[InstCombine] Fix miscompile in negation of select

[nikic]

Swapping the operands of a select is not valid if one hand is more poisonous that the other, because the negation zero contains poison elements.

Fix this by adding an extra parameter to isKnownNegation() to forbid poison elements.

I've implemented this using manual checks to avoid needing four variants for the NeedsNSW/AllowPoison combinations. Maybe there is a better way to do this...

Fixes #89669.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Nikita Popov (nikic)

Changes

Swapping the operands of a select is not valid if one hand is more poisonous that the other, because the negation zero contains poison elements.

Fix this by adding an extra parameter to isKnownNegation() to forbid poison elements.

I've implemented this using manual checks to avoid needing four variants for the NeedsNSW/AllowPoison combinations. Maybe there is a better way to do this...

Fixes #89669.

---

Full diff: https://github.com/llvm/llvm-project/pull/89698.diff

4 Files Affected:

• (modified) llvm/include/llvm/Analysis/ValueTracking.h (+2-1)
• (modified) llvm/lib/Analysis/ValueTracking.cpp (+17-6)
• (modified) llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp (+2-1)
• (modified) llvm/test/Transforms/InstCombine/sub-of-negatible.ll (+3-3)

diff --git a/llvm/include/llvm/Analysis/ValueTracking.h b/llvm/include/llvm/Analysis/ValueTracking.h
index bab7c8868532db..dfe04ff58aed86 100644
--- a/llvm/include/llvm/Analysis/ValueTracking.h
+++ b/llvm/include/llvm/Analysis/ValueTracking.h
@@ -130,7 +130,8 @@ bool isKnownNonZero(const Value *V, const SimplifyQuery &Q, unsigned Depth = 0);
 /// Currently can recoginze Value pair:
 /// 1: <X, Y> if X = sub (0, Y) or Y = sub (0, X)
 /// 2: <X, Y> if X = sub (A, B) and Y = sub (B, A)
-bool isKnownNegation(const Value *X, const Value *Y, bool NeedNSW = false);
+bool isKnownNegation(const Value *X, const Value *Y, bool NeedNSW = false,
+                     bool AllowPoison = true);
 
 /// Returns true if the give value is known to be non-negative.
 bool isKnownNonNegative(const Value *V, const SimplifyQuery &SQ,
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 21e3f8a4cc52c7..36d9010e34ba14 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -8042,17 +8042,28 @@ static SelectPatternResult matchMinMax(CmpInst::Predicate Pred,
   return {SPF_UNKNOWN, SPNB_NA, false};
 }
 
-bool llvm::isKnownNegation(const Value *X, const Value *Y, bool NeedNSW) {
+bool llvm::isKnownNegation(const Value *X, const Value *Y, bool NeedNSW,
+                           bool AllowPoison) {
   assert(X && Y && "Invalid operand");
 
-  // X = sub (0, Y) || X = sub nsw (0, Y)
-  if ((!NeedNSW && match(X, m_Sub(m_ZeroInt(), m_Specific(Y)))) ||
-      (NeedNSW && match(X, m_NSWNeg(m_Specific(Y)))))
+  auto IsNegationOf = [&](const Value *X, const Value *Y) {
+    if (!match(X, m_Neg(m_Specific(Y))))
+      return false;
+
+    auto *BO = cast<BinaryOperator>(X);
+    if (NeedNSW && !BO->hasNoSignedWrap())
+      return false;
+
+    auto *Zero = cast<Constant>(BO->getOperand(0));
+    if (!AllowPoison && !Zero->isNullValue())
+      return false;
+
     return true;
+  };
 
+  // X = sub (0, Y) || X = sub nsw (0, Y)
   // Y = sub (0, X) || Y = sub nsw (0, X)
-  if ((!NeedNSW && match(Y, m_Sub(m_ZeroInt(), m_Specific(X)))) ||
-      (NeedNSW && match(Y, m_NSWNeg(m_Specific(X)))))
+  if (IsNegationOf(X, Y) || IsNegationOf(Y, X))
     return true;
 
   // X = sub (A, B), Y = sub (B, A) || X = sub nsw (A, B), Y = sub nsw (B, A)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp b/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
index d697f361dec023..ed2a98ba4ae40e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
@@ -320,7 +320,8 @@ std::array<Value *, 2> Negator::getSortedOperandsOfBinOp(Instruction *I) {
     return NegatedPHI;
   }
   case Instruction::Select: {
-    if (isKnownNegation(I->getOperand(1), I->getOperand(2))) {
+    if (isKnownNegation(I->getOperand(1), I->getOperand(2), /*NeedNSW=*/false,
+                        /*AllowPoison=*/false)) {
       // Of one hand of select is known to be negation of another hand,
       // just swap the hands around.
       auto *NewSelect = cast<SelectInst>(I->clone());
diff --git a/llvm/test/Transforms/InstCombine/sub-of-negatible.ll b/llvm/test/Transforms/InstCombine/sub-of-negatible.ll
index 72fd7f7be2b04c..b2e14ceaca1b08 100644
--- a/llvm/test/Transforms/InstCombine/sub-of-negatible.ll
+++ b/llvm/test/Transforms/InstCombine/sub-of-negatible.ll
@@ -1385,12 +1385,12 @@ define i8 @dont_negate_ordinary_select(i8 %x, i8 %y, i8 %z, i1 %c) {
   ret i8 %t1
 }
 
-; FIXME: This is a miscompile.
 define <2 x i32> @negate_select_of_negation_poison(<2 x i1> %c, <2 x i32> %x) {
 ; CHECK-LABEL: @negate_select_of_negation_poison(
 ; CHECK-NEXT:    [[NEG:%.*]] = sub <2 x i32> <i32 0, i32 poison>, [[X:%.*]]
-; CHECK-NEXT:    [[TMP1:%.*]] = select <2 x i1> [[C:%.*]], <2 x i32> [[X]], <2 x i32> [[NEG]]
-; CHECK-NEXT:    ret <2 x i32> [[TMP1]]
+; CHECK-NEXT:    [[SEL:%.*]] = select <2 x i1> [[C:%.*]], <2 x i32> [[NEG]], <2 x i32> [[X]]
+; CHECK-NEXT:    [[NEG2:%.*]] = sub <2 x i32> zeroinitializer, [[SEL]]
+; CHECK-NEXT:    ret <2 x i32> [[NEG2]]
 ;
   %neg = sub <2 x i32> <i32 0, i32 poison>, %x
   %sel = select <2 x i1> %c, <2 x i32> %neg, <2 x i32> %x

[goldsteinn]

Think 2x comments is unnecessarily verbose, people will get it.

[goldsteinn]

This seems like a reasonable way to fix it to me.

LGTM.

[dtcxzyw]

LGTM.