[PATCH] D30311: [ValueTracking] Don't do an unchecked shift in ComputeNumSignBits

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy added inline comments.


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Comment at: lib/Analysis/ValueTracking.cpp:2209
     if (match(U->getOperand(1), m_APInt(ShAmt))) {
-      Tmp += ShAmt->getZExtValue();
+      Tmp += ShAmt->getLimitedValue(TyBits);
       if (Tmp > TyBits) Tmp = TyBits;
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majnemer wrote:
> sanjoy wrote:
> > majnemer wrote:
> > > Shouldn't we treat this in the same way we treat Instruction::Shl?
> > You mean return `1` for undefined shifts?  I don't see a good reason why we should do that (both here and in `shl`) -- `ashr` ing more than the bitwidth is `undef` anyway, so why not be maximally aggressive?
> It is undef but I don't think ComputeNumSignBits it allowed to assign known bits to undef values.
I see what you mean.  I'm a bit embarrassed now that it took me this long. :)


https://reviews.llvm.org/D30311