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

[Sanjay Patel via llvm-commits]

Ah - the degenerate case was based on:
https://bugs.llvm.org/show_bug.cgi?id=49785#c2

But I just reverted this locally:
https://reviews.llvm.org/rGbb907b26e2bf2a0d5ea3fcb13ba0ccef3420a38c

And neither the original C example from PR49785 nor the IR reduction are
taking forever to compile for me. Something else must have changed in the
past 2 weeks to mask the problem. So I'm not sure how to trigger this again.

On Wed, Apr 28, 2021 at 5:19 PM Philip Reames <listmail at philipreames.com>
wrote:

> Sanjay,
>
> Neither of these seem to be the degenerate test case you saw the compile
> time impact on.  Could you point me to that?  I'm looking for something to
> run to understand the compile time opportunities in this code.
>
> Philip
> On 4/15/21 6:48 AM, Sanjay Patel wrote:
>
> 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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