[Reassociate] prevent partial undef negation replacement

As shown in the examples in issue llvm#57683, we allow matching
vectors with poison (undef) in this transform (and possibly more),
but we can't then use the partially defined value as a replacement
value in other expressions blindly.

This seems to be avoided in simpler examples of reassociation,
and other passes should be able to clean up the redundant op
seen in these tests.

Author: rotateright

llvm/lib/Transforms/Scalar/Reassociate.cpp

@@ -887,6 +887,12 @@ static Value *NegateValue(Value *V, Instruction *BI,
     // be zapped by reassociate later, so we don't need much finesse here.
     Instruction *TheNeg = dyn_cast<Instruction>(U);
 
+    // We can't safely propagate a vector zero constant with poison/undef lanes.
+    Constant *C;
+    if (match(TheNeg, m_BinOp(m_Constant(C), m_Value())) &&
+        C->containsUndefOrPoisonElement())
+      continue;
+
     // Verify that the negate is in this function, V might be a constant expr.
     if (!TheNeg ||
         TheNeg->getParent()->getParent() != BI->getParent()->getParent())

llvm/test/Transforms/Reassociate/negation.ll

@@ -44,13 +44,14 @@ define <2 x i32> @negate_vec_undefs(<2 x i32> %a, <2 x i32> %b, <2 x i32> %z) {
   ret <2 x i32> %f
 }
 
-; FIXME: Replacing x with a partial undef negation is a miscompile.
+; Replacing %x with a partial undef negation is a miscompile.
 
 define <2 x i32> @PR57683(<2 x i32> %x) {
 ; CHECK-LABEL: @PR57683(
 ; CHECK-NEXT:    [[PARTIAL_NEG:%.*]] = sub <2 x i32> <i32 poison, i32 0>, [[X:%.*]]
 ; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <2 x i32> [[PARTIAL_NEG]], <2 x i32> [[X]], <2 x i32> <i32 1, i32 3>
-; CHECK-NEXT:    [[SUB:%.*]] = add <2 x i32> [[PARTIAL_NEG]], <i32 1, i32 1>
+; CHECK-NEXT:    [[X_NEG:%.*]] = sub <2 x i32> zeroinitializer, [[X]]
+; CHECK-NEXT:    [[SUB:%.*]] = add <2 x i32> [[X_NEG]], <i32 1, i32 1>
 ; CHECK-NEXT:    [[R:%.*]] = add <2 x i32> [[SUB]], [[SHUF]]
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -65,7 +66,8 @@ define <2 x float> @PR57683_FP(<2 x float> %x) {
 ; CHECK-LABEL: @PR57683_FP(
 ; CHECK-NEXT:    [[PARTIAL_NEG:%.*]] = fsub reassoc nsz <2 x float> <float poison, float 0.000000e+00>, [[X:%.*]]
 ; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <2 x float> [[PARTIAL_NEG]], <2 x float> [[X]], <2 x i32> <i32 1, i32 3>
-; CHECK-NEXT:    [[SUB:%.*]] = fadd reassoc nsz <2 x float> [[PARTIAL_NEG]], <float 1.000000e+00, float 1.000000e+00>
+; CHECK-NEXT:    [[X_NEG:%.*]] = fneg reassoc nsz <2 x float> [[X]]
+; CHECK-NEXT:    [[SUB:%.*]] = fadd reassoc nsz <2 x float> [[X_NEG]], <float 1.000000e+00, float 1.000000e+00>
 ; CHECK-NEXT:    [[R:%.*]] = fadd reassoc nsz <2 x float> [[SUB]], [[SHUF]]
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;