llvm dev - Aug 2016 - X86ISelLowering: Promote 'add nsw' to a wider type

Hi Sanjay,

Some time ago you implemented a sext(add_nsw(x, C)) --> add(sext(x), C_sext)
transformation in X86ISelLowering
https://reviews.llvm.org/D13757
Is there any reason why this transformation is limited to sexts and doesn’t
support zexts?

Thanks,
Artur
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Hi Artur -

I don't think there's any reason to limit the transform to sexts only;
that's just the case that was apparent in
https://llvm.org/bugs/show_bug.cgi?id=20134 , so I limited it to that
pattern.

It's probably worth noting that I'm currently fighting through casts of
all
kinds in IR (InstCombine) rather than the backend:
https://reviews.llvm.org/D22421
https://reviews.llvm.org/D22271
https://reviews.llvm.org/D22477
https://reviews.llvm.org/D20774
https://llvm.org/bugs/show_bug.cgi?id=27925

I'm very interested to see what kinds of patterns you're seeing and want
to
optimize. It's possible that IR transforms could eliminate the need for the
backend fixups...or it could make them harder. Are there bug reports for
the cases that you are looking at?


On Tue, Jul 19, 2016 at 9:13 AM, Artur Pilipenko <apilipenko at
azulsystems.com
> wrote:
> Hi Sanjay,
>
> Some time ago you implemented a sext(add_nsw(x, C)) --> add(sext(x),
> C_sext) transformation in X86ISelLowering
> https://reviews.llvm.org/D13757
> Is there any reason why this transformation is limited to sexts and
> doesn’t support zexts?
>
> Thanks,
> Artur
>
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I’m looking a a case very similar to the one in the bug you mentioned. The only
difference is that we don’t rely on LLVM index i64 canonicalization (done by
sign extension) but instead our front end does it using zero extension because
it knows that in our case indices can’t be negative. So, the resulting IR is
virtually the same except sexts are replaced by zexts.

I haven’t yet filed any bugs for that.

Artur

On 19 Jul 2016, at 18:54, Sanjay Patel <spatel at
rotateright.com<mailto:spatel at rotateright.com>> wrote:

Hi Artur -

I don't think there's any reason to limit the transform to sexts only;
that's just the case that was apparent in
https://llvm.org/bugs/show_bug.cgi?id=20134 , so I limited it to that pattern.

It's probably worth noting that I'm currently fighting through casts of
all kinds in IR (InstCombine) rather than the backend:
https://reviews.llvm.org/D22421
https://reviews.llvm.org/D22271
https://reviews.llvm.org/D22477
https://reviews.llvm.org/D20774
https://llvm.org/bugs/show_bug.cgi?id=27925

I'm very interested to see what kinds of patterns you're seeing and want
to optimize. It's possible that IR transforms could eliminate the need for
the backend fixups...or it could make them harder. Are there bug reports for the
cases that you are looking at?


On Tue, Jul 19, 2016 at 9:13 AM, Artur Pilipenko <apilipenko at
azulsystems.com<mailto:apilipenko at azulsystems.com>> wrote:
Hi Sanjay,

Some time ago you implemented a sext(add_nsw(x, C)) --> add(sext(x), C_sext)
transformation in X86ISelLowering
https://reviews.llvm.org/D13757
Is there any reason why this transformation is limited to sexts and doesn’t
support zexts?

Thanks,
Artur


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Hi Sanjay,
On 19 Jul 2016, at 18:54, Sanjay Patel <spatel at
rotateright.com<mailto:spatel at rotateright.com>> wrote:

Hi Artur -

I don't think there's any reason to limit the transform to sexts only;
that's just the case that was apparent in
https://llvm.org/bugs/show_bug.cgi?id=20134 , so I limited it to that pattern.

It's probably worth noting that I'm currently fighting through casts of
all kinds in IR (InstCombine) rather than the backend:
What is the current status of this work? Does it make sense to patch the
existing code in X86ISelLowering in order to support looking through zext or the
generic solution will be available soon?

Artur
https://reviews.llvm.org/D22421
https://reviews.llvm.org/D22271
https://reviews.llvm.org/D22477
https://reviews.llvm.org/D20774
https://llvm.org/bugs/show_bug.cgi?id=27925

I'm very interested to see what kinds of patterns you're seeing and want
to optimize. It's possible that IR transforms could eliminate the need for
the backend fixups...or it could make them harder. Are there bug reports for the
cases that you are looking at?


On Tue, Jul 19, 2016 at 9:13 AM, Artur Pilipenko <apilipenko at
azulsystems.com<mailto:apilipenko at azulsystems.com>> wrote:
Hi Sanjay,

Some time ago you implemented a sext(add_nsw(x, C)) --> add(sext(x), C_sext)
transformation in X86ISelLowering
https://reviews.llvm.org/D13757
Is there any reason why this transformation is limited to sexts and doesn’t
support zexts?

Thanks,
Artur


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