[llvm] 14b03b4 - GlobalISel: Check for powers of 2 for inverse funnel shift lowering

[Matt Arsenault via llvm-commits]

Author: Matt Arsenault
Date: 2021-04-20T11:30:22-04:00
New Revision: 14b03b4aadee8e6adcbb8c674c8d8f770b1bc4d6

URL: https://github.com/llvm/llvm-project/commit/14b03b4aadee8e6adcbb8c674c8d8f770b1bc4d6
DIFF: https://github.com/llvm/llvm-project/commit/14b03b4aadee8e6adcbb8c674c8d8f770b1bc4d6.diff

LOG: GlobalISel: Check for powers of 2 for inverse funnel shift lowering

This doesn't make a practical difference since it would only be broken
if a target actually had a legal non-power-of-2 inverse shift.

Added: 
    

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

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 99b6ea805d9c..0ab80de694a8 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -5392,6 +5392,10 @@ LegalizerHelper::lowerFunnelShiftWithInverse(MachineInstr &MI) {
   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;
 
@@ -5490,9 +5494,16 @@ LegalizerHelper::lowerFunnelShift(MachineInstr &MI) {
 
   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;
 }
 
 LegalizerHelper::LegalizeResult