[InstCombine] replace undef in vector constant for safe shift transfo…

…rm (PR45447)

As noted in PR45447, we have a vector-constant-with-undef-element transform bug:
https://bugs.llvm.org/show_bug.cgi?id=45447

We replace undefs with a safe constant (0 or -1) based on the (non-)negative
predicate constraint.

So this is correct:
http://volta.cs.utah.edu:8080/z/WZE36H
...but this is not:
http://volta.cs.utah.edu:8080/z/boj8gJ

Previously, we were relying on getSafeVectorConstantForBinop() in the related fold (D76800).
But that's making an assumption about what qualifies as "safe", and that assumption may
not always hold.

Differential Revision: https://reviews.llvm.org/D77739

llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

@@ -3071,16 +3071,22 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
     Constant *C;
     if (match(NotVal, m_AShr(m_Constant(C), m_Value(Y))) &&
         match(C, m_Negative())) {
-      Constant *NewC = ConstantExpr::getNot(C);
-      if (C->getType()->isVectorTy())
-        NewC = getSafeVectorConstantForBinop(Instruction::LShr, NewC, false);
-      return BinaryOperator::CreateLShr(NewC, Y);
+      // We matched a negative constant, so propagating undef is unsafe.
+      // Clamp undef elements to -1.
+      Type *EltTy = C->getType()->getScalarType();
+      C = Constant::replaceUndefsWith(C, ConstantInt::getAllOnesValue(EltTy));
+      return BinaryOperator::CreateLShr(ConstantExpr::getNot(C), Y);
     }
 
     // ~(C >>u Y) --> ~C >>s Y (when inverting the replicated sign bits)
     if (match(NotVal, m_LShr(m_Constant(C), m_Value(Y))) &&
-        match(C, m_NonNegative()))
+        match(C, m_NonNegative())) {
+      // We matched a non-negative constant, so propagating undef is unsafe.
+      // Clamp undef elements to 0.
+      Type *EltTy = C->getType()->getScalarType();
+      C = Constant::replaceUndefsWith(C, ConstantInt::getNullValue(EltTy));
       return BinaryOperator::CreateAShr(ConstantExpr::getNot(C), Y);
+    }
 
     // ~(X + C) --> -(C + 1) - X
     if (match(Op0, m_Add(m_Value(X), m_Constant(C))))

llvm/test/Transforms/InstCombine/vector-xor.ll

@@ -172,7 +172,7 @@ define <4 x i32> @test_v4i32_not_lshr_nonnegative_const(<4 x i32> %a0) {
 
 define <4 x i32> @test_v4i32_not_lshr_nonnegative_const_undef(<4 x i32> %a0) {
 ; CHECK-LABEL: @test_v4i32_not_lshr_nonnegative_const_undef(
-; CHECK-NEXT:    [[TMP1:%.*]] = ashr <4 x i32> <i32 -4, i32 -6, i32 undef, i32 -10>, [[A0:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = ashr <4 x i32> <i32 -4, i32 -6, i32 -1, i32 -10>, [[A0:%.*]]
 ; CHECK-NEXT:    ret <4 x i32> [[TMP1]]
 ;
   %1 = lshr <4 x i32> <i32  3, i32  5, i32 undef, i32  9>, %a0