[DivRemPairs] Do not freeze poisons that can't be undef (#92627)

Per comments in DivRemPairs, the rewrite from
```llvm
    %div = div %X, %Y
    %rem = rem %X, %Y
```
to
```llvm
    %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.

Author: krzysz00

llvm/lib/Transforms/Scalar/DivRemPairs.cpp

@@ -381,16 +381,15 @@ 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);
         Sub->setOperand(0, FrX);
       }
       // 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);

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

@@ -33,18 +33,14 @@ define i32 @no_freezes(ptr %p, i32 noundef %x, i32 noundef %y) {
   ret i32 %rem
 }
 
-; FIXME: There should be no need to `freeze` x2 and y2 since they have defined
-; but potentially poison values.
 define i32 @poison_does_not_freeze(ptr %p, i32 noundef %x, i32 noundef %y) {
 ; CHECK-LABEL: define i32 @poison_does_not_freeze(
 ; CHECK-SAME: ptr [[P:%.*]], i32 noundef [[X:%.*]], i32 noundef [[Y:%.*]]) {
 ; CHECK-NEXT:    [[X2:%.*]] = shl nuw nsw i32 [[X]], 5
 ; CHECK-NEXT:    [[Y2:%.*]] = add nuw nsw i32 [[Y]], 1
-; CHECK-NEXT:    [[X2_FROZEN:%.*]] = freeze i32 [[X2]]
-; CHECK-NEXT:    [[Y2_FROZEN:%.*]] = freeze i32 [[Y2]]
-; CHECK-NEXT:    [[DIV:%.*]] = udiv i32 [[X2_FROZEN]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X2_FROZEN]], [[TMP1]]
+; CHECK-NEXT:    [[DIV:%.*]] = udiv i32 [[X2]], [[Y2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2]]
+; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X2]], [[TMP1]]
 ; CHECK-NEXT:    store i32 [[DIV]], ptr [[P]], align 4
 ; CHECK-NEXT:    ret i32 [[REM_DECOMPOSED]]
 ;
@@ -61,11 +57,9 @@ define i32 @poison_does_not_freeze_signed(ptr %p, i32 noundef %x, i32 noundef %y
 ; CHECK-SAME: ptr [[P:%.*]], i32 noundef [[X:%.*]], i32 noundef [[Y:%.*]]) {
 ; CHECK-NEXT:    [[X2:%.*]] = shl nuw nsw i32 [[X]], 5
 ; CHECK-NEXT:    [[Y2:%.*]] = add nuw nsw i32 [[Y]], 1
-; CHECK-NEXT:    [[X2_FROZEN:%.*]] = freeze i32 [[X2]]
-; CHECK-NEXT:    [[Y2_FROZEN:%.*]] = freeze i32 [[Y2]]
-; CHECK-NEXT:    [[DIV:%.*]] = sdiv i32 [[X2_FROZEN]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X2_FROZEN]], [[TMP1]]
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i32 [[X2]], [[Y2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2]]
+; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X2]], [[TMP1]]
 ; CHECK-NEXT:    store i32 [[DIV]], ptr [[P]], align 4
 ; CHECK-NEXT:    ret i32 [[REM_DECOMPOSED]]
 ;
@@ -82,11 +76,9 @@ define <4 x i8> @poison_does_not_freeze_vector(ptr %p, <4 x i8> noundef %x, <4 x
 ; CHECK-SAME: ptr [[P:%.*]], <4 x i8> noundef [[X:%.*]], <4 x i8> noundef [[Y:%.*]]) {
 ; CHECK-NEXT:    [[X2:%.*]] = shl nuw nsw <4 x i8> [[X]], <i8 5, i8 5, i8 5, i8 5>
 ; CHECK-NEXT:    [[Y2:%.*]] = add nuw nsw <4 x i8> [[Y]], <i8 1, i8 1, i8 1, i8 1>
-; CHECK-NEXT:    [[X2_FROZEN:%.*]] = freeze <4 x i8> [[X2]]
-; CHECK-NEXT:    [[Y2_FROZEN:%.*]] = freeze <4 x i8> [[Y2]]
-; CHECK-NEXT:    [[DIV:%.*]] = udiv <4 x i8> [[X2_FROZEN]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[TMP1:%.*]] = mul <4 x i8> [[DIV]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub <4 x i8> [[X2_FROZEN]], [[TMP1]]
+; CHECK-NEXT:    [[DIV:%.*]] = udiv <4 x i8> [[X2]], [[Y2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = mul <4 x i8> [[DIV]], [[Y2]]
+; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub <4 x i8> [[X2]], [[TMP1]]
 ; CHECK-NEXT:    store <4 x i8> [[DIV]], ptr [[P]], align 4
 ; CHECK-NEXT:    ret <4 x i8> [[REM_DECOMPOSED]]
 ;
@@ -103,11 +95,9 @@ define i32 @explicit_poison_does_not_freeze(ptr %p, i32 noundef %y) {
 ; CHECK-SAME: ptr [[P:%.*]], i32 noundef [[Y:%.*]]) {
 ; CHECK-NEXT:    [[X:%.*]] = add i32 poison, 1
 ; CHECK-NEXT:    [[Y2:%.*]] = add nuw nsw i32 [[Y]], 1
-; CHECK-NEXT:    [[X_FROZEN:%.*]] = freeze i32 [[X]]
-; CHECK-NEXT:    [[Y2_FROZEN:%.*]] = freeze i32 [[Y2]]
-; CHECK-NEXT:    [[DIV:%.*]] = udiv i32 [[X_FROZEN]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X_FROZEN]], [[TMP1]]
+; CHECK-NEXT:    [[DIV:%.*]] = udiv i32 [[X]], [[Y2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2]]
+; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X]], [[TMP1]]
 ; CHECK-NEXT:    store i32 [[DIV]], ptr [[P]], align 4
 ; CHECK-NEXT:    ret i32 [[REM_DECOMPOSED]]
 ;
@@ -124,11 +114,9 @@ define i32 @explicit_poison_does_not_freeze_signed(ptr %p, i32 noundef %y) {
 ; CHECK-SAME: ptr [[P:%.*]], i32 noundef [[Y:%.*]]) {
 ; CHECK-NEXT:    [[X:%.*]] = add i32 poison, 1
 ; CHECK-NEXT:    [[Y2:%.*]] = add nuw nsw i32 [[Y]], 1
-; CHECK-NEXT:    [[X_FROZEN:%.*]] = freeze i32 [[X]]
-; CHECK-NEXT:    [[Y2_FROZEN:%.*]] = freeze i32 [[Y2]]
-; CHECK-NEXT:    [[DIV:%.*]] = sdiv i32 [[X_FROZEN]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2_FROZEN]]
-; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X_FROZEN]], [[TMP1]]
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i32 [[X]], [[Y2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[DIV]], [[Y2]]
+; CHECK-NEXT:    [[REM_DECOMPOSED:%.*]] = sub i32 [[X]], [[TMP1]]
 ; CHECK-NEXT:    store i32 [[DIV]], ptr [[P]], align 4
 ; CHECK-NEXT:    ret i32 [[REM_DECOMPOSED]]
 ;