src/transform/spirv.cc

// Copyright 2020 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/transform/spirv.h"

#include <string>
#include <utility>

#include "src/ast/call_statement.h"
#include "src/ast/return_statement.h"
#include "src/ast/stage_decoration.h"
#include "src/program_builder.h"
#include "src/semantic/function.h"
#include "src/semantic/statement.h"
#include "src/semantic/variable.h"

namespace tint {
namespace transform {

Spirv::Spirv() = default;
Spirv::~Spirv() = default;

Transform::Output Spirv::Run(const Program* in, const DataMap&) {
  ProgramBuilder out;
  CloneContext ctx(&out, in);
  HandleEntryPointIOTypes(ctx);
  ctx.Clone();

  // TODO(jrprice): Look into combining these transforms into a single clone.
  Program tmp(std::move(out));

  ProgramBuilder out2;
  CloneContext ctx2(&out2, &tmp);
  HandleSampleMaskBuiltins(ctx2);
  AddEmptyEntryPoint(ctx2);
  ctx2.Clone();

  return Output{Program(std::move(out2))};
}

void Spirv::HandleEntryPointIOTypes(CloneContext& ctx) const {
  // Hoist entry point parameters, return values, and struct members out to
  // global variables. Declare and construct struct parameters in the function
  // body. Replace entry point return statements with calls to a function that
  // assigns the return value to the global output variables.
  //
  // Before:
  // ```
  // struct FragmentInput {
  //   [[builtin(sample_index)]] sample_index : u32;
  //   [[builtin(sample_mask_in)]] sample_mask_in : u32;
  // };
  // struct FragmentOutput {
  //   [[builtin(frag_depth)]] depth: f32;
  //   [[builtin(sample_mask_out)]] mask_out : u32;
  // };
  //
  // [[stage(fragment)]]
  // fn frag_main(
  //   [[builtin(frag_coord)]] coord : vec4<f32>,
  //   samples : FragmentInput
  // ) -> FragmentOutput {
  //   var output : FragmentOutput = FragmentOutput(1.0,
  //                                                samples.sample_mask_in);
  //   return output;
  // }
  // ```
  //
  // After:
  // ```
  // struct FragmentInput {
  //   sample_index : u32;
  //   sample_mask_in : u32;
  // };
  // struct FragmentOutput {
  //   depth: f32;
  //   mask_out : u32;
  // };
  //
  // [[builtin(frag_coord)]] var<in> coord : vec4<f32>,
  // [[builtin(sample_index)]] var<in> sample_index : u32,
  // [[builtin(sample_mask_in)]] var<in> sample_mask_in : u32,
  // [[builtin(frag_depth)]] var<out> depth: f32;
  // [[builtin(sample_mask_out)]] var<out> mask_out : u32;
  //
  // fn frag_main_ret(retval : FragmentOutput) {
  //   depth = reval.depth;
  //   mask_out = retval.mask_out;
  // }
  //
  // [[stage(fragment)]]
  // fn frag_main() {
  //   let samples : FragmentInput(sample_index, sample_mask_in);
  //   var output : FragmentOutput = FragmentOutput(1.0,
  //                                                samples.sample_mask_in);
  //   frag_main_ret(output);
  //   return;
  // }
  // ```

  // Strip entry point IO decorations from struct declarations.
  for (auto* ty : ctx.src->AST().ConstructedTypes()) {
    if (auto* struct_ty = ty->As<type::Struct>()) {
      // Build new list of struct members without entry point IO decorations.
      ast::StructMemberList new_struct_members;
      for (auto* member : struct_ty->impl()->members()) {
        ast::DecorationList new_decorations = RemoveDecorations(
            &ctx, member->decorations(), [](const ast::Decoration* deco) {
              return deco
                  ->IsAnyOf<ast::BuiltinDecoration, ast::LocationDecoration>();
            });
        new_struct_members.push_back(
            ctx.dst->Member(ctx.Clone(member->symbol()),
                            ctx.Clone(member->type()), new_decorations));
      }

