[clang] [llvm] [HLSL] Implement SV_GroupThreadId semantic (PR #117781)

[Zhengxing li via llvm-commits]

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@@ -2851,7 +2851,21 @@ bool SPIRVInstructionSelector::selectIntrinsic(Register ResVReg,
           .constrainAllUses(TII, TRI, RBI);
     break;
   case Intrinsic::spv_thread_id:
-    return selectSpvThreadId(ResVReg, ResType, I);
+    // The HLSL SV_DispatchThreadID semantic is lowered to llvm.spv.thread.id
+    // intrinsic in LLVM IR for SPIR-V backend.
+    //
+    // In SPIR-V backend, llvm.spv.thread.id is now correctly translated to a
+    // `GlobalInvocationId` builtin variable
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lizhengxing wrote:

@tex3d Those comments are based on the original comments in `selectSpvThreadId` function with minimal changes. They were added by @sudonatalie. 

If there's no objections from other reviewers, I think it's fine to keep those comments here.

https://github.com/llvm/llvm-project/pull/117781