src/tint/transform/zero_init_workgroup_memory.cc - dawn - Git at Google

 // Copyright 2021 The Tint Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 #include "src/tint/transform/zero_init_workgroup_memory.h"

 #include <algorithm>
 #include <map>
 #include <unordered_map>
 #include <utility>
 #include <vector>

 #include "src/tint/ast/workgroup_attribute.h"
 #include "src/tint/program_builder.h"
 #include "src/tint/sem/atomic.h"
 #include "src/tint/sem/function.h"
 #include "src/tint/sem/variable.h"
 #include "src/tint/utils/map.h"
 #include "src/tint/utils/unique_vector.h"

 TINT_INSTANTIATE_TYPEINFO(tint::transform::ZeroInitWorkgroupMemory);

 namespace tint::transform {

 using StatementList = utils::Vector<const ast::Statement*, 8>;

 /// PIMPL state for the ZeroInitWorkgroupMemory transform
 struct ZeroInitWorkgroupMemory::State {
     /// The clone context
     CloneContext& ctx;

     /// An alias to *ctx.dst
     ProgramBuilder& b = *ctx.dst;

     /// The constant size of the workgroup. If 0, then #workgroup_size_expr should
     /// be used instead.
     uint32_t workgroup_size_const = 0;
     /// The size of the workgroup as an expression generator. Use if
     /// #workgroup_size_const is 0.
     std::function<const ast::Expression*()> workgroup_size_expr;

     /// ArrayIndex represents a function on the local invocation index, of
     /// the form: `array_index = (local_invocation_index % modulo) / division`
     struct ArrayIndex {
         /// The RHS of the modulus part of the expression
         uint32_t modulo = 1;
         /// The RHS of the division part of the expression
         uint32_t division = 1;

         /// Equality operator
         /// @param i the ArrayIndex to compare to this ArrayIndex
         /// @returns true if `i` and this ArrayIndex are equal
         bool operator==(const ArrayIndex& i) const {
             return modulo == i.modulo && division == i.division;
         }

         /// Hash function for the ArrayIndex type
         struct Hasher {
             /// @param i the ArrayIndex to calculate a hash for
             /// @returns the hash value for the ArrayIndex `i`
             size_t operator()(const ArrayIndex& i) const {
                 return utils::Hash(i.modulo, i.division);
             }
         };
     };

     /// A list of unique ArrayIndex
     using ArrayIndices = utils::UniqueVector<ArrayIndex, 4, ArrayIndex::Hasher>;

     /// Expression holds information about an expression that is being built for a
     /// statement will zero workgroup values.
     struct Expression {
         /// The AST expression node
         const ast::Expression* expr = nullptr;
         /// The number of iterations required to zero the value
         uint32_t num_iterations = 0;
         /// All array indices used by this expression
         ArrayIndices array_indices;
     };

     /// Statement holds information about a statement that will zero workgroup
     /// values.
     struct Statement {
         /// The AST statement node
         const ast::Statement* stmt;
         /// The number of iterations required to zero the value
         uint32_t num_iterations;
         /// All array indices used by this statement
         ArrayIndices array_indices;
     };

     /// All statements that zero workgroup memory
     std::vector<Statement> statements;

     /// A map of ArrayIndex to the name reserved for the `let` declaration of that
     /// index.
     std::unordered_map<ArrayIndex, Symbol, ArrayIndex::Hasher> array_index_names;

     /// Constructor
     /// @param c the CloneContext used for the transform
     explicit State(CloneContext& c) : ctx(c) {}

     /// Run inserts the workgroup memory zero-initialization logic at the top of
     /// the given function
     /// @param fn a compute shader entry point function
     void Run(const ast::Function* fn) {
         auto& sem = ctx.src->Sem();

         CalculateWorkgroupSize(ast::GetAttribute<ast::WorkgroupAttribute>(fn->attributes));

         // Generate a list of statements to zero initialize each of the
         // workgroup storage variables used by `fn`. This will populate #statements.
         auto* func = sem.Get(fn);
         for (auto* var : func->TransitivelyReferencedGlobals()) {
             if (var->AddressSpace() == ast::AddressSpace::kWorkgroup) {
                 BuildZeroingStatements(var->Type()->UnwrapRef(), [&](uint32_t num_values) {
                     auto var_name = ctx.Clone(var->Declaration()->symbol);
                     return Expression{b.Expr(var_name), num_values, ArrayIndices{}};
                 });
             }
         }

         if (statements.empty()) {
             return;  // No workgroup variables to initialize.
         }