      // Redeclare the struct.
      auto* new_struct = ctx.dst->create<type::Struct>(
          ctx.Clone(struct_ty->symbol()),
          ctx.dst->create<ast::Struct>(
              new_struct_members, ctx.Clone(struct_ty->impl()->decorations())));
      ctx.Replace(struct_ty, new_struct);
    }
  }

  for (auto* func : ctx.src->AST().Functions()) {
    if (!func->IsEntryPoint()) {
      continue;
    }

    for (auto* param : func->params()) {
      Symbol new_var = HoistToInputVariables(
          ctx, func, ctx.src->Sem().Get(param)->Type(), param->decorations());

      // Replace all uses of the function parameter with the new variable.
      for (auto* user : ctx.src->Sem().Get(param)->Users()) {
        ctx.Replace<ast::Expression>(user->Declaration(),
                                     ctx.dst->Expr(new_var));
      }
    }

    if (!func->return_type()->Is<type::Void>()) {
      ast::StatementList stores;
      auto store_value_symbol = ctx.dst->Symbols().New();
      HoistToOutputVariables(ctx, func, func->return_type(),
                             func->return_type_decorations(), {},
                             store_value_symbol, stores);

      // Create a function that writes a return value to all output variables.
      auto* store_value =
          ctx.dst->Const(store_value_symbol, ctx.Clone(func->return_type()));
      auto return_func_symbol = ctx.dst->Symbols().New();
      auto* return_func = ctx.dst->create<ast::Function>(
          return_func_symbol, ast::VariableList{store_value},
          ctx.dst->ty.void_(), ctx.dst->create<ast::BlockStatement>(stores),
          ast::DecorationList{}, ast::DecorationList{});
      ctx.InsertBefore(ctx.src->AST().GlobalDeclarations(), func, return_func);

      // Replace all return statements with calls to the output function.
      auto* sem_func = ctx.src->Sem().Get(func);
      for (auto* ret : sem_func->ReturnStatements()) {
        auto* ret_sem = ctx.src->Sem().Get(ret);
        auto* call = ctx.dst->Call(return_func_symbol, ctx.Clone(ret->value()));
        ctx.InsertBefore(ret_sem->Block()->statements(), ret,
                         ctx.dst->create<ast::CallStatement>(call));
        ctx.Replace(ret, ctx.dst->create<ast::ReturnStatement>());
      }
    }

    // Rewrite the function header to remove the parameters and return value.
    auto* new_func = ctx.dst->create<ast::Function>(
        func->source(), ctx.Clone(func->symbol()), ast::VariableList{},
        ctx.dst->ty.void_(), ctx.Clone(func->body()),
        ctx.Clone(func->decorations()), ast::DecorationList{});
    ctx.Replace(func, new_func);
  }
}

void Spirv::HandleSampleMaskBuiltins(CloneContext& ctx) const {
  // Find global variables decorated with [[builtin(sample_mask_{in,out})]] and
  // change their type from `u32` to `array<u32, 1>`, as required by Vulkan.
  //
  // Before:
  // ```
  // [[builtin(sample_mask_out)]] var<out> mask_out : u32;
  // fn main() {
  //   mask_out = 1u;
  // }
  // ```
  // After:
  // ```
  // [[builtin(sample_mask_out)]] var<out> mask_out : array<u32, 1>;
  // fn main() {
  //   mask_out[0] = 1u;
  // }
  // ```

  for (auto* var : ctx.src->AST().GlobalVariables()) {
    for (auto* deco : var->decorations()) {
      if (auto* builtin = deco->As<ast::BuiltinDecoration>()) {
        if (builtin->value() != ast::Builtin::kSampleMaskIn &&
            builtin->value() != ast::Builtin::kSampleMaskOut) {
          continue;
        }

        // Use the same name as the old variable.
        auto var_name = ctx.Clone(var->symbol());
        // Use `array<u32, 1>` for the new variable.
        auto* type = ctx.dst->ty.array(ctx.dst->ty.u32(), 1u);
        // Create the new variable.
        auto* var_arr = ctx.dst->Var(var->source(), var_name, type,
                                     var->declared_storage_class(), nullptr,
                                     ctx.Clone(var->decorations()));
        // Replace the variable with the arrayed version.
        ctx.Replace(var, var_arr);

