Instcombine's ShrinkDemandedConstant may strip bits out of constants,
obscuring what would otherwise be a low-bits mask. Use ComputeMaskedBits
to compute what ShrinkDemandedConstant knew about to reconstruct a
low-bits mask value.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73540 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index ca805bd..3731fdf 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -68,6 +68,7 @@
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CommandLine.h"
@@ -2394,14 +2395,28 @@
if (CI->isAllOnesValue())
return getSCEV(U->getOperand(0));
const APInt &A = CI->getValue();
- unsigned Ones = A.countTrailingOnes();
- if (APIntOps::isMask(Ones, A))
+
+ // Instcombine's ShrinkDemandedConstant may strip bits out of
+ // constants, obscuring what would otherwise be a low-bits mask.
+ // Use ComputeMaskedBits to compute what ShrinkDemandedConstant
+ // knew about to reconstruct a low-bits mask value.
+ unsigned LZ = A.countLeadingZeros();
+ unsigned BitWidth = A.getBitWidth();
+ APInt AllOnes = APInt::getAllOnesValue(BitWidth);
+ APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
+ ComputeMaskedBits(U->getOperand(0), AllOnes, KnownZero, KnownOne, TD);
+
+ APInt EffectiveMask = APInt::getLowBitsSet(BitWidth, BitWidth - LZ);
+
+ if (LZ != 0 && !((~A & ~KnownZero) & EffectiveMask)) {
return
getZeroExtendExpr(getTruncateExpr(getSCEV(U->getOperand(0)),
- IntegerType::get(Ones)),
+ IntegerType::get(BitWidth - LZ)),
U->getType());
+ }
}
break;
+
case Instruction::Or:
// If the RHS of the Or is a constant, we may have something like:
// X*4+1 which got turned into X*4|1. Handle this as an Add so loop