         // Scan the entry point for an existing local_invocation_index builtin
         // parameter
         std::function<const ast::Expression*()> local_index;
         for (auto* param : fn->params) {
             if (auto* builtin = ast::GetAttribute<ast::BuiltinAttribute>(param->attributes)) {
                 if (builtin->builtin == ast::BuiltinValue::kLocalInvocationIndex) {
                     local_index = [=] { return b.Expr(ctx.Clone(param->symbol)); };
                     break;
                 }
             }

             if (auto* str = sem.Get(param)->Type()->As<sem::Struct>()) {
                 for (auto* member : str->Members()) {
                     if (auto* builtin = ast::GetAttribute<ast::BuiltinAttribute>(
                             member->Declaration()->attributes)) {
                         if (builtin->builtin == ast::BuiltinValue::kLocalInvocationIndex) {
                             local_index = [=] {
                                 auto* param_expr = b.Expr(ctx.Clone(param->symbol));
                                 auto member_name = ctx.Clone(member->Declaration()->symbol);
                                 return b.MemberAccessor(param_expr, member_name);
                             };
                             break;
                         }
                     }
                 }
             }
         }
         if (!local_index) {
             // No existing local index parameter. Append one to the entry point.
             auto* param = b.Param(b.Symbols().New("local_invocation_index"), b.ty.u32(),
                                   utils::Vector{
                                       b.Builtin(ast::BuiltinValue::kLocalInvocationIndex),
                                   });
             ctx.InsertBack(fn->params, param);
             local_index = [=] { return b.Expr(param->symbol); };
         }

         // Take the zeroing statements and bin them by the number of iterations
         // required to zero the workgroup data. We then emit these in blocks,
         // possibly wrapped in if-statements or for-loops.
         std::unordered_map<uint32_t, std::vector<Statement>> stmts_by_num_iterations;
         std::vector<uint32_t> num_sorted_iterations;
         for (auto& s : statements) {
             auto& stmts = stmts_by_num_iterations[s.num_iterations];
             if (stmts.empty()) {
                 num_sorted_iterations.emplace_back(s.num_iterations);
             }
             stmts.emplace_back(s);
         }
         std::sort(num_sorted_iterations.begin(), num_sorted_iterations.end());

         // Loop over the statements, grouped by num_iterations.
         for (auto num_iterations : num_sorted_iterations) {
             auto& stmts = stmts_by_num_iterations[num_iterations];

             // Gather all the array indices used by all the statements in the block.
             ArrayIndices array_indices;
             for (auto& s : stmts) {
                 for (auto& idx : s.array_indices) {
                     array_indices.Add(idx);
                 }
             }

             // Determine the block type used to emit these statements.

             if (workgroup_size_const == 0 || num_iterations > workgroup_size_const) {
                 // Either the workgroup size is dynamic, or smaller than num_iterations.
                 // In either case, we need to generate a for loop to ensure we
                 // initialize all the array elements.
                 //
                 //  for (var idx : u32 = local_index;
                 //           idx < num_iterations;
                 //           idx += workgroup_size) {
                 //    ...
                 //  }
                 auto idx = b.Symbols().New("idx");
                 auto* init = b.Decl(b.Var(idx, b.ty.u32(), local_index()));
                 auto* cond = b.create<ast::BinaryExpression>(ast::BinaryOp::kLessThan, b.Expr(idx),
                                                              b.Expr(u32(num_iterations)));
                 auto* cont = b.Assign(
                     idx, b.Add(idx, workgroup_size_const ? b.Expr(u32(workgroup_size_const))
                                                          : workgroup_size_expr()));

