[llvm] [AMDGPU][Attributor] Make `AAAMDFlatWorkGroupSize` honor existing attribute (PR #114357)

[Matt Arsenault via llvm-commits]

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@@ -168,9 +168,18 @@ class AMDGPUInformationCache : public InformationCache {
     return ST.supportsGetDoorbellID();
   }
 
-  std::pair<unsigned, unsigned> getFlatWorkGroupSizes(const Function &F) {
+  std::optional<std::pair<unsigned, unsigned>>
+  getFlatWorkGroupSizeAttr(const Function &F) const {
+    auto R = AMDGPU::getIntegerPairAttribute(F, "amdgpu-flat-work-group-size");
+    if (!R)
+      return std::nullopt;
+    return std::make_pair(R->first, *(R->second));
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arsenm wrote:

Extra parens? 

https://github.com/llvm/llvm-project/pull/114357