[llvm] 7ef2c68 - [InstSimplify] improve efficiency for detecting non-zero value

[Sanjay Patel via llvm-commits]

I'm not understanding the question. 🤔

I suspect this doesn't answer it, but more of the trail that led here:
There was a gap where isKnownNonEqual was succeeding where
simplifyICmpWithZero() did not - it was plugged with:
https://reviews.llvm.org/rG5ae5d25e38efad1d59ed97d969a5e930b58a5e16

That was exposed by this test (and that's what the unittest in the above
commit is based on):
https://github.com/llvm/llvm-project/blob/9345f9fa5d6401bf9b98ce915bf4fbde9d183ce7/llvm/test/Analysis/ValueTracking/monotonic-phi.ll#L185


On Wed, Apr 14, 2021 at 6:04 PM Philip Reames <listmail at philipreames.com>
wrote:

> Hm, this seems interesting.
>
> I had been thinking about inverting this API by having the shared code
> return the two pointers which imply equality/inequality and having
> instcombine rewrite the condition.  However, I couldn't find a case
> where the existing logic didn't already do that inversion through some
> other (more general) path.  Do you happen to have an IR fragment where
> we trip this example?  I'd like to take a look at it with that lens.
>
> Philip
>
> On 4/14/21 6:12 AM, Sanjay Patel via llvm-commits wrote:
> > Author: Sanjay Patel
> > Date: 2021-04-14T09:04:15-04:00
> > New Revision: 7ef2c68a3d24af0b0d540e748e8b564180f4e18a
> >
> > URL:
> https://github.com/llvm/llvm-project/commit/7ef2c68a3d24af0b0d540e748e8b564180f4e18a
> > DIFF:
> https://github.com/llvm/llvm-project/commit/7ef2c68a3d24af0b0d540e748e8b564180f4e18a.diff
> >
> > LOG: [InstSimplify] improve efficiency for detecting non-zero value
> >
> > Stepping through callstacks in the example from D99759 reveals
> > this potential compile-time improvement.
> >
> > The savings come from avoiding ValueTracking's computing known
> > bits if we have already dealt with special-case patterns.
> >
> > Further improvements in this direction seem possible.
> >
> > This makes a degenerate test based on PR49785 about 40x faster
> > (25 sec -> 0.6 sec), but it does not address the larger question
> > of how to limit computeKnownBitsFromAssume(). Ie, the original
> > test there is still infinite-time for all practical purposes.
> >
> > Differential Revision: https://reviews.llvm.org/D100408
> >
> > Added:
> >
> >
> > Modified:
> >      llvm/lib/Analysis/InstructionSimplify.cpp
> >
> > Removed:
> >
> >
> >
> >
> ################################################################################
> > diff  --git a/llvm/lib/Analysis/InstructionSimplify.cpp
> b/llvm/lib/Analysis/InstructionSimplify.cpp
> > index b233a0f3eb2d..08f504a0ce37 100644
> > --- a/llvm/lib/Analysis/InstructionSimplify.cpp
> > +++ b/llvm/lib/Analysis/InstructionSimplify.cpp
> > @@ -3432,6 +3432,8 @@ static Value *SimplifyICmpInst(unsigned Predicate,
> Value *LHS, Value *RHS,
> >     if (Value *V = simplifyICmpOfBools(Pred, LHS, RHS, Q))
> >       return V;
> >
> > +  // TODO: Sink/common this with other potentially expensive calls that
> use
> > +  //       ValueTracking? See comment below for isKnownNonEqual().
> >     if (Value *V = simplifyICmpWithZero(Pred, LHS, RHS, Q))
> >       return V;
> >
> > @@ -3637,7 +3639,9 @@ static Value *SimplifyICmpInst(unsigned Predicate,
> Value *LHS, Value *RHS,
> >     }
> >
> >     // icmp eq|ne X, Y -> false|true if X != Y
> > -  if (ICmpInst::isEquality(Pred) &&
> > +  // This is potentially expensive, and we have already
> computedKnownBits for
> > +  // compares with 0 above here, so only try this for a non-zero
> compare.
> > +  if (ICmpInst::isEquality(Pred) && !match(RHS, m_Zero()) &&
> >         isKnownNonEqual(LHS, RHS, Q.DL, Q.AC, Q.CxtI, Q.DT,
> Q.IIQ.UseInstrInfo)) {
> >       return Pred == ICmpInst::ICMP_NE ? getTrue(ITy) : getFalse(ITy);
> >     }
> >
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
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