[PATCH] D144333: [DivRemPairs] Strip division's poison generating flagGiven this transformation: X % Y -> X - (X / Y) * YThis patch strips off the poison-generating flag of X / Y such as exact, because it may make the optimized form result poison whereas X % Y...

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune created this revision.
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...does not.

The issue was reported here: https://github.com/llvm/llvm-project/issues/60748


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D144333

Files:
  llvm/lib/Transforms/Scalar/DivRemPairs.cpp
  llvm/test/Transforms/DivRemPairs/AArch64/div-rem-pairs.ll


Index: llvm/test/Transforms/DivRemPairs/AArch64/div-rem-pairs.ll
===================================================================
--- llvm/test/Transforms/DivRemPairs/AArch64/div-rem-pairs.ll
+++ llvm/test/Transforms/DivRemPairs/AArch64/div-rem-pairs.ll
@@ -5,7 +5,7 @@
 ; CHECK-LABEL: @f(
 ; CHECK-NEXT:    [[DOTFROZEN:%.*]] = freeze i8 [[TMP1:%.*]]
 ; CHECK-NEXT:    [[DOTFROZEN1:%.*]] = freeze i8 [[TMP2:%.*]]
-; CHECK-NEXT:    [[TMP4:%.*]] = udiv exact i8 [[DOTFROZEN]], [[DOTFROZEN1]]
+; CHECK-NEXT:    [[TMP4:%.*]] = udiv i8 [[DOTFROZEN]], [[DOTFROZEN1]]
 ; CHECK-NEXT:    [[TMP5:%.*]] = mul i8 [[TMP4]], [[DOTFROZEN1]]
 ; CHECK-NEXT:    [[DOTDECOMPOSED:%.*]] = sub i8 [[DOTFROZEN]], [[TMP5]]
 ; CHECK-NEXT:    store i8 [[DOTDECOMPOSED]], ptr [[TMP0:%.*]], align 1
Index: llvm/lib/Transforms/Scalar/DivRemPairs.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/DivRemPairs.cpp
+++ llvm/lib/Transforms/Scalar/DivRemPairs.cpp
@@ -371,6 +371,11 @@
       Mul->insertAfter(RemInst);
       Sub->insertAfter(Mul);
 
+      // If DivInst has the exact flag, remove it. Otherwise this optimization
+      // may replace a well-defined value 'X % Y' with poison.
+      if (DivInst->hasPoisonGeneratingFlags())
+        DivInst->dropPoisonGeneratingFlags();
+
       // If X can be undef, X should be frozen first.
       // For example, let's assume that Y = 1 & X = undef:
       //   %div = sdiv undef, 1 // %div = undef


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