[DivRemPairs] Do not freeze poisons that can't be undef

[krzysz00]

Per comments in DivRemPairs, the rewrite from

    %div = div %X, %Y
    %rem = rem %X, %Y

to

    %div = div %X, %Y
    %.mul = mul %div, %Y
    %rem = sub %X, %mul

is unsound when %X or %Y are undef.

However, it is known to be sound if %X or %Y are poison but can't be undef, since both the pre- and post-rewrite %rem are poison.

Additionally, proofs: https://alive2.llvm.org/ce/z/xtNQ8j

A comment in the pass listed a TODO for changing a usage of isGuaranteedNotToBeUndefOrPoison() in the pass to something that only detects undef. Such a function has been implemented since the time that TODO was written, but has not been used.

Therefore, this commit updates DivRemPairs to use
isGuaranteedNotToBeUndef() instead.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

@llvm/pr-subscribers-llvm-transforms

Author: Krzysztof Drewniak (krzysz00)

Changes

Per comments in DivRemPairs, the rewrite from

    %div = div %X, %Y
    %rem = rem %X, %Y

to

    %div = div %X, %Y
    %.mul = mul %div, %Y
    %rem = sub %X, %mul

is unsound when %X or %Y are undef.

However, it is known to be sound if %X or %Y are poison but can't be undef, since both the pre- and post-rewrite %rem are poison.

A comment in the pass listed a TODO for changing a usage of isGuaranteedNotToBeUndefOrPoison() in the pass to something that only detects undef. Such a function has been implemented since the time that TODO was written, but has not been used.

Therefore, this commit updates DivRemPairs to use
isGuaranteedNotToBeUndef() instead.

---

Full diff: https://github.com/llvm/llvm-project/pull/92627.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/DivRemPairs.cpp (+2-3)

diff --git a/llvm/lib/Transforms/Scalar/DivRemPairs.cpp b/llvm/lib/Transforms/Scalar/DivRemPairs.cpp
index 45f36a36b5dd0..f7ada9fb8eb8a 100644
--- a/llvm/lib/Transforms/Scalar/DivRemPairs.cpp
+++ b/llvm/lib/Transforms/Scalar/DivRemPairs.cpp
@@ -381,8 +381,7 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
       //   %mul = mul %div, 1   // %mul = undef
       //   %rem = sub %x, %mul  // %rem = undef - undef = undef
       // If X is not frozen, %rem becomes undef after transformation.
-      // TODO: We need a undef-specific checking function in ValueTracking
-      if (!isGuaranteedNotToBeUndefOrPoison(X, nullptr, DivInst, &DT)) {
+      if (!isGuaranteedNotToBeUndef(X, nullptr, DivInst, &DT)) {
         auto *FrX =
             new FreezeInst(X, X->getName() + ".frozen", DivInst->getIterator());
         DivInst->setOperand(0, FrX);
@@ -390,7 +389,7 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
       }
       // Same for Y. If X = 1 and Y = (undef | 1), %rem in src is either 1 or 0,
       // but %rem in tgt can be one of many integer values.
-      if (!isGuaranteedNotToBeUndefOrPoison(Y, nullptr, DivInst, &DT)) {
+      if (!isGuaranteedNotToBeUndef(Y, nullptr, DivInst, &DT)) {
         auto *FrY =
             new FreezeInst(Y, Y->getName() + ".frozen", DivInst->getIterator());
         DivInst->setOperand(1, FrY);