Constant fold x == undef to undef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107074 91177308-0d34-0410-b5e6-96231b3b80d8

lib/VMCore/ConstantFold.cpp

@@ -1817,8 +1817,15 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
     return Constant::getAllOnesValue(ResultTy);
 
   // Handle some degenerate cases first
-  if (isa<UndefValue>(C1) || isa<UndefValue>(C2))
+  if (isa<UndefValue>(C1) || isa<UndefValue>(C2)) {
+    // For EQ and NE, we can always pick a value for the undef to make the
+    // predicate pass or fail, so we can return undef.
+    if (ICmpInst::isEquality(ICmpInst::Predicate(pred)))
+      return UndefValue::get(ResultTy);
+    // Otherwise, pick the same value as the non-undef operand, and fold
+    // it to true or false.
     return ConstantInt::get(ResultTy, CmpInst::isTrueWhenEqual(pred));
+  }
 
   // No compile-time operations on this type yet.
   if (C1->getType()->isPPC_FP128Ty())

test/Transforms/InstCombine/icmp.ll

@@ -131,3 +131,26 @@ entry:
 ; CHECK: ret i1 false
 }
 
+define i1 @test14(i8 %X) nounwind readnone {
+entry:
+        %cmp = icmp slt i8 undef, -128
+        ret i1 %cmp
+; CHECK: @test14
+; CHECK: ret i1 false
+}
+
+define i1 @test15() nounwind readnone {
+entry:
+        %cmp = icmp eq i8 undef, -128
+        ret i1 %cmp
+; CHECK: @test15
+; CHECK: ret i1 undef
+}
+
+define i1 @test16() nounwind readnone {
+entry:
+        %cmp = icmp ne i8 undef, -128
+        ret i1 %cmp
+; CHECK: @test16
+; CHECK: ret i1 undef
+}