[InstCombine] Fix infinite loop in llvm#67273

Closes llvm#67783.

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -5450,8 +5450,16 @@ Instruction *InstCombinerImpl::foldICmpEquality(ICmpInst &I) {
   Constant *Cst;
   if (match(&I, m_c_ICmp(PredUnused,
                          m_OneUse(m_Xor(m_Value(A), m_ImmConstant(Cst))),
-                         m_Value(B))))
-    return new ICmpInst(Pred, Builder.CreateXor(A, B), Cst);
+                         m_Value(B)))) {
+    // Special case:
+    // icmp eq/ne OneUse(A ^ Cst1), Cst2 --> icmp eq/ne A, Cst1 ^ Cst2
+    // We handle this to avoid infinite loops.
+    if (match(B, m_ImmConstant())) {
+      if (Value *V = simplifyXorInst(B, Cst, SQ.getWithInstruction(&I)))
+        return new ICmpInst(Pred, A, V);
+    } else
+      return new ICmpInst(Pred, Builder.CreateXor(A, B), Cst);
+  }
 
   return nullptr;
 }

llvm/test/Transforms/InstCombine/icmp-equality-xor.ll

@@ -131,3 +131,16 @@ define i1 @foo2(i32 %x, i32 %y) {
   %cmp = icmp eq i32 %and, %and1
   ret i1 %cmp
 }
+
+; tests from PR67783
+define <2 x i1> @foo3(<2 x i8> %x) {
+; CHECK-LABEL: @foo3(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne <2 x i8> [[X:%.*]], <i8 -9, i8 -80>
+; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+;
+entry:
+  %xor = xor <2 x i8> %x, <i8 -2, i8 -1>
+  %cmp = icmp ne <2 x i8> %xor, <i8 9, i8 79>
+  ret <2 x i1> %cmp
+}