Really check that the bits that will become zero are actually already zero

before eliminating the operation that zeros them. This fixes rdar://8739316.

llvm-svn: 121353

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -168,9 +168,8 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
     // We can always turn lshr(c1)+shl(c2) -> lshr(c3)+and(c4), but it isn't
     // profitable unless we know the and'd out bits are already zero.
     if (CI->getZExtValue() > NumBits) {
-      unsigned LowBits = CI->getZExtValue() - NumBits;
       if (MaskedValueIsZero(I->getOperand(0),
-                          APInt::getLowBitsSet(TypeWidth, LowBits) << NumBits))
+                            APInt::getLowBitsSet(TypeWidth, NumBits)))
         return true;
     }
       

llvm/test/Transforms/InstCombine/2010-11-01-lshr-mask.ll

 ; RUN: opt -instcombine -S < %s | FileCheck %s
-; <rdar://problem/8606771>
 
+; <rdar://problem/8606771>
+; CHECK: @main
 define i32 @main(i32 %argc) nounwind ssp {
 entry:
   %tmp3151 = trunc i32 %argc to i8
@@ -18,3 +19,26 @@ entry:
 ; CHECK: ret i32
   ret i32 %tmp4086
 }
+
+; rdar://8739316
+; CHECK: @foo
+define i8 @foo(i8 %arg, i8 %arg1) nounwind {
+bb:
+  %tmp = shl i8 %arg, 7
+  %tmp2 = and i8 %arg1, 84
+  %tmp3 = and i8 %arg1, -118
+  %tmp4 = and i8 %arg1, 33
+  %tmp5 = sub i8 -88, %tmp2
+  %tmp6 = and i8 %tmp5, 84
+  %tmp7 = or i8 %tmp4, %tmp6
+  %tmp8 = xor i8 %tmp, %tmp3
+  %tmp9 = or i8 %tmp7, %tmp8
+  %tmp10 = lshr i8 %tmp8, 7
+  %tmp11 = shl i8 %tmp10, 5
+
+; CHECK: %0 = lshr i8 %tmp8, 2
+; CHECK: %tmp11 = and i8 %0, 32
+
+  %tmp12 = xor i8 %tmp11, %tmp9
+  ret i8 %tmp12
+}