[HLSL] Implement default constant buffer $Globals (2nd attempt) (#128589)

All variable declarations in the global scope that are not resources,
static or empty are implicitly added to implicit constant buffer
`$Globals`. They are created in `hlsl_constant` address space and
collected in an implicit `HLSLBufferDecl` node that is added to the AST
at the end of the translation unit. Codegen is the same as for explicit
constant buffers.

Fixes #123801

This is a second attempt to implement this feature. The first attempt
had to be reverted because of memory leaks. The problem was adding a
`SmallVector` member on `HLSLBufferDecl` node to represent a list of
default buffer declarations. When this vector needed to grow, it
allocated memory that was never released, because all memory used by AST
nodes must be allocated by `ASTContext` allocator and is released all at
once. Destructors on AST nodes are never called.

It this change the list of default buffer declarations is collected in a
`SmallVector` instance on `SemaHLSL`. The `HLSLBufDecl` representing
`$Globals` is created at the end of the translation unit when the number
of declarations is known, and the list is copied into an array allocated
by the `ASTContext` allocator.

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 7924c32..1b7d0ac 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -5513,6 +5513,11 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D,
   if (getLangOpts().OpenCL && ASTTy->isSamplerT())
     return;
 
+  // HLSL default buffer constants will be emitted during HLSLBufferDecl codegen
+  if (getLangOpts().HLSL &&
+      D->getType().getAddressSpace() == LangAS::hlsl_constant)
+    return;
+
   // If this is OpenMP device, check if it is legal to emit this global
   // normally.
   if (LangOpts.OpenMPIsTargetDevice && OpenMPRuntime &&