writer/hlsl: Don't emit structs for storage buffer usage

Use the semantic StorageClassUsage() to determine whether the structure is used only for storage buffer usage. If it is, don't emit a struct definition for it.
This fixes issues with attempting to generate runtime arrays - they're only legal for storage buffer usage. Storage buffers use ByteAddressBuffer instead of structured loads / stores.

Bug: tint:185
Fixed: tint:682
Change-Id: I5b58a133eee2fe036a84e028fa85b611e4895b1a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/46382
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/writer/hlsl/generator_impl.cc

@@ -2590,9 +2590,17 @@ bool GeneratorImpl::EmitType(std::ostream& out,
 bool GeneratorImpl::EmitStructType(std::ostream& out,
                                    const type::Struct* str,
                                    const std::string& name) {
-  // TODO(dsinclair): Block decoration?
-  // if (str->impl()->decoration() != ast::Decoration::kNone) {
-  // }
+  auto* sem_str = builder_.Sem().Get(str);
+
+  auto storage_class_uses = sem_str->StorageClassUsage();
+  if (storage_class_uses.size() ==
+      storage_class_uses.count(ast::StorageClass::kStorage)) {
+    // The only use of the structure is as a storage buffer.
+    // Structures used as storage buffer are read and written to via a
+    // ByteAddressBuffer instead of true structure.
+    return true;
+  }
+
   out << "struct " << name << " {" << std::endl;
 
   increment_indent();

src/writer/hlsl/generator_impl_alias_type_test.cc

@@ -42,15 +42,13 @@ TEST_F(HlslGeneratorImplTest_Alias, EmitAlias_NameCollision) {
 }
 
 TEST_F(HlslGeneratorImplTest_Alias, EmitAlias_Struct) {
-  auto* str = create<ast::Struct>(
-      ast::StructMemberList{
+  auto* s = Structure("A", {
                                Member("a", ty.f32()),
                                Member("b", ty.i32()),
-      },
-      ast::DecorationList{});
-
-  auto* s = ty.struct_("A", str);
+                           });
   auto* alias = ty.alias("B", s);
+  AST().AddConstructedType(alias);
+  Global("g", alias, ast::StorageClass::kPrivate);
 
   GeneratorImpl& gen = Build();
 

src/writer/hlsl/generator_impl_function_test.cc

@@ -916,10 +916,7 @@ TEST_F(HlslGeneratorImplTest_Function,
   GeneratorImpl& gen = Build();
 
   ASSERT_TRUE(gen.Generate(out)) << gen.error();
-  EXPECT_EQ(result(), R"(struct Data {
-  float d;
-};
-
+  EXPECT_EQ(result(), R"(
 RWByteAddressBuffer data : register(u0, space0);
 
 [numthreads(1, 1, 1)]

src/writer/hlsl/generator_impl_type_test.cc

@@ -171,6 +171,7 @@ TEST_F(HlslGeneratorImplTest_Type, EmitType_StructDecl) {
                                Member("a", ty.i32()),
                                Member("b", ty.f32()),
                            });
+  Global("g", s, ast::StorageClass::kPrivate);
 
   GeneratorImpl& gen = Build();
 
@@ -182,11 +183,25 @@ TEST_F(HlslGeneratorImplTest_Type, EmitType_StructDecl) {
 )");
 }
 
+TEST_F(HlslGeneratorImplTest_Type, EmitType_StructDecl_OmittedIfStorageBuffer) {
+  auto* s = Structure("S", {
+                               Member("a", ty.i32()),
+                               Member("b", ty.f32()),
+                           });
+  Global("g", s, ast::StorageClass::kStorage);
+
+  GeneratorImpl& gen = Build();
+
+  ASSERT_TRUE(gen.EmitStructType(out, s, "S")) << gen.error();
+  EXPECT_EQ(result(), "");
+}
+
 TEST_F(HlslGeneratorImplTest_Type, EmitType_Struct) {
   auto* s = Structure("S", {
                                Member("a", ty.i32()),
                                Member("b", ty.f32()),
                            });
+  Global("g", s, ast::StorageClass::kPrivate);
 
   GeneratorImpl& gen = Build();
 
@@ -203,6 +218,7 @@ TEST_F(HlslGeneratorImplTest_Type, DISABLED_EmitType_Struct_InjectPadding) {
                Member("b", ty.f32()),
                Member("c", ty.f32(), {MemberAlign(128), MemberSize(128)}),
            });
+  Global("g", s, ast::StorageClass::kPrivate);
 
   GeneratorImpl& gen = Build();
 
@@ -223,6 +239,7 @@ TEST_F(HlslGeneratorImplTest_Type, EmitType_Struct_NameCollision) {
                                Member("double", ty.i32()),
                                Member("float", ty.f32()),
                            });
+  Global("g", s, ast::StorageClass::kPrivate);
 
   GeneratorImpl& gen = Build();
 
@@ -242,6 +259,7 @@ TEST_F(HlslGeneratorImplTest_Type, DISABLED_EmitType_Struct_WithDecoration) {
                           Member("b", ty.f32()),
                       },
                       {create<ast::StructBlockDecoration>()});
+  Global("g", s, ast::StorageClass::kPrivate);
 
   GeneratorImpl& gen = Build();