[llvm] [AMDGPU] Allow amdgpu-waves-per-eu to lower target occupancy range (PR #168358)

[Matt Arsenault via llvm-commits]

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@@ -190,23 +190,29 @@ std::pair<unsigned, unsigned> AMDGPUSubtarget::getEffectiveWavesPerEU(
   // sizes limits the achievable maximum, and we aim to support enough waves per
   // EU so that we can concurrently execute all waves of a single workgroup of
   // maximum size on a CU.
-  std::pair<unsigned, unsigned> Default = {
+  std::pair<unsigned, unsigned> WavesPerEU = {
       getWavesPerEUForWorkGroup(FlatWorkGroupSizes.second),
       getOccupancyWithWorkGroupSizes(LDSBytes, FlatWorkGroupSizes).second};
-  Default.first = std::min(Default.first, Default.second);
-
-  // Make sure requested minimum is within the default range and lower than the
-  // requested maximum. The latter must not violate target specification.
-  if (RequestedWavesPerEU.first < Default.first ||
-      RequestedWavesPerEU.first > Default.second ||
-      RequestedWavesPerEU.first > RequestedWavesPerEU.second ||
-      RequestedWavesPerEU.second > getMaxWavesPerEU())
-    return Default;
-
-  // We cannot exceed maximum occupancy implied by flat workgroup size and LDS.
-  RequestedWavesPerEU.second =
-      std::min(RequestedWavesPerEU.second, Default.second);
-  return RequestedWavesPerEU;
+  WavesPerEU.first = std::min(WavesPerEU.first, WavesPerEU.second);
+
+  // Requested minimum must not violate subtarget's specifications and be no
+  // greater than maximum.
+  if (RequestedWavesPerEU.first &&
+      (RequestedWavesPerEU.first < getMinWavesPerEU() ||
+       RequestedWavesPerEU.first > RequestedWavesPerEU.second))
+    return WavesPerEU;
+  // Requested maximum must not violate subtarget's specifications.
+  if (RequestedWavesPerEU.second > getMaxWavesPerEU())
+    return WavesPerEU;
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arsenm wrote:

Not in the low level query; I think we already do emit such a remark in the asm printer 

https://github.com/llvm/llvm-project/pull/168358