llvm dev - May 2020 - RFC: [GlobalISel] propagating int/float type information

> On May 4, 2020, at 2:01 PM, Matt Arsenault via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> 
> 
>> On May 1, 2020, at 14:00, Amara Emerson <aemerson at apple.com
<mailto:aemerson at apple.com>> wrote:
>> 
>> The other thing we could do is to assign speculative regbanks to vregs
during translation (if the target wants to opt-in), and then RBS can finalize
the regbanks, changing some if it deems it necessary/optimal.
> 
> This seems reasonable to me. It wouldn’t require any new infrastructure
How would that work with illegal operations?

Would the legalizer have to preserve them?

Ditto with combines.

What I am saying is although this does not require any new infrastructure, this
is not free adopt.

Cheers,
-Quentin
> 
> -Matt
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200504/e922cafc/attachment.html>

My thinking was no, any regbanks assigned before RBS are only hints and do not
have to be preserved. But for good code it would help if the legalizer preserves
them.

This issue would be there with the other approaches too. Propagating information
through destructive passes like the legalizer/combined is not going to be free
in any case.

Amara
> On May 4, 2020, at 2:53 PM, Quentin Colombet <qcolombet at apple.com>
wrote:
> 
> 
> 
>>> On May 4, 2020, at 2:01 PM, Matt Arsenault via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>>> 
>>> 
>>> 
>>> On May 1, 2020, at 14:00, Amara Emerson <aemerson at
apple.com> wrote:
>>> 
>>> The other thing we could do is to assign speculative regbanks to
vregs during translation (if the target wants to opt-in), and then RBS can
finalize the regbanks, changing some if it deems it necessary/optimal.
>> 
>> This seems reasonable to me. It wouldn’t require any new infrastructure
> 
> How would that work with illegal operations?
> 
> Would the legalizer have to preserve them?
> 
> Ditto with combines.
> 
> What I am saying is although this does not require any new infrastructure,
this is not free adopt.
> 
> Cheers,
> -Quentin
> 
>> 
>> -Matt
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200504/201ed0d2/attachment.html>

Hmm, I feel we would be better at fixing RBS.

I don’t think it would be necessarily expensive compile time wise to do
something more sensible in RBS when running in fast:
We could defer the assignment of a register bank until we see the first real use
(i.e., not a PHI or copy).

As with every heuristics, there are downsides too, but it feels to me that this
is more robust than channeling metadata through a bunch of passes.
> On May 4, 2020, at 3:07 PM, Amara Emerson <aemerson at apple.com>
wrote:
> 
> My thinking was no, any regbanks assigned before RBS are only hints and do
not have to be preserved. But for good code it would help if the legalizer
preserves them.
> 
> This issue would be there with the other approaches too. Propagating
information through destructive passes like the legalizer/combined is not going
to be free in any case.
> 
> Amara
> 
>> On May 4, 2020, at 2:53 PM, Quentin Colombet <qcolombet at
apple.com> wrote:
>> 
>> 
>> 
>>> On May 4, 2020, at 2:01 PM, Matt Arsenault via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>> 
>>> 
>>> 
>>>> On May 1, 2020, at 14:00, Amara Emerson <aemerson at
apple.com <mailto:aemerson at apple.com>> wrote:
>>>> 
>>>> The other thing we could do is to assign speculative regbanks
to vregs during translation (if the target wants to opt-in), and then RBS can
finalize the regbanks, changing some if it deems it necessary/optimal.
>>> 
>>> This seems reasonable to me. It wouldn’t require any new
infrastructure
>> 
>> How would that work with illegal operations?
>> 
>> Would the legalizer have to preserve them?
>> 
>> Ditto with combines.
>> 
>> What I am saying is although this does not require any new
infrastructure, this is not free adopt.
>> 
>> Cheers,
>> -Quentin
>> 
>>> 
>>> -Matt
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> 
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200504/20d76b58/attachment-0001.html>