[llvm] a1b1c4a - [InstCombine] Fix miscompile in negation of select (#89698)

[via llvm-commits]

Author: Nikita Popov
Date: 2024-04-24T10:56:26+09:00
New Revision: a1b1c4a6d1d52916c5d885170a5f54632d579cdc

URL: https://github.com/llvm/llvm-project/commit/a1b1c4a6d1d52916c5d885170a5f54632d579cdc
DIFF: https://github.com/llvm/llvm-project/commit/a1b1c4a6d1d52916c5d885170a5f54632d579cdc.diff

LOG: [InstCombine] Fix miscompile in negation of select (#89698)

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 https://github.com/llvm/llvm-project/issues/89669.

Added: 
    

Modified: 
    llvm/include/llvm/Analysis/ValueTracking.h
    llvm/lib/Analysis/ValueTracking.cpp
    llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
    llvm/test/Transforms/InstCombine/sub-of-negatible.ll

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Analysis/ValueTracking.h b/llvm/include/llvm/Analysis/ValueTracking.h
index a2fa8f6064e116..571e44cdac2650 100644
--- a/llvm/include/llvm/Analysis/ValueTracking.h
+++ b/llvm/include/llvm/Analysis/ValueTracking.h
@@ -131,7 +131,8 @@ 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..7501f78ca23b7b 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -8042,17 +8042,27 @@ 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;
+  };
 
-  // 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)))))
+  // X = -Y or Y = -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