()->As<core::type::Pointer>()) {
if (load_rule) {
if (ptr->AddressSpace() == core::AddressSpace::kFunction ||
ptr->Access() == core::Access::kRead) {
// We are loading from a pointer to a function-scope variable or an
// immutable module-scope variable, so add an edge to its contents.
node->AddEdge(current_value);
} else {
// We are loading from a pointer to a mutable module-scope variable,
// which always has non-uniform contents.
node->AddEdge(current_function_->may_be_non_uniform);
}
} else {
// This is a pointer parameter that we are not loading from, so add an
// edge to the pointer value itself.
node->AddEdge(current_function_->parameters[param->Index()].value);
}
} else {
// The parameter is a value, so add an edge to it.
node->AddEdge(current_value);
}
return std::make_pair(cf, node);
}
},
[&](const sem::GlobalVariable* global) {
// Loads from global read-write variables may be non-uniform.
if (global->Declaration()->Is<ast::Var>() &&
global->Access() != core::Access::kRead && load_rule) {
node->AddEdge(current_function_->may_be_non_uniform);
} else {
node->AddEdge(cf);
}
return std::make_pair(cf, node);
},
[&](const sem::LocalVariable* local) {
node->AddEdge(cf);
auto* local_value = current_function_->variables.Get(local);
if (local->Type()->Is<core::type::Pointer>()) {
if (load_rule) {
// We are loading from the pointer, so add an edge to its contents.
auto* root = var_user->RootIdentifier();
if (root->Is<sem::GlobalVariable>()) {
if (root->Access() != core::Access::kRead) {
// The contents of a mutable global variable is always non-uniform.
node->AddEdge(current_function_->may_be_non_uniform);
}
} else {
node->AddEdge(current_function_->variables.Get(root));
}
// The uniformity of the contents also depends on the uniformity of the
// pointer itself. For a pointer captured in a let declaration, this will
// come from the value node of that declaration.
node->AddEdge(local_value);
} else {
// The variable is a pointer that we are not loading from, so add an edge to
// the pointer value itself.
node->AddEdge(local_value);
}
} else if (local->Type()->Is<core::type::Reference>()) {
if (load_rule) {
// We are loading from the reference, so add an edge to its contents.
node->AddEdge(local_value);
} else {
// References to local variables (i.e. var declarations) are always uniform,
// so no other edges needed.
}
} else {
// The identifier is a value declaration, so add an edge to it.
node->AddEdge(local_value);
}
return std::make_pair(cf, node);
},
TINT_ICE_ON_NO_MATCH);
}
/// Process an expression.
/// @param cf the input control flow node
/// @param expr the expression to process
/// @param load_rule true if the load rule is being invoked on this expression
/// @returns a pair of (control flow node, value node)
std::pair<Node*, Node*> ProcessExpression(Node* cf,
const ast::Expression* expr,
bool load_rule = false) {
if (sem_.Get<sem::Load>(expr)) {
// Set the load-rule flag to indicate that identifier expressions in this sub-tree
// should add edges to the contents of the variables that they refer to.
load_rule = true;
}
return Switch(
expr,
[&](const ast::BinaryExpression* e) {
if (e->IsLogical()) {
// Short-circuiting binary operators are a special case.
auto [cf1, v1] = ProcessExpression(cf, e->lhs);
// Add a diagnostic node to capture the control flow change.
