Add support for computing "zext of value" in KnownBits. NFCI
Summary:
The description of KnownBits::zext() and
KnownBits::zextOrTrunc() has confusingly been telling
that the operation is equivalent to zero extending the
value we're tracking. That has not been true, instead
the user has been forced to explicitly set the extended
bits as known zero afterwards.
This patch adds a second argument to KnownBits::zext()
and KnownBits::zextOrTrunc() to control if the extended
bits should be considered as known zero or as unknown.
Reviewers: craig.topper, RKSimon
Reviewed By: RKSimon
Subscribers: javed.absar, hiraditya, jdoerfert, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D58650
llvm-svn: 355099
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 80a08e2..f6e3886 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1128,12 +1128,9 @@
Q.DL.getTypeSizeInBits(ScalarTy);
assert(SrcBitWidth && "SrcBitWidth can't be zero");
- Known = Known.zextOrTrunc(SrcBitWidth);
+ Known = Known.zextOrTrunc(SrcBitWidth, false);
computeKnownBits(I->getOperand(0), Known, Depth + 1, Q);
- Known = Known.zextOrTrunc(BitWidth);
- // Any top bits are known to be zero.
- if (BitWidth > SrcBitWidth)
- Known.Zero.setBitsFrom(SrcBitWidth);
+ Known = Known.zextOrTrunc(BitWidth, true /* ExtendedBitsAreKnownZero */);
break;
}
case Instruction::BitCast: {