                 auto block =
                     DeclareArrayIndices(num_iterations, array_indices, [&] { return b.Expr(idx); });
                 for (auto& s : stmts) {
                     block.Push(s.stmt);
                 }
                 auto* for_loop = b.For(init, cond, cont, b.Block(block));
                 ctx.InsertFront(fn->body->statements, for_loop);
             } else if (num_iterations < workgroup_size_const) {
                 // Workgroup size is a known constant, but is greater than
                 // num_iterations. Emit an if statement:
                 //
                 //  if (local_index < num_iterations) {
                 //    ...
                 //  }
                 auto* cond = b.create<ast::BinaryExpression>(
                     ast::BinaryOp::kLessThan, local_index(), b.Expr(u32(num_iterations)));
                 auto block = DeclareArrayIndices(num_iterations, array_indices,
                                                  [&] { return b.Expr(local_index()); });
                 for (auto& s : stmts) {
                     block.Push(s.stmt);
                 }
                 auto* if_stmt = b.If(cond, b.Block(block));
                 ctx.InsertFront(fn->body->statements, if_stmt);
             } else {
                 // Workgroup size exactly equals num_iterations.
                 // No need for any conditionals. Just emit a basic block:
                 //
                 // {
                 //    ...
                 // }
                 auto block = DeclareArrayIndices(num_iterations, array_indices,
                                                  [&] { return b.Expr(local_index()); });
                 for (auto& s : stmts) {
                     block.Push(s.stmt);
                 }
                 ctx.InsertFront(fn->body->statements, b.Block(block));
             }
         }

         // Append a single workgroup barrier after the zero initialization.
         ctx.InsertFront(fn->body->statements, b.CallStmt(b.Call("workgroupBarrier")));
     }

     /// BuildZeroingExpr is a function that builds a sub-expression used to zero
     /// workgroup values. `num_values` is the number of elements that the
     /// expression will be used to zero. Returns the expression.
     using BuildZeroingExpr = std::function<Expression(uint32_t num_values)>;

     /// BuildZeroingStatements() generates the statements required to zero
     /// initialize the workgroup storage expression of type `ty`.
     /// @param ty the expression type
     /// @param get_expr a function that builds the AST nodes for the expression.
     void BuildZeroingStatements(const sem::Type* ty, const BuildZeroingExpr& get_expr) {
         if (CanTriviallyZero(ty)) {
             auto var = get_expr(1u);
             auto* zero_init = b.Construct(CreateASTTypeFor(ctx, ty));
             statements.emplace_back(
                 Statement{b.Assign(var.expr, zero_init), var.num_iterations, var.array_indices});
             return;
         }

         if (auto* atomic = ty->As<sem::Atomic>()) {
             auto* zero_init = b.Construct(CreateASTTypeFor(ctx, atomic->Type()));
             auto expr = get_expr(1u);
             auto* store = b.Call("atomicStore", b.AddressOf(expr.expr), zero_init);
             statements.emplace_back(
                 Statement{b.CallStmt(store), expr.num_iterations, expr.array_indices});
             return;
         }

         if (auto* str = ty->As<sem::Struct>()) {
             for (auto* member : str->Members()) {
                 auto name = ctx.Clone(member->Declaration()->symbol);
                 BuildZeroingStatements(member->Type(), [&](uint32_t num_values) {
                     auto s = get_expr(num_values);
                     return Expression{b.MemberAccessor(s.expr, name), s.num_iterations,
                                       s.array_indices};
                 });
             }
             return;
         }

         if (auto* arr = ty->As<sem::Array>()) {
             BuildZeroingStatements(arr->ElemType(), [&](uint32_t num_values) {
                 // num_values is the number of values to zero for the element type.
                 // The number of iterations required to zero the array and its elements
                 // is:
                 //      `num_values * arr->Count()`
                 // The index for this array is:
                 //      `(idx % modulo) / division`
                 auto count = arr->ConstantCount();
                 if (!count) {
                     ctx.dst->Diagnostics().add_error(diag::System::Transform,
                                                      sem::Array::kErrExpectedConstantCount);
                     return Expression{};
                 }
                 auto modulo = num_values * count.value();
                 auto division = num_values;
                 auto a = get_expr(modulo);
                 auto array_indices = a.array_indices;
                 array_indices.Add(ArrayIndex{modulo, division});
                 auto index = utils::GetOrCreate(array_index_names, ArrayIndex{modulo, division},
                                                 [&] { return b.Symbols().New("i"); });
                 return Expression{b.IndexAccessor(a.expr, index), a.num_iterations, array_indices};
             });
             return;
         }

         TINT_UNREACHABLE(Transform, b.Diagnostics())
             << "could not zero workgroup type: " << ty->FriendlyName(ctx.src->Symbols());
     }

