[llvm] [AMDGPU] Defaults for missing dimensions in SYCL required wg size (PR #68872)

[Jakub Chlanda via llvm-commits]

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@@ -317,10 +317,38 @@ static bool processUse(CallInst *CI, bool IsV5OrAbove) {
   return MadeChange;
 }
 
+// SYCL allows required work-group size attribute to be partially specified
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jchlanda wrote:

I guess in principle you are right, but, we do not want to alter the metadata in clang, as it would loos the dimensionality information that is important.

> If we really needed to support 2 forms of the metadata, I would expect to just have a utility function to read the effective value. You don't need to rewrite it

Since this feature is borrowed from OpenCL, and OpenCL assumptions are held in the ROCm toolchain (see examples in the comment below) I feel it has to be rewritten to converge to OpenCL version; and this spot seems very convenient for it. 

https://github.com/llvm/llvm-project/pull/68872