auto* v1_cf = CreateNode({"short_circuit_op"}, e);
v1_cf->affects_control_flow = true;
v1_cf->AddEdge(v1);
auto [cf2, v2] = ProcessExpression(v1_cf, e->rhs);
return std::pair<Node*, Node*>(cf, v2);
} else {
auto [cf1, v1] = ProcessExpression(cf, e->lhs);
auto [cf2, v2] = ProcessExpression(cf1, e->rhs);
auto* result = CreateNode({"binary_expr_result"}, e);
result->AddEdge(v1);
result->AddEdge(v2);
return std::pair<Node*, Node*>(cf2, result);
}
},
[&](const ast::CallExpression* c) { return ProcessCall(cf, c); },
[&](const ast::IdentifierExpression* i) {
return ProcessIdentExpression(cf, i, load_rule);
},
[&](const ast::IndexAccessorExpression* i) {
auto [cf1, v1] = ProcessExpression(cf, i->object, load_rule);
auto [cf2, v2] = ProcessExpression(cf1, i->index);
auto* result = CreateNode({"index_accessor_result"});
result->AddEdge(v1);
result->AddEdge(v2);
return std::pair<Node*, Node*>(cf2, result);
},
[&](const ast::LiteralExpression*) { return std::make_pair(cf, cf); },
[&](const ast::MemberAccessorExpression* m) {
return ProcessExpression(cf, m->object, load_rule);
},
[&](const ast::UnaryOpExpression* u) {
return ProcessExpression(cf, u->expr, load_rule);
},
TINT_ICE_ON_NO_MATCH);
}
/// @param u unary expression with op == kIndirection
/// @returns true if `u` is an indirection unary expression that ultimately dereferences a
/// partial pointer, false otherwise.
bool IsDerefOfPartialPointer(const ast::UnaryOpExpression* u) {
TINT_ASSERT(u->op == core::UnaryOp::kIndirection);
// To determine if we're dereferencing a partial pointer, unwrap *&
// chains; if the final expression is an identifier, see if it's a
// partial pointer. If it's not an identifier, then it must be an
// index/member accessor expression, and thus a partial pointer.
auto* e = UnwrapIndirectAndAddressOfChain(u);
if (auto* var_user = sem_.Get<sem::VariableUser>(e)) {
if (current_function_->partial_ptrs.Contains(var_user->Variable())) {
return true;
}
} else {
TINT_ASSERT(e->Is<ast::AccessorExpression>());
return true;
}
return false;
}
/// LValue holds the Nodes returned by ProcessLValueExpression()
struct LValue {
/// The control-flow node for an LValue expression
Node* cf = nullptr;
/// The new value node for an LValue expression
Node* new_val = nullptr;
/// The root identifier for an LValue expression.
const sem::Variable* root_identifier = nullptr;
};
/// Process an LValue expression.
/// @param cf the input control flow node
/// @param expr the expression to process
/// @param is_dereferencing `true` if we are dereferencing a pointer
/// @param is_partial_reference `true` if we are referencing a subset of a variable
/// @returns a tuple of (control flow node, variable node, root identifier)
LValue ProcessLValueExpression(Node* cf,
const ast::Expression* expr,
bool is_dereferencing = false,
bool is_partial_reference = false) {
return Switch(
expr,
[&](const ast::IdentifierExpression* i) {
auto* sem = sem_.GetVal(i)->UnwrapLoad()->As<sem::VariableUser>();
auto result = Switch(
sem->Variable(),
[&](const sem::GlobalVariable*) {
// Pointers cannot be stored in module-scope variables, so we should never
// be dereferencing here.
TINT_ASSERT(!is_dereferencing);
return LValue{cf, current_function_->may_be_non_uniform, nullptr};
},
[&](const sem::LocalVariable* local) {
Node* value = nullptr;
const sem::Variable* root_ident = local;
if (is_dereferencing) {
// If we are dereferencing then we must have a pointer, and the only
// declaration that can hold a pointer is a `let`.
TINT_ASSERT(local->Declaration()->Is<ast::Let>() &&
local->Type()->Is<core::type::Pointer>());
// Determine the root identifier for the contents of the pointer.
root_ident = local->Initializer()->RootIdentifier();
// Create a new value node for the contents of the pointer.
value = CreateNode({NameFor(root_ident), "_contents"});
// The uniformity of the value depends on the pointer itself.
value->AddEdge(current_function_->variables.Get(local));
} else {
// Create a new value node for this variable.
value = CreateNode({NameFor(i), "_lvalue"});
}
return LValue{cf, value, root_ident};
},
[&](const sem::Parameter* param) {
// Parameters can only be LValues when we are dereferencing a pointer.