     /// DeclareArrayIndices returns a list of statements that contain the `let`
     /// declarations for all of the ArrayIndices.
     /// @param num_iterations the number of iterations for the block
     /// @param array_indices the list of array indices to generate `let`
     ///        declarations for
     /// @param iteration a function that returns the index of the current
     ///         iteration.
     /// @returns the list of `let` statements that declare the array indices
     StatementList DeclareArrayIndices(uint32_t num_iterations,
                                       const ArrayIndices& array_indices,
                                       const std::function<const ast::Expression*()>& iteration) {
         StatementList stmts;
         std::map<Symbol, ArrayIndex> indices_by_name;
         for (auto index : array_indices) {
             auto name = array_index_names.at(index);
             auto* mod = (num_iterations > index.modulo)
                             ? b.create<ast::BinaryExpression>(ast::BinaryOp::kModulo, iteration(),
                                                               b.Expr(u32(index.modulo)))
                             : iteration();
             auto* div = (index.division != 1u) ? b.Div(mod, u32(index.division)) : mod;
             auto* decl = b.Decl(b.Let(name, b.ty.u32(), div));
             stmts.Push(decl);
         }
         return stmts;
     }

     /// CalculateWorkgroupSize initializes the members #workgroup_size_const and
     /// #workgroup_size_expr with the linear workgroup size.
     /// @param attr the workgroup attribute applied to the entry point function
     void CalculateWorkgroupSize(const ast::WorkgroupAttribute* attr) {
         bool is_signed = false;
         workgroup_size_const = 1u;
         workgroup_size_expr = nullptr;
         for (auto* expr : attr->Values()) {
             if (!expr) {
                 continue;
             }
             auto* sem = ctx.src->Sem().Get(expr);
             if (auto* c = sem->ConstantValue()) {
                 workgroup_size_const *= c->As<AInt>();
                 continue;
             }
             // Constant value could not be found. Build expression instead.
             workgroup_size_expr = [this, expr, size = workgroup_size_expr] {
                 auto* e = ctx.Clone(expr);
                 if (ctx.src->TypeOf(expr)->UnwrapRef()->Is<sem::I32>()) {
                     e = b.Construct<u32>(e);
                 }
                 return size ? b.Mul(size(), e) : e;
             };
         }
         if (workgroup_size_expr) {
             if (workgroup_size_const != 1) {
                 // Fold workgroup_size_const in to workgroup_size_expr
                 workgroup_size_expr = [this, is_signed, const_size = workgroup_size_const,
                                        expr_size = workgroup_size_expr] {
                     return is_signed ? b.Mul(expr_size(), i32(const_size))
                                      : b.Mul(expr_size(), u32(const_size));
                 };
             }
             // Indicate that workgroup_size_expr should be used instead of the
             // constant.
             workgroup_size_const = 0;
         }
     }

     /// @returns true if a variable with store type `ty` can be efficiently zeroed
     /// by assignment of a type initializer without operands. If
     /// CanTriviallyZero() returns false, then the type needs to be
     /// initialized by decomposing the initialization into multiple
     /// sub-initializations.
     /// @param ty the type to inspect
     bool CanTriviallyZero(const sem::Type* ty) {
         if (ty->Is<sem::Atomic>()) {
             return false;
         }
         if (auto* str = ty->As<sem::Struct>()) {
             for (auto* member : str->Members()) {
                 if (!CanTriviallyZero(member->Type())) {
                     return false;
                 }
             }
         }
         if (ty->Is<sem::Array>()) {
             return false;
         }
         // True for all other storable types
         return true;
     }
 };

 ZeroInitWorkgroupMemory::ZeroInitWorkgroupMemory() = default;

 ZeroInitWorkgroupMemory::~ZeroInitWorkgroupMemory() = default;

 bool ZeroInitWorkgroupMemory::ShouldRun(const Program* program, const DataMap&) const {
     for (auto* global : program->AST().GlobalVariables()) {
         if (auto* var = global->As<ast::Var>()) {
             if (var->declared_address_space == ast::AddressSpace::kWorkgroup) {
                 return true;
             }
         }
     }
     return false;
 }

 void ZeroInitWorkgroupMemory::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
     for (auto* fn : ctx.src->AST().Functions()) {
         if (fn->PipelineStage() == ast::PipelineStage::kCompute) {
             State{ctx}.Run(fn);
         }
     }
     ctx.Clone();
 }

