When folding away a (shl (shr)) pair, we need to check that the bits that will BECOME the low

bits are zero, not that the current low bits are zero.  Fixes <rdar://problem/8606771>.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117953 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 21 insertions, 1 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineShifts.cpp b/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 27716b886a..012d238729 100644
--- a/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -157,7 +157,7 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
     if (CI->getZExtValue() > NumBits) {
       unsigned LowBits = CI->getZExtValue() - NumBits;
       if (MaskedValueIsZero(I->getOperand(0),
-                            APInt::getLowBitsSet(TypeWidth, LowBits)))
+                          APInt::getLowBitsSet(TypeWidth, LowBits) << NumBits))
         return true;
     }
       
diff --git a/test/Transforms/InstCombine/2010-11-01-lshr-mask.ll b/test/Transforms/InstCombine/2010-11-01-lshr-mask.ll
new file mode 100644
index 0000000000..d74c70e4f7
--- /dev/null
+++ b/test/Transforms/InstCombine/2010-11-01-lshr-mask.ll
@@ -0,0 +1,20 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+; <rdar://problem/8606771>
+
+define i32 @main(i32 %argc) nounwind ssp {
+entry:
+  %tmp3151 = trunc i32 %argc to i8
+  %tmp3161 = or i8 %tmp3151, -17
+  %tmp3162 = and i8 %tmp3151, 122
+  %tmp3163 = xor i8 %tmp3162, -17
+  %tmp4114 = shl i8 %tmp3163, 6
+  %tmp4115 = xor i8 %tmp4114, %tmp3163
+  %tmp4120 = xor i8 %tmp3161, %tmp4115
+; CHECK: lshr i8 %tmp4115, 1
+; CHECK-NOT: shl i8 %tmp4126, 6
+  %tmp4126 = lshr i8 %tmp4120, 7
+  %tmp4127 = mul i8 %tmp4126, 64
+  %tmp4086 = zext i8 %tmp4127 to i32
+; CHECK: ret i32
+  ret i32 %tmp4086
+}