TINT_ASSERT(is_dereferencing && param->Type()->Is<core::type::Pointer>());
// Create a new value node for the contents of the pointer.
auto* value = CreateNode({NameFor(i), "_contents"});
// The uniformity of the value depends on the pointer itself.
value->AddEdge(current_function_->parameters[param->Index()].value);
return LValue{cf, value, param};
},
[&](Default) -> LValue {
TINT_ICE() << "unknown lvalue identifier expression type: "
<< std::string(sem->Variable()->TypeInfo().name);
});
// If the identifier is part of an expression that is a partial reference to a
// variable (e.g. index or member access), we link back to the variable's previous
// value. If the previous value was non-uniform, a partial assignment will not make
// it uniform.
auto* old_value = current_function_->variables.Get(result.root_identifier);
if (is_partial_reference && old_value) {
result.new_val->AddEdge(old_value);
}
return result;
},
[&](const ast::IndexAccessorExpression* i) {
// If the source object is a pointer, there is an implicit dereference due to the
// pointer_composite_access language feature.
is_dereferencing =
is_dereferencing || sem_.GetVal(i->object)->Type()->Is<core::type::Pointer>();
auto [cf1, l1, root_ident] =
ProcessLValueExpression(cf, i->object, is_dereferencing,
/*is_partial_reference*/ true);
auto [cf2, v2] = ProcessExpression(cf1, i->index);
l1->AddEdge(v2);
return LValue{cf2, l1, root_ident};
},
[&](const ast::MemberAccessorExpression* m) {
// If the source object is a pointer, there is an implicit dereference due to the
// pointer_composite_access language feature.
is_dereferencing =
is_dereferencing || sem_.GetVal(m->object)->Type()->Is<core::type::Pointer>();
return ProcessLValueExpression(cf, m->object, is_dereferencing,
/*is_partial_reference*/ true);
},
[&](const ast::UnaryOpExpression* u) {
if (u->op == core::UnaryOp::kIndirection) {
return ProcessLValueExpression(
cf, u->expr,
/* is_dereferencing */ true,
/* is_partial_reference */ is_partial_reference ||
IsDerefOfPartialPointer(u));
}
return ProcessLValueExpression(cf, u->expr,
/* is_dereferencing */ false,
/* is_partial_reference */ is_partial_reference);
},
TINT_ICE_ON_NO_MATCH);
}
/// Process a function call expression.
/// @param cf the input control flow node
/// @param call the function call to process
/// @returns a pair of (control flow node, value node)
std::pair<Node*, Node*> ProcessCall(Node* cf, const ast::CallExpression* call) {
std::string name = NameFor(call->target);
// Process call arguments
Node* cf_last_arg = cf;
Vector<Node*, 8> args;
Vector<Node*, 8> ptrarg_contents;
ptrarg_contents.Resize(call->args.Length());
for (size_t i = 0; i < call->args.Length(); i++) {
auto [cf_i, arg_i] = ProcessExpression(cf_last_arg, call->args[i]);
// Capture the index of this argument in a new node.
// Note: This is an additional node that isn't described in the specification, for the
// purpose of providing diagnostic information.
Node* arg_node = CreateNode({name, "_arg_", std::to_string(i)}, call);
arg_node->type = Node::kFunctionCallArgumentValue;
arg_node->arg_index = static_cast<uint32_t>(i);
arg_node->AddEdge(arg_i);
// For pointer arguments, create an additional node to represent the contents of that
// pointer prior to the function call.
auto* sem_arg = sem_.GetVal(call->args[i]);
if (sem_arg->Type()->Is<core::type::Pointer>()) {
auto* arg_contents =
CreateNode({name, "_ptrarg_", std::to_string(i), "_contents"}, call);
arg_contents->type = Node::kFunctionCallArgumentContents;
arg_contents->arg_index = static_cast<uint32_t>(i);
auto* root = sem_arg->RootIdentifier();
if (root->Is<sem::GlobalVariable>()) {
if (root->Access() != core::Access::kRead) {
// The contents of a mutable global variable is always non-uniform.
arg_contents->AddEdge(current_function_->may_be_non_uniform);
}
} else {
arg_contents->AddEdge(current_function_->variables.Get(root));
}
arg_contents->AddEdge(arg_node);
ptrarg_contents[i] = arg_contents;
}
cf_last_arg = cf_i;
args.Push(arg_node);
}
// Note: This is an additional node that isn't described in the specification, for the
// purpose of providing diagnostic information.
