[hlsl-writer] Emit specialization variables

This CL adds specialization to the HLSL backend. The specialization is
emulated using defined preprocessor macros.

Bug: tint:154
Change-Id: I73ab42360558967eee9a0da6bbe74d513fe4cc57
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/29720
Commit-Queue: David Neto <dneto@google.com>
Reviewed-by: David Neto <dneto@google.com>

src/writer/hlsl/generator_impl.cc

diff --git a/src/writer/hlsl/generator_impl.cc b/src/writer/hlsl/generator_impl.cc
index 86eef79..1ed6c02 100644
--- a/src/writer/hlsl/generator_impl.cc
+++ b/src/writer/hlsl/generator_impl.cc
@@ -2053,7 +2053,7 @@
                                              const ast::Variable* var) {
   make_indent(out);
 
-  if (var->IsDecorated()) {
+  if (var->IsDecorated() && !var->AsDecorated()->HasConstantIdDecoration()) {
     error_ = "Decorated const values not valid";
     return false;
   }
@@ -2064,8 +2064,6 @@
 
   std::ostringstream constructor_out;
   if (var->constructor() != nullptr) {
-    constructor_out << " = ";
-
     std::ostringstream pre;
     if (!EmitExpression(pre, constructor_out, var->constructor())) {
       return false;
@@ -2073,14 +2071,40 @@
     out << pre.str();
   }
 
-  out << "static const ";
-  if (!EmitType(out, var->type(), var->name())) {
-    return false;
+  if (var->IsDecorated() && var->AsDecorated()->HasConstantIdDecoration()) {
+    auto const_id = var->AsDecorated()->constant_id();
+
+    out << "#ifndef WGSL_SPEC_CONSTANT_" << const_id << std::endl;
+
+    if (var->constructor() != nullptr) {
+      out << "#define WGSL_SPEC_CONSTANT_" << const_id << " "
+          << constructor_out.str() << std::endl;
+    } else {
+      out << "#error spec constant required for constant id " << const_id
+          << std::endl;
+    }
+    out << "#endif" << std::endl;
+    out << "static const ";
+    if (!EmitType(out, var->type(), var->name())) {
+      return false;
+    }
+    out << " " << var->name() << " = WGSL_SPEC_CONSTANT_" << const_id << ";"
+        << std::endl;
+    out << "#undef WGSL_SPEC_CONSTANT_" << const_id << std::endl;
+  } else {
+    out << "static const ";
+    if (!EmitType(out, var->type(), var->name())) {
+      return false;
+    }
+    if (!var->type()->IsArray()) {
+      out << " " << var->name();
+    }
+
+    if (var->constructor() != nullptr) {
+      out << " = " << constructor_out.str();
+    }
+    out << ";" << std::endl;
   }
-  if (!var->type()->IsArray()) {
-    out << " " << var->name();
-  }
-  out << constructor_out.str() << ";" << std::endl;
 
   return true;
 }

src/writer/hlsl/generator_impl_module_constant_test.cc

diff --git a/src/writer/hlsl/generator_impl_module_constant_test.cc b/src/writer/hlsl/generator_impl_module_constant_test.cc
index abad8ea..94ffb8e 100644
--- a/src/writer/hlsl/generator_impl_module_constant_test.cc
+++ b/src/writer/hlsl/generator_impl_module_constant_test.cc
@@ -15,6 +15,8 @@
 #include <memory>
 #include <vector>
 
+#include "src/ast/constant_id_decoration.h"
+#include "src/ast/decorated_variable.h"
 #include "src/ast/float_literal.h"
 #include "src/ast/module.h"
 #include "src/ast/scalar_constructor_expression.h"
@@ -56,6 +58,50 @@
       "static const float pos[3] = {1.00000000f, 2.00000000f, 3.00000000f};\n");
 }
 
+TEST_F(HlslGeneratorImplTest_ModuleConstant, Emit_SpecConstant) {
+  ast::type::F32Type f32;
+
+  ast::VariableDecorationList decos;
+  decos.push_back(std::make_unique<ast::ConstantIdDecoration>(23));
+
+  auto var = std::make_unique<ast::DecoratedVariable>(
+      std::make_unique<ast::Variable>("pos", ast::StorageClass::kNone, &f32));
+  var->set_decorations(std::move(decos));
+  var->set_is_const(true);
+  var->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 3.0f)));
+
+  ASSERT_TRUE(gen().EmitProgramConstVariable(out(), var.get()))
+      << gen().error();
+  EXPECT_EQ(result(), R"(#ifndef WGSL_SPEC_CONSTANT_23
+#define WGSL_SPEC_CONSTANT_23 3.00000000f
+#endif
+static const float pos = WGSL_SPEC_CONSTANT_23;
+#undef WGSL_SPEC_CONSTANT_23
+)");
+}
+
+TEST_F(HlslGeneratorImplTest_ModuleConstant, Emit_SpecConstant_NoConstructor) {
+  ast::type::F32Type f32;
+
+  ast::VariableDecorationList decos;
+  decos.push_back(std::make_unique<ast::ConstantIdDecoration>(23));
+
+  auto var = std::make_unique<ast::DecoratedVariable>(
+      std::make_unique<ast::Variable>("pos", ast::StorageClass::kNone, &f32));
+  var->set_decorations(std::move(decos));
+  var->set_is_const(true);
+
+  ASSERT_TRUE(gen().EmitProgramConstVariable(out(), var.get()))
+      << gen().error();
+  EXPECT_EQ(result(), R"(#ifndef WGSL_SPEC_CONSTANT_23
+#error spec constant required for constant id 23
+#endif
+static const float pos = WGSL_SPEC_CONSTANT_23;
+#undef WGSL_SPEC_CONSTANT_23
+)");
+}
+
 }  // namespace
 }  // namespace hlsl
 }  // namespace writer