        // Replace all uses of the old variable with `var_arr[0]`.
        for (auto* user : ctx.src->Sem().Get(var)->Users()) {
          auto* new_ident = ctx.dst->IndexAccessor(
              ctx.dst->Expr(var_arr->symbol()), ctx.dst->Expr(0));
          ctx.Replace<ast::Expression>(user->Declaration(), new_ident);
        }
      }
    }
  }
}

void Spirv::AddEmptyEntryPoint(CloneContext& ctx) const {
  for (auto* func : ctx.src->AST().Functions()) {
    if (func->IsEntryPoint()) {
      return;
    }
  }
  ctx.dst->Func(
      "_tint_unused_entry_point", {}, ctx.dst->ty.void_(), {},
      {ctx.dst->create<ast::StageDecoration>(ast::PipelineStage::kCompute)});
}

Symbol Spirv::HoistToInputVariables(
    CloneContext& ctx,
    const ast::Function* func,
    type::Type* ty,
    const ast::DecorationList& decorations) const {
  if (!ty->UnwrapAliasIfNeeded()->Is<type::Struct>()) {
    // Base case: create a global variable and return.
    ast::DecorationList new_decorations =
        RemoveDecorations(&ctx, decorations, [](const ast::Decoration* deco) {
          return !deco->IsAnyOf<ast::BuiltinDecoration,
                                ast::LocationDecoration>();
        });
    auto global_var_symbol = ctx.dst->Symbols().New();
    auto* global_var =
        ctx.dst->Var(global_var_symbol, ctx.Clone(ty),
                     ast::StorageClass::kInput, nullptr, new_decorations);
    ctx.InsertBefore(ctx.src->AST().GlobalDeclarations(), func, global_var);
    return global_var_symbol;
  }

  // Recurse into struct members and build the initializer list.
  ast::ExpressionList init_values;
  auto* struct_ty = ty->UnwrapAliasIfNeeded()->As<type::Struct>();
  for (auto* member : struct_ty->impl()->members()) {
    auto member_var =
        HoistToInputVariables(ctx, func, member->type(), member->decorations());
    init_values.push_back(ctx.dst->Expr(member_var));
  }

  auto func_var_symbol = ctx.dst->Symbols().New();
  if (func->body()->empty()) {
    // The return value should never get used.
    return func_var_symbol;
  }

  // Create a function-scope variable for the struct.
  auto* initializer = ctx.dst->Construct(ctx.Clone(ty), init_values);
  auto* func_var = ctx.dst->Const(func_var_symbol, ctx.Clone(ty), initializer);
  ctx.InsertBefore(func->body()->statements(), *func->body()->begin(),
                   ctx.dst->WrapInStatement(func_var));
  return func_var_symbol;
}

void Spirv::HoistToOutputVariables(CloneContext& ctx,
                                   const ast::Function* func,
                                   type::Type* ty,
                                   const ast::DecorationList& decorations,
                                   std::vector<Symbol> member_accesses,
                                   Symbol store_value,
                                   ast::StatementList& stores) const {
  // Base case.
  if (!ty->UnwrapAliasIfNeeded()->Is<type::Struct>()) {
    // Create a global variable.
    ast::DecorationList new_decorations =
        RemoveDecorations(&ctx, decorations, [](const ast::Decoration* deco) {
          return !deco->IsAnyOf<ast::BuiltinDecoration,
                                ast::LocationDecoration>();
        });
    auto global_var_symbol = ctx.dst->Symbols().New();
    auto* global_var =
        ctx.dst->Var(global_var_symbol, ctx.Clone(ty),
                     ast::StorageClass::kOutput, nullptr, new_decorations);
    ctx.InsertBefore(ctx.src->AST().GlobalDeclarations(), func, global_var);

    // Create the assignment instruction.
    ast::Expression* rhs = ctx.dst->Expr(store_value);
    for (auto member : member_accesses) {
      rhs = ctx.dst->MemberAccessor(rhs, member);
    }
    stores.push_back(ctx.dst->Assign(ctx.dst->Expr(global_var_symbol), rhs));

    return;
  }

  // Recurse into struct members.
  auto* struct_ty = ty->UnwrapAliasIfNeeded()->As<type::Struct>();
  for (auto* member : struct_ty->impl()->members()) {
    member_accesses.push_back(ctx.Clone(member->symbol()));
    HoistToOutputVariables(ctx, func, member->type(), member->decorations(),
                           member_accesses, store_value, stores);
    member_accesses.pop_back();
  }
}

}  // namespace transform
}  // namespace tint