Node* call_node = CreateNode({name, "_call"}, call);
call_node->AddEdge(cf_last_arg);
Node* result = CreateNode({name, "_return_value"}, call);
result->type = Node::kFunctionCallReturnValue;
Node* cf_after = CreateNode({"CF_after_", name}, call);
auto default_severity = kUniformityFailuresAsError ? wgsl::DiagnosticSeverity::kError
: wgsl::DiagnosticSeverity::kWarning;
// Get tags for the callee.
CallSiteTag callsite_tag = {CallSiteTag::CallSiteNoRestriction};
FunctionTag function_tag = NoRestriction;
auto* sem = SemCall(call);
const FunctionInfo* func_info = nullptr;
Switch(
sem->Target(),
[&](const sem::BuiltinFn* builtin) {
// Most builtins have no restrictions. The exceptions are barriers, derivatives,
// some texture sampling builtins, and atomics.
if (builtin->IsBarrier()) {
callsite_tag = {CallSiteTag::CallSiteRequiredToBeUniform, default_severity};
} else if (builtin->Fn() == wgsl::BuiltinFn::kWorkgroupUniformLoad) {
callsite_tag = {CallSiteTag::CallSiteRequiredToBeUniform, default_severity};
} else if (builtin->IsDerivative() ||
builtin->Fn() == wgsl::BuiltinFn::kTextureSample ||
builtin->Fn() == wgsl::BuiltinFn::kTextureSampleBias ||
builtin->Fn() == wgsl::BuiltinFn::kTextureSampleCompare) {
// Get the severity of derivative uniformity violations in this context.
auto severity = sem_.DiagnosticSeverity(
call, wgsl::CoreDiagnosticRule::kDerivativeUniformity);
if (severity != wgsl::DiagnosticSeverity::kOff) {
callsite_tag = {CallSiteTag::CallSiteRequiredToBeUniform, severity};
}
function_tag = ReturnValueMayBeNonUniform;
} else if (builtin->IsAtomic()) {
callsite_tag = {CallSiteTag::CallSiteNoRestriction};
function_tag = ReturnValueMayBeNonUniform;
} else if (builtin->Fn() == wgsl::BuiltinFn::kTextureLoad) {
// Loading from a read-write storage texture may produce a non-uniform value.
auto* storage =
builtin->Parameters()[0]->Type()->As<core::type::StorageTexture>();
if (storage && storage->Access() == core::Access::kReadWrite) {
callsite_tag = {CallSiteTag::CallSiteNoRestriction};
function_tag = ReturnValueMayBeNonUniform;
}
} else if (builtin->IsSubgroup()) {
// Get the severity of subgroup uniformity violations in this context.
auto severity = sem_.DiagnosticSeverity(
call, wgsl::CoreDiagnosticRule::kSubgroupUniformity);
if (severity != wgsl::DiagnosticSeverity::kOff) {
callsite_tag = {CallSiteTag::CallSiteRequiredToBeUniform, severity};
}
function_tag = ReturnValueMayBeNonUniform;
}
},
[&](const sem::Function* func) {
// We must have already analyzed the user-defined function since we process
// functions in dependency order.