 }  // namespace tint::transform
	// Copyright 2021 The Tint Authors.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	#include "src/tint/transform/zero_init_workgroup_memory.h"

	#include <algorithm>
	#include <map>
	#include <unordered_map>
	#include <utility>
	#include <vector>

	#include "src/tint/ast/workgroup_attribute.h"
	#include "src/tint/program_builder.h"
	#include "src/tint/sem/atomic.h"
	#include "src/tint/sem/function.h"
	#include "src/tint/sem/variable.h"
	#include "src/tint/utils/map.h"
	#include "src/tint/utils/unique_vector.h"

	TINT_INSTANTIATE_TYPEINFO(tint::transform::ZeroInitWorkgroupMemory);

	namespace tint::transform {

	using StatementList = utils::Vector<const ast::Statement*, 8>;

	/// PIMPL state for the ZeroInitWorkgroupMemory transform
	struct ZeroInitWorkgroupMemory::State {
	/// The clone context
	CloneContext& ctx;

	/// An alias to *ctx.dst
	ProgramBuilder& b = *ctx.dst;

	/// The constant size of the workgroup. If 0, then #workgroup_size_expr should
	/// be used instead.
	uint32_t workgroup_size_const = 0;
	/// The size of the workgroup as an expression generator. Use if
	/// #workgroup_size_const is 0.
	std::function<const ast::Expression*()> workgroup_size_expr;

	/// ArrayIndex represents a function on the local invocation index, of
	/// the form: `array_index = (local_invocation_index % modulo) / division`
	struct ArrayIndex {
	/// The RHS of the modulus part of the expression
	uint32_t modulo = 1;
	/// The RHS of the division part of the expression
	uint32_t division = 1;

	/// Equality operator
	/// @param i the ArrayIndex to compare to this ArrayIndex
	/// @returns true if `i` and this ArrayIndex are equal
	bool operator==(const ArrayIndex& i) const {
	return modulo == i.modulo && division == i.division;
	}

	/// Hash function for the ArrayIndex type
	struct Hasher {
	/// @param i the ArrayIndex to calculate a hash for
	/// @returns the hash value for the ArrayIndex `i`
	size_t operator()(const ArrayIndex& i) const {
	return utils::Hash(i.modulo, i.division);
	}
	};
	};

	/// A list of unique ArrayIndex
	using ArrayIndices = utils::UniqueVector<ArrayIndex, 4, ArrayIndex::Hasher>;

	/// Expression holds information about an expression that is being built for a
	/// statement will zero workgroup values.
	struct Expression {
	/// The AST expression node
	const ast::Expression* expr = nullptr;
	/// The number of iterations required to zero the value
	uint32_t num_iterations = 0;
	/// All array indices used by this expression
	ArrayIndices array_indices;
	};

	/// Statement holds information about a statement that will zero workgroup
	/// values.
	struct Statement {
	/// The AST statement node
	const ast::Statement* stmt;
	/// The number of iterations required to zero the value
	uint32_t num_iterations;
	/// All array indices used by this statement
	ArrayIndices array_indices;
	};

	/// All statements that zero workgroup memory
	std::vector<Statement> statements;

	/// A map of ArrayIndex to the name reserved for the `let` declaration of that
	/// index.
	std::unordered_map<ArrayIndex, Symbol, ArrayIndex::Hasher> array_index_names;

	/// Constructor
	/// @param c the CloneContext used for the transform
	explicit State(CloneContext& c) : ctx(c) {}

	/// Run inserts the workgroup memory zero-initialization logic at the top of
	/// the given function
	/// @param fn a compute shader entry point function
	void Run(const ast::Function* fn) {
	auto& sem = ctx.src->Sem();

	CalculateWorkgroupSize(ast::GetAttribute<ast::WorkgroupAttribute>(fn->attributes));

