[PATCH] D99541: GlobalISel: Check for powers of 2 for inverse funnel shift lowering

[Matt Arsenault via Phabricator via llvm-commits]

arsenm updated this revision to Diff 334281.
arsenm added a comment.

Fix being totally wrong


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D99541/new/

https://reviews.llvm.org/D99541

Files:
  llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp


Index: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -5248,6 +5248,10 @@
   LLT ShTy = MRI.getType(Z);
 
   unsigned BW = Ty.getScalarSizeInBits();
+
+  if (!isPowerOf2_32(BW))
+    return UnableToLegalize;
+
   const bool IsFSHL = MI.getOpcode() == TargetOpcode::G_FSHL;
   unsigned RevOpcode = IsFSHL ? TargetOpcode::G_FSHR : TargetOpcode::G_FSHL;
 
@@ -5346,9 +5350,16 @@
 
   bool IsFSHL = MI.getOpcode() == TargetOpcode::G_FSHL;
   unsigned RevOpcode = IsFSHL ? TargetOpcode::G_FSHR : TargetOpcode::G_FSHL;
+
+  // TODO: Use smarter heuristic that accounts for vector legalization.
   if (LI.getAction({RevOpcode, {Ty, ShTy}}).Action == Lower)
     return lowerFunnelShiftAsShifts(MI);
-  return lowerFunnelShiftWithInverse(MI);
+
+  // This only works for powers of 2, fallback to shifts if it fails.
+  LegalizerHelper::LegalizeResult Result = lowerFunnelShiftWithInverse(MI);
+  if (Result == UnableToLegalize)
+    return lowerFunnelShiftAsShifts(MI);
+  return Result;
 }
 
 // Expand s32 = G_UITOFP s64 using bit operations to an IEEE float


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