InstCombine: Don't just copy known bits from the first operand of an srem.

That's obviously wrong. Conservatively restrict it to the sign bit, which
matches the original intention of this analysis. Fixes PR15940.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181518 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 8add1ea..a7bfe09 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -754,7 +754,7 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
       ComputeMaskedBits(I->getOperand(0), LHSKnownZero, LHSKnownOne, Depth+1);
       // If it's known zero, our sign bit is also zero.
       if (LHSKnownZero.isNegative())
-        KnownZero |= LHSKnownZero;
+        KnownZero.setBit(KnownZero.getBitWidth() - 1);
     }
     break;
   case Instruction::URem: {

diff --git a/test/Transforms/InstCombine/icmp.ll b/test/Transforms/InstCombine/icmp.ll
index c912a57..ed1cd1e 100644 (file)
--- a/test/Transforms/InstCombine/icmp.ll
+++ b/test/Transforms/InstCombine/icmp.ll
@@ -707,6 +707,18 @@ define i1 @test69(i32 %c) nounwind uwtable {
   ret i1 %3
 }
 
+; PR15940
+; CHECK: @test70
+; CHECK-NEXT: %A = srem i32 5, %X
+; CHECK-NEXT: %C = icmp ne i32 %A, 2
+; CHECK-NEXT: ret i1 %C
+define i1 @test70(i32 %X) {
+  %A = srem i32 5, %X
+  %B = add i32 %A, 2
+  %C = icmp ne i32 %B, 4
+  ret i1 %C
+}
+
 ; CHECK: @icmp_sext16trunc
 ; CHECK-NEXT: %1 = trunc i32 %x to i16
 ; CHECK-NEXT: %cmp = icmp slt i16 %1, 36