[clang] [llvm] [HLSL] Implement explicit layout for default constant buffer ($Globals) (PR #128991)

[Ashley Coleman via cfe-commits]

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@@ -174,21 +176,51 @@ createBufferHandleType(const HLSLBufferDecl *BufDecl) {
   return cast<HLSLAttributedResourceType>(QT.getTypePtr());
 }
 
+// Iterates over all declarations in the HLSL buffer and based on the
+// packoffset or register(c#) annotations it fills outs the Layout
+// vector with the user-specified layout offsets.
+// The buffer offsets can be specified 2 ways:
+// 1. declarations in cbuffer {} block can have a packoffset annotation
+//    (translates to HLSLPackOffsetAttr)
+// 2. default constant buffer declarations at global scope can have
+//    register(c#) annotations (translates to HLSLResourceBindingAttr with
+//    RegisterType::C)
+// It is not guaranteed that all declarations in a buffer have an annotation.
+// For those where it is not specified a -1 value is added to the Layout
+// vector. In the final layout these declarations will be placed at the end
+// of the HLSL buffer after all of the elements with specified offset.
 static void fillPackoffsetLayout(const HLSLBufferDecl *BufDecl,
-                                 SmallVector<unsigned> &Layout) {
+                                 SmallVector<int32_t> &Layout) {
   assert(Layout.empty() && "expected empty vector for layout");
   assert(BufDecl->hasValidPackoffset());
 
-  for (Decl *D : BufDecl->decls()) {
+  for (Decl *D : BufDecl->buffer_decls()) {
     if (isa<CXXRecordDecl, EmptyDecl>(D) || isa<FunctionDecl>(D)) {
       continue;
     }
     VarDecl *VD = dyn_cast<VarDecl>(D);
     if (!VD || VD->getType().getAddressSpace() != LangAS::hlsl_constant)
       continue;
-    assert(VD->hasAttr<HLSLPackOffsetAttr>() &&
-           "expected packoffset attribute on every declaration");
-    size_t Offset = VD->getAttr<HLSLPackOffsetAttr>()->getOffsetInBytes();
+
+    if (!VD->hasAttrs()) {
+      Layout.push_back(-1);
+      continue;
+    }
+
+    size_t Offset = -1;
+    for (auto *Attr : VD->getAttrs()) {
+      if (auto *POA = dyn_cast<HLSLPackOffsetAttr>(Attr)) {
+        Offset = POA->getOffsetInBytes();
+        break;
+      }
+      auto *RBA = dyn_cast<HLSLResourceBindingAttr>(Attr);
+      if (RBA &&
+          RBA->getRegisterType() == HLSLResourceBindingAttr::RegisterType::C) {
+        // size of constant buffer row is 16 bytes
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V-FEXrt wrote:

nit:  I don't think this comment is providing anything now that there is a constant for the value

https://github.com/llvm/llvm-project/pull/128991