auto info = functions_.Get(func->Declaration());
TINT_ASSERT(info);
callsite_tag = info->callsite_tag;
function_tag = info->function_tag;
func_info = info.value;
},
[&](const sem::ValueConstructor*) {
callsite_tag = {CallSiteTag::CallSiteNoRestriction};
function_tag = NoRestriction;
},
[&](const sem::ValueConversion*) {
callsite_tag = {CallSiteTag::CallSiteNoRestriction};
function_tag = NoRestriction;
}, //
TINT_ICE_ON_NO_MATCH);
cf_after->AddEdge(call_node);
if (function_tag == ReturnValueMayBeNonUniform) {
result->AddEdge(current_function_->may_be_non_uniform);
}
result->AddEdge(cf_after);
// For each argument, add edges based on parameter tags.
for (size_t i = 0; i < args.Length(); i++) {
if (func_info) {
auto& param_info = func_info->parameters[i];
// Capture the direct uniformity requirements.
switch (param_info.tag_direct.tag) {
case ParameterTag::ParameterValueRequiredToBeUniform:
current_function_->RequiredToBeUniform(param_info.tag_direct.severity)
->AddEdge(args[i]);
break;
case ParameterTag::ParameterContentsRequiredToBeUniform: {
current_function_->RequiredToBeUniform(param_info.tag_direct.severity)
->AddEdge(ptrarg_contents[i]);
break;
}
case ParameterTag::ParameterNoRestriction:
break;
}
// Capture the effects of this parameter on the return value.
switch (param_info.tag_retval.tag) {
case ParameterTag::ParameterValueRequiredToBeUniform:
result->AddEdge(args[i]);
break;
case ParameterTag::ParameterContentsRequiredToBeUniform: {
result->AddEdge(ptrarg_contents[i]);
break;
}
case ParameterTag::ParameterNoRestriction:
break;
}
// Capture the effects of other call parameters on the contents of this parameter
// after the call returns.
auto* sem_arg = sem_.GetVal(call->args[i]);
if (sem_arg->Type()->Is<core::type::Pointer>()) {
auto* ptr_result =
CreateNode({name, "_ptrarg_", std::to_string(i), "_result"}, call);
ptr_result->type = Node::kFunctionCallPointerArgumentResult;
ptr_result->arg_index = static_cast<uint32_t>(i);
if (param_info.pointer_may_become_non_uniform) {
ptr_result->AddEdge(current_function_->may_be_non_uniform);
} else {
// Add edge to the call to catch when it's called in non-uniform control
// flow.
ptr_result->AddEdge(call_node);
// Add edges from the resulting pointer value to any other arguments that
// feed it. We distinguish between requirements on the source arguments
// value versus its contents for pointer arguments.
for (auto* source : param_info.ptr_output_source_param_values) {
ptr_result->AddEdge(args[source->Index()]);
}
for (auto* source : param_info.ptr_output_source_param_contents) {
ptr_result->AddEdge(ptrarg_contents[source->Index()]);
}
}
// Update the current stored value for this pointer argument.
auto* root_ident = sem_arg->RootIdentifier();
TINT_ASSERT(root_ident);
current_function_->variables.Set(root_ident, ptr_result);
}
} else {
auto* builtin = sem->Target()->As<sem::BuiltinFn>();
if (builtin && builtin->Fn() == wgsl::BuiltinFn::kWorkgroupUniformLoad) {
// The workgroupUniformLoad builtin requires its parameter to be uniform.
current_function_->RequiredToBeUniform(default_severity)->AddEdge(args[i]);
} else if (builtin &&
(builtin->Fn() == wgsl::BuiltinFn::kSubgroupShuffleDown ||
builtin->Fn() == wgsl::BuiltinFn::kSubgroupShuffleUp ||
builtin->Fn() == wgsl::BuiltinFn::kSubgroupShuffleXor) &&
i == 1) {
// The subgroupShuffle{Down,Up,Xor} builtins require their `delta` parameters to
// be uniform.
// Get the severity of subgroup uniformity violations in this context.
auto severity = sem_.DiagnosticSeverity(
call->args[i], wgsl::CoreDiagnosticRule::kSubgroupUniformity);
if (severity != wgsl::DiagnosticSeverity::kOff) {
current_function_->RequiredToBeUniform(severity)->AddEdge(args[i]);
}
} else {
// All other builtin function parameters are RequiredToBeUniformForReturnValue,
// as are parameters for value constructors and value conversions.
result->AddEdge(args[i]);
}
}
}
// Add the callsite requirement last.
