Just mark the sign bit as known zero, rather than any other irrelevan…

…t bits known zero in the LHS. Fixes PR12541. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155818 91177308-0d34-0410-b5e6-96231b3b80d8
darkbasic · Apr 30, 2012 · 5ff30e7 · 5ff30e7
1 parent bfbab99
commit 5ff30e7
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Showing 2 changed files with 13 additions and 1 deletion.
diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
@@ -564,7 +564,7 @@ void llvm::ComputeMaskedBits(Value *V, APInt &KnownZero, APInt &KnownOne,
                         Depth+1);
       // If it's known zero, our sign bit is also zero.
       if (LHSKnownZero.isNegative())
-        KnownZero |= LHSKnownZero;
+        KnownZero.setBit(BitWidth - 1);
     }
 
     break;

diff --git a/test/Transforms/InstCombine/2012-04-30-SRem.ll b/test/Transforms/InstCombine/2012-04-30-SRem.ll
@@ -0,0 +1,12 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+; PR12541
+
+define i32 @foo(i32 %x) {
+  %y = xor i32 %x, 3
+  %z = srem i32 1656690544, %y
+  %sext = shl i32 %z, 24
+  %s = ashr exact i32 %sext, 24
+  ret i32 %s
+; CHECK-NOT: and
+; The shifts were wrongly being turned into an and with 112
+}