src/tint/lang/wgsl/ast/transform/demote_to_helper.cc

// Copyright 2022 The Dawn & Tint Authors
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#include "src/tint/lang/wgsl/ast/transform/demote_to_helper.h"

#include <unordered_map>
#include <unordered_set>
#include <utility>

#include "src/tint/lang/core/type/reference.h"
#include "src/tint/lang/wgsl/ast/transform/hoist_to_decl_before.h"
#include "src/tint/lang/wgsl/program/clone_context.h"
#include "src/tint/lang/wgsl/program/program_builder.h"
#include "src/tint/lang/wgsl/resolver/resolve.h"
#include "src/tint/lang/wgsl/sem/block_statement.h"
#include "src/tint/lang/wgsl/sem/call.h"
#include "src/tint/lang/wgsl/sem/function.h"
#include "src/tint/lang/wgsl/sem/statement.h"
#include "src/tint/utils/containers/map.h"
#include "src/tint/utils/rtti/switch.h"

TINT_INSTANTIATE_TYPEINFO(tint::ast::transform::DemoteToHelper);

using namespace tint::core::number_suffixes;  // NOLINT

namespace tint::ast::transform {

DemoteToHelper::DemoteToHelper() = default;

DemoteToHelper::~DemoteToHelper() = default;

Transform::ApplyResult DemoteToHelper::Apply(const Program& src, const DataMap&, DataMap&) const {
    auto& sem = src.Sem();

    // Collect the set of functions that need to be processed.
    // A function needs to be processed if it is reachable by a shader that contains a discard at
    // any point in its call hierarchy.
    std::unordered_set<const sem::Function*> functions_to_process;
    for (auto* func : src.AST().Functions()) {
        if (!func->IsEntryPoint()) {
            continue;
        }

        // Determine whether this entry point and its callees need to be transformed.
        bool needs_transform = false;
        if (sem.Get(func)->DiscardStatement()) {
            needs_transform = true;
        } else {
            for (auto* callee : sem.Get(func)->TransitivelyCalledFunctions()) {
                if (callee->DiscardStatement()) {
                    needs_transform = true;
                    break;
                }
            }
        }
        if (!needs_transform) {
            continue;
        }

        // Process the entry point and its callees.
        functions_to_process.insert(sem.Get(func));
        for (auto* callee : sem.Get(func)->TransitivelyCalledFunctions()) {
            functions_to_process.insert(callee);
        }
    }

    if (functions_to_process.empty()) {
        return SkipTransform;
    }

    ProgramBuilder b;
    program::CloneContext ctx{&b, &src, /* auto_clone_symbols */ true};

    // Create a module-scope flag that indicates whether the current invocation has been discarded.
    auto flag = b.Symbols().New("tint_discarded");
    b.GlobalVar(flag, core::AddressSpace::kPrivate, b.Expr(false));

    // Replace all discard statements with a statement that marks the invocation as discarded.
    ctx.ReplaceAll(
        [&](const DiscardStatement*) -> const Statement* { return b.Assign(flag, b.Expr(true)); });

    // Insert a conditional discard at the end of each entry point that does not end with a return.
    for (auto* func : functions_to_process) {
        if (func->Declaration()->IsEntryPoint()) {
            auto* sem_body = sem.Get(func->Declaration()->body);
            if (sem_body->Behaviors().Contains(sem::Behavior::kNext)) {
                ctx.InsertBack(func->Declaration()->body->statements,
                               b.If(flag, b.Block(b.Discard())));
            }
        }
    }

    HoistToDeclBefore hoist_to_decl_before(ctx);

    // Mask all writes to host-visible memory using the discarded flag.
    // We also insert a discard statement before all return statements in entry points for shaders
    // that discard.
    std::unordered_map<const core::type::Type*, Symbol> atomic_cmpxchg_result_types;
    for (auto* node : src.ASTNodes().Objects()) {
        Switch(
            node,

            // Mask assignments to storage buffer variables.
            [&](const AssignmentStatement* assign) {
                // Skip writes in functions that are not called from shaders that discard.
                auto* func = sem.Get(assign)->Function();
                if (functions_to_process.count(func) == 0) {
                    return;
                }