// We traverse edges in reverse order, so this makes the callsite requirement take highest
// priority when reporting violations.
if (callsite_tag.tag == CallSiteTag::CallSiteRequiredToBeUniform) {
current_function_->RequiredToBeUniform(callsite_tag.severity)->AddEdge(call_node);
}
return {cf_after, result};
}
/// Traverse a graph starting at `source`, inserting all visited nodes into `reachable` and
/// recording which node they were reached from.
/// @param source the starting node
/// @param reachable the set of reachable nodes to populate, if required
void Traverse(Node* source, UniqueVector<Node*, 4>* reachable = nullptr) {
Vector<Node*, 8> to_visit{source};
while (!to_visit.IsEmpty()) {
auto* node = to_visit.Back();
to_visit.Pop();
if (reachable) {
reachable->Add(node);
}
for (auto* to : node->edges) {
if (to->visited_from == nullptr) {
to->visited_from = node;
to_visit.Push(to);
}
}
}
}
/// Trace back along a path from `start` until finding a node that matches a predicate.
/// @param start the starting node
/// @param pred the predicate function
/// @returns the first node found that matches the predicate, or nullptr
template <typename F>
Node* TraceBackAlongPathUntil(Node* start, F&& pred) {
auto* current = start;
while (current) {
if (pred(current)) {
break;
}
current = current->visited_from;
}
return current;
}
/// Recursively descend through the function called by `call` and the functions that it calls in
/// order to find a call to a builtin function that requires uniformity with the given severity.
const ast::CallExpression* FindBuiltinThatRequiresUniformity(
const ast::CallExpression* call,
wgsl::DiagnosticSeverity severity) {
auto* target = SemCall(call)->Target();
if (target->Is<sem::BuiltinFn>()) {
// This is a call to a builtin, so we must be done.
return call;
} else if (auto* user = target->As<sem::Function>()) {
// This is a call to a user-defined function, so inspect the functions called by that
// function and look for one whose node has an edge from the RequiredToBeUniform node.
auto target_info = functions_.Get(user->Declaration());
for (auto* call_node : target_info->RequiredToBeUniform(severity)->edges) {
if (call_node->type == Node::kRegular) {
auto* child_call = call_node->ast->As<ast::CallExpression>();
return FindBuiltinThatRequiresUniformity(child_call, severity);
}
}
TINT_UNREACHABLE() << "unable to find child call with uniformity requirement";
} else {
TINT_UNREACHABLE() << "unexpected call expression type";
}
}
/// Add diagnostic notes to show where control flow became non-uniform on the way to a node.
/// @param function the function being analyzed
/// @param required_to_be_uniform the node to traverse from
/// @param may_be_non_uniform the node to traverse to
void ShowControlFlowDivergence(FunctionInfo& function,
Node* required_to_be_uniform,
Node* may_be_non_uniform) {
// Traverse the graph to generate a path from the node to the source of non-uniformity.
function.ResetVisited();
Traverse(required_to_be_uniform);
// Get the source of the non-uniform value.
auto* non_uniform_source = may_be_non_uniform;
if (non_uniform_source == function.may_be_non_uniform) {
non_uniform_source = non_uniform_source->visited_from;
}
TINT_ASSERT(non_uniform_source);
// Show where the non-uniform value results in non-uniform control flow.
auto* control_flow = TraceBackAlongPathUntil(
non_uniform_source, [](Node* node) { return node->affects_control_flow; });
if (control_flow) {
diagnostics_.AddNote(control_flow->ast->source)
<< "control flow depends on possibly non-uniform value";
// TODO(jrprice): There are cases where the function with uniformity requirements is not
// actually inside this control flow construct, for example:
// - A conditional interrupt (e.g. break), with a barrier elsewhere in the loop
// - A conditional assignment to a variable, which is later used to guard a barrier
// In these cases, the diagnostics are not entirely accurate as they may not highlight
// the actual cause of divergence.