	// Generate a list of statements to zero initialize each of the
	// workgroup storage variables used by `fn`. This will populate #statements.
	auto* func = sem.Get(fn);
	for (auto* var : func->TransitivelyReferencedGlobals()) {
	if (var->AddressSpace() == ast::AddressSpace::kWorkgroup) {
	BuildZeroingStatements(var->Type()->UnwrapRef(), [&](uint32_t num_values) {
	auto var_name = ctx.Clone(var->Declaration()->symbol);
	return Expression{b.Expr(var_name), num_values, ArrayIndices{}};
	});
	}
	}

	if (statements.empty()) {
	return; // No workgroup variables to initialize.
	}

	// Scan the entry point for an existing local_invocation_index builtin
	// parameter
	std::function<const ast::Expression*()> local_index;
	for (auto* param : fn->params) {
	if (auto* builtin = ast::GetAttribute<ast::BuiltinAttribute>(param->attributes)) {
	if (builtin->builtin == ast::BuiltinValue::kLocalInvocationIndex) {
	local_index = [=] { return b.Expr(ctx.Clone(param->symbol)); };
	break;
	}
	}

	if (auto* str = sem.Get(param)->Type()->As<sem::Struct>()) {
	for (auto* member : str->Members()) {
	if (auto* builtin = ast::GetAttribute<ast::BuiltinAttribute>(
	member->Declaration()->attributes)) {
	if (builtin->builtin == ast::BuiltinValue::kLocalInvocationIndex) {
	local_index = [=] {
	auto* param_expr = b.Expr(ctx.Clone(param->symbol));
	auto member_name = ctx.Clone(member->Declaration()->symbol);
	return b.MemberAccessor(param_expr, member_name);
	};
	break;
	}
	}
	}
	}
	}
	if (!local_index) {
	// No existing local index parameter. Append one to the entry point.
	auto* param = b.Param(b.Symbols().New("local_invocation_index"), b.ty.u32(),
	utils::Vector{
	b.Builtin(ast::BuiltinValue::kLocalInvocationIndex),
	});
	ctx.InsertBack(fn->params, param);
	local_index = [=] { return b.Expr(param->symbol); };
	}

	// Take the zeroing statements and bin them by the number of iterations
	// required to zero the workgroup data. We then emit these in blocks,
	// possibly wrapped in if-statements or for-loops.
	std::unordered_map<uint32_t, std::vector<Statement>> stmts_by_num_iterations;
	std::vector<uint32_t> num_sorted_iterations;
	for (auto& s : statements) {
	auto& stmts = stmts_by_num_iterations[s.num_iterations];
	if (stmts.empty()) {
	num_sorted_iterations.emplace_back(s.num_iterations);
	}
	stmts.emplace_back(s);
	}
	std::sort(num_sorted_iterations.begin(), num_sorted_iterations.end());

	// Loop over the statements, grouped by num_iterations.
	for (auto num_iterations : num_sorted_iterations) {
	auto& stmts = stmts_by_num_iterations[num_iterations];

	// Gather all the array indices used by all the statements in the block.
	ArrayIndices array_indices;
	for (auto& s : stmts) {
	for (auto& idx : s.array_indices) {
	array_indices.Add(idx);
	}
	}

	// Determine the block type used to emit these statements.

	if (workgroup_size_const == 0 \|\| num_iterations > workgroup_size_const) {
	// Either the workgroup size is dynamic, or smaller than num_iterations.
	// In either case, we need to generate a for loop to ensure we
	// initialize all the array elements.
	//
	// for (var idx : u32 = local_index;
	// idx < num_iterations;
	// idx += workgroup_size) {
	// ...
	// }
	auto idx = b.Symbols().New("idx");
	auto* init = b.Decl(b.Var(idx, b.ty.u32(), local_index()));
	auto* cond = b.create<ast::BinaryExpression>(ast::BinaryOp::kLessThan, b.Expr(idx),
	b.Expr(u32(num_iterations)));
	auto* cont = b.Assign(
	idx, b.Add(idx, workgroup_size_const ? b.Expr(u32(workgroup_size_const))
	: workgroup_size_expr()));