                // Skip phony assignments.
                if (assign->lhs->Is<PhonyExpression>()) {
                    return;
                }

                // Skip writes to invocation-private address spaces.
                auto* ref = sem.GetVal(assign->lhs)->Type()->As<core::type::Reference>();
                switch (ref->AddressSpace()) {
                    case core::AddressSpace::kStorage:
                        // Need to mask these.
                        break;
                    case core::AddressSpace::kFunction:
                    case core::AddressSpace::kPrivate:
                    case core::AddressSpace::kOut:
                        // Skip these.
                        return;
                    default:
                        TINT_UNREACHABLE()
                            << "write to unhandled address space: " << ref->AddressSpace();
                }

                // If the RHS has side effects (which may contain derivative operations), we need to
                // hoist it out to a separate declaration so that it does not get masked.
                auto* rhs = sem.GetVal(assign->rhs);
                if (rhs->HasSideEffects()) {
                    hoist_to_decl_before.Add(rhs, assign->rhs,
                                             HoistToDeclBefore::VariableKind::kLet);
                }

                // Mask the assignment using the invocation-discarded flag.
                ctx.Replace(assign, b.If(b.Not(flag), b.Block(ctx.Clone(assign))));
            },

            // Mask builtins that write to host-visible memory.
            [&](const CallExpression* call) {
                auto* sem_call = sem.Get<sem::Call>(call);
                auto* stmt = sem_call ? sem_call->Stmt() : nullptr;
                auto* func = stmt ? stmt->Function() : nullptr;
                auto* builtin = sem_call ? sem_call->Target()->As<sem::BuiltinFn>() : nullptr;
                if (functions_to_process.count(func) == 0 || !builtin) {
                    return;
                }

                if (builtin->Fn() == wgsl::BuiltinFn::kTextureStore) {
                    // A call to textureStore() will always be a statement.
                    // Wrap it inside a conditional block.
                    auto* masked_call = b.If(b.Not(flag), b.Block(ctx.Clone(stmt->Declaration())));
                    ctx.Replace(stmt->Declaration(), masked_call);
                } else if (builtin->IsAtomic() && builtin->Fn() != wgsl::BuiltinFn::kAtomicLoad) {
                    // A call to an atomic builtin can be a statement or an expression.
                    if (auto* call_stmt = stmt->Declaration()->As<CallStatement>();
                        call_stmt && call_stmt->expr == call) {
                        // This call is a statement.
                        // Wrap it inside a conditional block.
                        auto* masked_call = b.If(b.Not(flag), b.Block(ctx.Clone(call_stmt)));
                        ctx.Replace(stmt->Declaration(), masked_call);
                    } else {
                        // This call is an expression.
                        // We transform:
                        //   let y = x + atomicAdd(&p, 1);
                        // Into:
                        //   var tmp : i32;
                        //   if (!tint_discarded) {
                        //     tmp = atomicAdd(&p, 1);
                        //   }
                        //   let y = x + tmp;
                        auto result = b.Sym();
                        auto result_ty = CreateASTTypeFor(ctx, sem_call->Type());
                        auto* masked_call =
                            b.If(b.Not(flag),
                                 b.Block(b.Assign(result, ctx.CloneWithoutTransform(call))));
                        auto* result_decl = b.Decl(b.Var(result, result_ty));
                        hoist_to_decl_before.Prepare(sem_call);
                        hoist_to_decl_before.InsertBefore(stmt, result_decl);
                        hoist_to_decl_before.InsertBefore(stmt, masked_call);
                        ctx.Replace(call, b.Expr(result));
                    }
                }
            },

            // Insert a conditional discard before all return statements in entry points.
            [&](const ReturnStatement* ret) {
                auto* func = sem.Get(ret)->Function();
                if (func->Declaration()->IsEntryPoint() && functions_to_process.count(func)) {
                    auto* discard = b.If(flag, b.Block(b.Discard()));
                    ctx.InsertBefore(sem.Get(ret)->Block()->Declaration()->statements, ret,
                                     discard);
                }
            });
    }

    ctx.Clone();
    return resolver::Resolve(b);
}

}  // namespace tint::ast::transform