}
ShowSourceOfNonUniformity(non_uniform_source);
}
/// Add a diagnostic note to show the origin of a non-uniform value.
/// @param non_uniform_source the node that represents a non-uniform value
void ShowSourceOfNonUniformity(Node* non_uniform_source) {
TINT_ASSERT(non_uniform_source);
auto var_type = [&](const sem::Variable* var) {
switch (var->AddressSpace()) {
case core::AddressSpace::kStorage:
return "read_write storage buffer ";
case core::AddressSpace::kWorkgroup:
return "workgroup storage variable ";
case core::AddressSpace::kPrivate:
return "module-scope private variable ";
default:
return "";
}
};
auto param_type = [&](const ast::Parameter* param) {
if (ast::HasAttribute<ast::BuiltinAttribute>(param->attributes)) {
return "builtin ";
} else if (ast::HasAttribute<ast::LocationAttribute>(param->attributes)) {
return "user-defined input ";
} else {
return "parameter ";
}
};
// Show the source of the non-uniform value.
Switch(
non_uniform_source->ast,
[&](const ast::IdentifierExpression* ident) {
auto* var = sem_.GetVal(ident)->UnwrapLoad()->As<sem::VariableUser>()->Variable();
if (auto* param = var->As<sem::Parameter>()) {
auto* func = param->Owner()->As<sem::Function>();
diagnostics_.AddNote(ident->source)
<< param_type(param->Declaration()) << "'" << NameFor(ident) << "' of '"
<< NameFor(func) << "' may be non-uniform";
} else {
diagnostics_.AddNote(ident->source)
<< "reading from " << var_type(var) << "'" << NameFor(ident)
<< "' may result in a non-uniform value";
}
},
[&](const ast::Parameter* p) {
auto* param = sem_.Get(p);
auto* func = param->Owner()->As<sem::Function>();
if (non_uniform_source->type == Node::kFunctionPointerParameterContents) {
diagnostics_.AddNote(p->source)
<< "parameter '" << NameFor(p) << "' of '" << NameFor(func)
<< "' may point to a non-uniform value";
} else {
diagnostics_.AddNote(p->source)
<< param_type(p) << "'" << NameFor(p) << "' of '" << NameFor(func)
<< "' may be non-uniform";
}
},
[&](const ast::Variable* v) {
auto* var = sem_.Get(v);
diagnostics_.AddNote(v->source)
<< "reading from " << var_type(var) << "'" << NameFor(v)
<< "' may result in a non-uniform value";
},
[&](const ast::CallExpression* c) {
auto target_name = NameFor(c->target);
switch (non_uniform_source->type) {
case Node::kFunctionCallReturnValue: {
diagnostics_.AddNote(c->source)
<< "return value of '" + target_name + "' may be non-uniform";
break;
}
case Node::kFunctionCallArgumentContents: {
auto* arg = c->args[non_uniform_source->arg_index];
auto* var = sem_.GetVal(arg)->RootIdentifier();
diagnostics_.AddNote(var->Declaration()->source)
<< "reading from " << var_type(var) << "'" << NameFor(var)
<< "' may result in a non-uniform value";
break;
}
case Node::kFunctionCallArgumentValue: {
auto* arg = c->args[non_uniform_source->arg_index];
// TODO(jrprice): Which output? (return value vs another pointer argument).
diagnostics_.AddNote(arg->source)
<< "passing non-uniform pointer to '" << target_name
<< "' may produce a non-uniform output";
break;
}
case Node::kFunctionCallPointerArgumentResult: {
diagnostics_.AddNote(c->args[non_uniform_source->arg_index]->source)
<< "contents of pointer may become non-uniform after calling '"
<< target_name << "'";
break;
}
default: {
TINT_ICE() << "unhandled source of non-uniformity";
}
}
},
[&](const ast::Expression* e) {
diagnostics_.AddNote(e->source) << "result of expression may be non-uniform";
}, //
TINT_ICE_ON_NO_MATCH);
}
/// Generate a diagnostic message for a uniformity issue.