	auto block =
	DeclareArrayIndices(num_iterations, array_indices, [&] { return b.Expr(idx); });
	for (auto& s : stmts) {
	block.Push(s.stmt);
	}
	auto* for_loop = b.For(init, cond, cont, b.Block(block));
	ctx.InsertFront(fn->body->statements, for_loop);
	} else if (num_iterations < workgroup_size_const) {
	// Workgroup size is a known constant, but is greater than
	// num_iterations. Emit an if statement:
	//
	// if (local_index < num_iterations) {
	// ...
	// }
	auto* cond = b.create<ast::BinaryExpression>(
	ast::BinaryOp::kLessThan, local_index(), b.Expr(u32(num_iterations)));
	auto block = DeclareArrayIndices(num_iterations, array_indices,
	[&] { return b.Expr(local_index()); });
	for (auto& s : stmts) {
	block.Push(s.stmt);
	}
	auto* if_stmt = b.If(cond, b.Block(block));
	ctx.InsertFront(fn->body->statements, if_stmt);
	} else {
	// Workgroup size exactly equals num_iterations.
	// No need for any conditionals. Just emit a basic block:
	//
	// {
	// ...
	// }
	auto block = DeclareArrayIndices(num_iterations, array_indices,
	[&] { return b.Expr(local_index()); });
	for (auto& s : stmts) {
	block.Push(s.stmt);
	}
	ctx.InsertFront(fn->body->statements, b.Block(block));
	}
	}

	// Append a single workgroup barrier after the zero initialization.
	ctx.InsertFront(fn->body->statements, b.CallStmt(b.Call("workgroupBarrier")));
	}

	/// BuildZeroingExpr is a function that builds a sub-expression used to zero
	/// workgroup values. `num_values` is the number of elements that the
	/// expression will be used to zero. Returns the expression.
	using BuildZeroingExpr = std::function<Expression(uint32_t num_values)>;

	/// BuildZeroingStatements() generates the statements required to zero
	/// initialize the workgroup storage expression of type `ty`.
	/// @param ty the expression type
	/// @param get_expr a function that builds the AST nodes for the expression.
	void BuildZeroingStatements(const sem::Type* ty, const BuildZeroingExpr& get_expr) {
	if (CanTriviallyZero(ty)) {
	auto var = get_expr(1u);
	auto* zero_init = b.Construct(CreateASTTypeFor(ctx, ty));
	statements.emplace_back(
	Statement{b.Assign(var.expr, zero_init), var.num_iterations, var.array_indices});
	return;
	}

	if (auto* atomic = ty->As<sem::Atomic>()) {
	auto* zero_init = b.Construct(CreateASTTypeFor(ctx, atomic->Type()));
	auto expr = get_expr(1u);
	auto* store = b.Call("atomicStore", b.AddressOf(expr.expr), zero_init);
	statements.emplace_back(
	Statement{b.CallStmt(store), expr.num_iterations, expr.array_indices});
	return;
	}

	if (auto* str = ty->As<sem::Struct>()) {
	for (auto* member : str->Members()) {
	auto name = ctx.Clone(member->Declaration()->symbol);
	BuildZeroingStatements(member->Type(), [&](uint32_t num_values) {
	auto s = get_expr(num_values);
	return Expression{b.MemberAccessor(s.expr, name), s.num_iterations,
	s.array_indices};
	});
	}
	return;
	}

	if (auto* arr = ty->As<sem::Array>()) {
	BuildZeroingStatements(arr->ElemType(), [&](uint32_t num_values) {
	// num_values is the number of values to zero for the element type.
	// The number of iterations required to zero the array and its elements
	// is:
	// `num_values * arr->Count()`
	// The index for this array is:
	// `(idx % modulo) / division`
	auto count = arr->ConstantCount();
	if (!count) {
	ctx.dst->Diagnostics().add_error(diag::System::Transform,
	sem::Array::kErrExpectedConstantCount);
	return Expression{};
	}
	auto modulo = num_values * count.value();
	auto division = num_values;
	auto a = get_expr(modulo);
	auto array_indices = a.array_indices;
	array_indices.Add(ArrayIndex{modulo, division});
	auto index = utils::GetOrCreate(array_index_names, ArrayIndex{modulo, division},
	[&] { return b.Symbols().New("i"); });
	return Expression{b.IndexAccessor(a.expr, index), a.num_iterations, array_indices};
	});
	return;
	}

	TINT_UNREACHABLE(Transform, b.Diagnostics())
	<< "could not zero workgroup type: " << ty->FriendlyName(ctx.src->Symbols());
	}

