[spirv] Add support for vk::offset attribute (#922)

Added support for the vk::offset attribute to manually
specify offsets for struct members.

This commit only handles non-nested structs and it
does not validate the offset provided either.

tools/clang/include/clang/Basic/Attr.td

@@ -914,6 +914,13 @@ def VKPushConstant : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
+def VKOffset : InheritableAttr {
+  let Spellings = [CXX11<"vk", "offset">];
+  let Args = [IntArgument<"Offset">];
+  let LangOpts = [SPIRV];
+  let Documentation = [Undocumented];
+}
+
 // SPIRV Change Ends
 
 def C11NoReturn : InheritableAttr {

tools/clang/lib/SPIRV/TypeTranslator.cpp

@@ -619,7 +619,11 @@ TypeTranslator::getLayoutDecorations(const DeclContext *decl, LayoutRule rule) {
         getAlignmentAndSize(fieldType, rule, isRowMajor, &stride);
 
     // Each structure-type member must have an Offset Decoration.
-    roundToPow2(&offset, memberAlignment);
+    const auto *offsetAttr = field->getAttr<VKOffsetAttr>();
+    if (offsetAttr)
+        offset = offsetAttr->getOffset();
+    else
+        roundToPow2(&offset, memberAlignment);
     decorations.push_back(Decoration::getOffset(*spirvContext, offset, index));
     offset += memberSize;
 

tools/clang/lib/Sema/SemaHLSL.cpp

@@ -10381,6 +10381,10 @@ void hlsl::HandleDeclAttributeForHLSL(Sema &S, Decl *D, const AttributeList &A,
     declAttr = ::new (S.Context) VKPushConstantAttr(A.getRange(), S.Context,
       A.getAttributeSpellingListIndex());
     break;
+  case AttributeList::AT_VKOffset:
+    declAttr = ::new (S.Context) VKOffsetAttr(A.getRange(), S.Context,
+      ValidateAttributeIntArg(S, A), A.getAttributeSpellingListIndex());
+    break;
   default:
     Handled = false;
     return;

tools/clang/test/CodeGenSPIRV/vk.push-constant.offset.hlsl

@@ -0,0 +1,20 @@
+// Run: %dxc -T vs_6_0 -E main
+
+// CHECK: OpMemberDecorate %type_PushConstant_S 0 Offset 0
+// CHECK: OpMemberDecorate %type_PushConstant_S 1 Offset 8
+// CHECK: OpMemberDecorate %type_PushConstant_S 2 Offset 32
+
+struct S {
+    float a;
+    [[vk::offset(8)]]
+    float2 b;
+    [[vk::offset(32)]]
+    float4 f;
+};
+
+[[vk::push_constant]]
+S pcs1;
+
+float main() : A {
+    return 1.0;
+}

tools/clang/unittests/SPIRV/CodeGenSPIRVTest.cpp

@@ -1035,6 +1035,7 @@ TEST_F(FileTest, VulkanStructuredBufferCounter) {
 }
 
 TEST_F(FileTest, VulkanPushConstant) { runFileTest("vk.push-constant.hlsl"); }
+TEST_F(FileTest, VulkanPushConstantOffset) { runFileTest("vk.push-constant.offset.hlsl"); }
 TEST_F(FileTest, VulkanMultiplePushConstant) {
   runFileTest("vk.push-constant.multiple.hlsl", FileTest::Expect::Failure);
 }