[clang] [llvm] [llvm][AMDGPU] Fold `llvm.amdgcn.wavefrontsize` early (PR #114481)

[Matt Arsenault via cfe-commits]

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@@ -1024,6 +1024,15 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
     }
     break;
   }
+  case Intrinsic::amdgcn_wavefrontsize: {
+    // TODO: this is a workaround for the pseudo-generic target one gets with no
+    // specified mcpu, which spoofs its wave size to 64; it should be removed.
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arsenm wrote:

We already do some light 64->32 folds, that are only sort of correct.

Technically we could make exec_hi an allocatable scratch register in wave32, but what we do now bakes in an assumption that exec_hi must always be 0.

But yes, the only way to really avoid any possible edge cases (and support a future of machine linked libraries) requires just having totally separate builds 

https://github.com/llvm/llvm-project/pull/114481