[DivRemPairs] Strip division's poison generating flag

[aqjune]

Summary

Given this transformation: X % Y -> X - (X / Y) * Y

This 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 does not.

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

[nikic]

LGTM

llvm/lib/Transforms/Scalar/DivRemPairs.cpp

376

Not really necessary to check has first.

llvm/test/Transforms/DivRemPairs/AArch64/div-rem-pairs.ll

18

Please don't use unnamed instructions (opt -S -passes=instnamer).

[aqjune]

Reflect comments

llvm/lib/Transforms/Scalar/DivRemPairs.cpp

if (HasDivRemOp) {
       // If the div and rem are in the same block, we do the same transform,
       // but any code movement would be within the same block.
 
       if (!DivDominates)
         DivInst->moveBefore(RemInst);
       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.
+      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
       //   %rem = srem undef, 1 // %rem = 0
       // =>
       //   %div = sdiv undef, 1 // %div = undef
       //   %mul = mul %div, 1   // %mul = undef
       //   %rem = sub %x, %mul  // %rem = undef - undef = undef

llvm/test/Transforms/DivRemPairs/AArch64/div-rem-pairs.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=div-rem-pairs -S -mtriple=aarch64-unknown-unknown    | FileCheck %s
 
 define i8 @f(ptr %p, i8 %x, i8 %y) {
 ; CHECK-LABEL: @f(
 ; CHECK-NEXT:    [[X_FROZEN:%.*]] = freeze i8 [[X:%.*]]
 ; CHECK-NEXT:    [[Y_FROZEN:%.*]] = freeze i8 [[Y:%.*]]
-; CHECK-NEXT:    [[XDIVY:%.*]] = udiv exact i8 [[X_FROZEN]], [[Y_FROZEN]]
+; CHECK-NEXT:    [[XDIVY:%.*]] = udiv i8 [[X_FROZEN]], [[Y_FROZEN]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = mul i8 [[XDIVY]], [[Y_FROZEN]]
 ; CHECK-NEXT:    [[XMODY_DECOMPOSED:%.*]] = sub i8 [[X_FROZEN]], [[TMP1]]
 ; CHECK-NEXT:    store i8 [[XMODY_DECOMPOSED]], ptr [[P:%.*]], align 1
 ; CHECK-NEXT:    ret i8 [[XDIVY]]
 ;
   %xmody = urem i8 %x, %y
   %xdivy = udiv exact i8 %x, %y
   store i8 %xmody, ptr %p, align 1
   ret i8 %xdivy
 }