	/// DeclareArrayIndices returns a list of statements that contain the `let`
	/// declarations for all of the ArrayIndices.
	/// @param num_iterations the number of iterations for the block
	/// @param array_indices the list of array indices to generate `let`
	/// declarations for
	/// @param iteration a function that returns the index of the current
	/// iteration.
	/// @returns the list of `let` statements that declare the array indices
	StatementList DeclareArrayIndices(uint32_t num_iterations,
	const ArrayIndices& array_indices,
	const std::function<const ast::Expression*()>& iteration) {
	StatementList stmts;
	std::map<Symbol, ArrayIndex> indices_by_name;
	for (auto index : array_indices) {
	auto name = array_index_names.at(index);
	auto* mod = (num_iterations > index.modulo)
	? b.create<ast::BinaryExpression>(ast::BinaryOp::kModulo, iteration(),
	b.Expr(u32(index.modulo)))
	: iteration();
	auto* div = (index.division != 1u) ? b.Div(mod, u32(index.division)) : mod;
	auto* decl = b.Decl(b.Let(name, b.ty.u32(), div));
	stmts.Push(decl);
	}
	return stmts;
	}

	/// CalculateWorkgroupSize initializes the members #workgroup_size_const and
	/// #workgroup_size_expr with the linear workgroup size.
	/// @param attr the workgroup attribute applied to the entry point function
	void CalculateWorkgroupSize(const ast::WorkgroupAttribute* attr) {
	bool is_signed = false;
	workgroup_size_const = 1u;
	workgroup_size_expr = nullptr;
	for (auto* expr : attr->Values()) {
	if (!expr) {
	continue;
	}
	auto* sem = ctx.src->Sem().Get(expr);
	if (auto* c = sem->ConstantValue()) {
	workgroup_size_const *= c->As<AInt>();
	continue;
	}
	// Constant value could not be found. Build expression instead.
	workgroup_size_expr = [this, expr, size = workgroup_size_expr] {
	auto* e = ctx.Clone(expr);
	if (ctx.src->TypeOf(expr)->UnwrapRef()->Is<sem::I32>()) {
	e = b.Construct<u32>(e);
	}
	return size ? b.Mul(size(), e) : e;
	};
	}
	if (workgroup_size_expr) {
	if (workgroup_size_const != 1) {
	// Fold workgroup_size_const in to workgroup_size_expr
	workgroup_size_expr = [this, is_signed, const_size = workgroup_size_const,
	expr_size = workgroup_size_expr] {
	return is_signed ? b.Mul(expr_size(), i32(const_size))
	: b.Mul(expr_size(), u32(const_size));
	};
	}
	// Indicate that workgroup_size_expr should be used instead of the
	// constant.
	workgroup_size_const = 0;
	}
	}

	/// @returns true if a variable with store type `ty` can be efficiently zeroed
	/// by assignment of a type initializer without operands. If
	/// CanTriviallyZero() returns false, then the type needs to be
	/// initialized by decomposing the initialization into multiple
	/// sub-initializations.
	/// @param ty the type to inspect
	bool CanTriviallyZero(const sem::Type* ty) {
	if (ty->Is<sem::Atomic>()) {
	return false;
	}
	if (auto* str = ty->As<sem::Struct>()) {
	for (auto* member : str->Members()) {
	if (!CanTriviallyZero(member->Type())) {
	return false;
	}
	}
	}
	if (ty->Is<sem::Array>()) {
	return false;
	}
	// True for all other storable types
	return true;
	}
	};

	ZeroInitWorkgroupMemory::ZeroInitWorkgroupMemory() = default;

	ZeroInitWorkgroupMemory::~ZeroInitWorkgroupMemory() = default;

	bool ZeroInitWorkgroupMemory::ShouldRun(const Program* program, const DataMap&) const {
	for (auto* global : program->AST().GlobalVariables()) {
	if (auto* var = global->As<ast::Var>()) {
	if (var->declared_address_space == ast::AddressSpace::kWorkgroup) {
	return true;
	}
	}
	}
	return false;
	}

	void ZeroInitWorkgroupMemory::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
	for (auto* fn : ctx.src->AST().Functions()) {
	if (fn->PipelineStage() == ast::PipelineStage::kCompute) {
	State{ctx}.Run(fn);
	}
	}
	ctx.Clone();
	}

	} // namespace tint::transform