/// @param function the function that the diagnostic is being produced for
/// @param source_node the node that has caused a uniformity issue in `function`
/// @param severity the severity of the diagnostic
void MakeError(FunctionInfo& function, Node* source_node, wgsl::DiagnosticSeverity severity) {
// Helper to produce a diagnostic message, as a note or with the global failure severity.
auto report = [&](Source source, std::string msg, bool note) {
diag::Diagnostic error{};
error.severity = note ? diag::Severity::Note : wgsl::ToSeverity(severity);
error.source = source;
error.message = msg;
diagnostics_.Add(std::move(error));
};
// Traverse the graph to generate a path from RequiredToBeUniform to the source node.
function.ResetVisited();
Traverse(function.RequiredToBeUniform(severity));
TINT_ASSERT(source_node->visited_from);
// Find a node that is required to be uniform that has a path to the source node.
auto* cause = TraceBackAlongPathUntil(source_node, [&](Node* node) {
return node->visited_from == function.RequiredToBeUniform(severity);
});
// The node will always have a corresponding call expression.
auto* call = cause->ast->As<ast::CallExpression>();
TINT_ASSERT(call);
auto* target = SemCall(call)->Target();
auto func_name = NameFor(call->target);
if (cause->type == Node::kFunctionCallArgumentValue ||
cause->type == Node::kFunctionCallArgumentContents) {
bool is_value = (cause->type == Node::kFunctionCallArgumentValue);
auto* user_func = target->As<sem::Function>();
if (user_func) {
// Recurse into the called function to show the reason for the requirement.
auto next_function = functions_.Get(user_func->Declaration());
auto& param_info = next_function->parameters[cause->arg_index];
MakeError(*next_function,
is_value ? param_info.value : param_info.ptr_input_contents, severity);
// Show the place where the non-uniform argument was passed.
// If this is a builtin, this will be the trigger location for the failure.
StringStream ss;
ss << "possibly non-uniform value passed" << (is_value ? "" : " via pointer")
<< " here";
report(call->args[cause->arg_index]->source, ss.str(), /* note */ true);
} else {
// The uniformity requirement must come from a builtin function.
auto* builtin = target->As<sem::BuiltinFn>();
TINT_ASSERT(builtin);
StringStream ss;
ss << "'" << builtin->Fn() << "' requires argument " << cause->arg_index << " to "
<< (is_value ? "be uniform" : "have uniform contents");
report(call->args[cause->arg_index]->source, ss.str(), /* note */ false);
}
// Show the origin of non-uniformity for the value or data that is being passed.
ShowSourceOfNonUniformity(source_node->visited_from);
} else {
auto* builtin_call = FindBuiltinThatRequiresUniformity(call, severity);
{
// Show a builtin was reachable from this call (which may be the call itself).
// This will be the trigger location for the failure.
StringStream ss;
ss << "'" << NameFor(builtin_call->target)
<< "' must only be called from uniform control flow";
report(builtin_call->source, ss.str(), /* note */ false);
}
if (builtin_call != call) {
// The call was to a user function, so show that call too.
StringStream ss;
ss << "called ";
if (target->As<sem::Function>() != SemCall(builtin_call)->Stmt()->Function()) {
ss << "indirectly ";
}
ss << "by '" << func_name << "' from '" << function.name << "'";
report(call->source, ss.str(), /* note */ true);
}
// Show the point at which control-flow depends on a non-uniform value.
ShowControlFlowDivergence(function, cause, source_node);
}
}
// Helper for obtaining the sem::Call node for the ast::CallExpression
const sem::Call* SemCall(const ast::CallExpression* expr) const {
return sem_.Get(expr)->UnwrapMaterialize()->As<sem::Call>();
}
};
} // namespace
bool AnalyzeUniformity(ProgramBuilder& builder, const DependencyGraph& dependency_graph) {
UniformityGraph graph(builder);
return graph.Build(dependency_graph);
}
} // namespace tint::resolver