llvm dev - Apr 2019 - Disable combining of loads and stores in instcombine

I’m writing a pass for some custom hardware — we’d like to split arrays across
hardware elements; this doesn’t work if consecutive writes to characters get
combined to a word.
On Apr 16, 2019, 8:17 PM -0700, Tom Stellard <tstellar at redhat.com>,
wrote:
> On 04/16/2019 11:38 AM, Neil Ryan via llvm-dev wrote:
> > LLVM's optimizer combines stores to consecutive characters into a
write of a single word. For instance, if I have char A[4] and I write some
static value to each element, these writes would be combined into a single
32-bit word write. I found this thread
<http://llvm.1065342.n5.nabble.com/disabling-combining-load-stores-in-optimizer-td37560.html>
from 2009 -- it seems like it wasn't possible then. Has anything changed
since?
> >
>
> Why do you want to disable this optimization?
>
> -Tom
>
>
> > Neil
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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On 04/16/2019 09:11 PM, Neil Ryan wrote:
> I’m writing a pass for some custom hardware — we’d like to split arrays
across hardware elements; this doesn’t work if consecutive writes to characters
get combined to a word.
Do you have an LLVM IR example where this happens when you don't want
it to?

-Tom
> On Apr 16, 2019, 8:17 PM -0700, Tom Stellard <tstellar at
redhat.com>, wrote:
>> On 04/16/2019 11:38 AM, Neil Ryan via llvm-dev wrote:
>>> LLVM's optimizer combines stores to consecutive characters into
a write of a single word. For instance, if I have char A[4] and I write some
static value to each element, these writes would be combined into a single
32-bit word write. I found this thread
<http://llvm.1065342.n5.nabble.com/disabling-combining-load-stores-in-optimizer-td37560.html>
from 2009 -- it seems like it wasn't possible then. Has anything changed
since?
>>>
>>
>> Why do you want to disable this optimization?
>>
>> -Tom
>>
>>
>>> Neil
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>

This is really a codegen problem. You can decompose the load/store however you
like in the backend. InstCombine should still combine the loads as a
canonicalization.

-Matt

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of llvm-dev
<llvm-dev at lists.llvm.org>
Reply-To: Neil Ryan <neilryan at cs.washington.edu>
Date: Wednesday, April 17, 2019 at 9:28 PM
To: llvm-dev <llvm-dev at lists.llvm.org>, "tstellar at
redhat.com" <tstellar at redhat.com>
Subject: Re: [llvm-dev] Disable combining of loads and stores in instcombine

I’m writing a pass for some custom hardware — we’d like to split arrays across
hardware elements; this doesn’t work if consecutive writes to characters get
combined to a word.
On Apr 16, 2019, 8:17 PM -0700, Tom Stellard <tstellar at redhat.com>,
wrote:

On 04/16/2019 11:38 AM, Neil Ryan via llvm-dev wrote:

LLVM's optimizer combines stores to consecutive characters into a write of a
single word. For instance, if I have char A[4] and I write some static value to
each element, these writes would be combined into a single 32-bit word write. I
found this thread
<http://llvm.1065342.n5.nabble.com/disabling-combining-load-stores-in-optimizer-td37560.html>
from 2009 -- it seems like it wasn't possible then. Has anything changed
since?

Why do you want to disable this optimization?

-Tom



Neil


_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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IIRC it’s not strictly possible to determine what array a load/store is based
on. I don’t believe the decomposition is always possible, as information is lost
when accesses are combined.

Neil
On Apr 17, 2019, 12:31 PM -0700, Arsenault, Matthew <Matthew.Arsenault at
amd.com>, wrote:
> This is really a codegen problem. You can decompose the load/store however
you like in the backend. InstCombine should still combine the loads as a
canonicalization.
>
> -Matt
>
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
llvm-dev <llvm-dev at lists.llvm.org>
> Reply-To: Neil Ryan <neilryan at cs.washington.edu>
> Date: Wednesday, April 17, 2019 at 9:28 PM
> To: llvm-dev <llvm-dev at lists.llvm.org>, "tstellar at
redhat.com" <tstellar at redhat.com>
> Subject: Re: [llvm-dev] Disable combining of loads and stores in
instcombine
>
> I’m writing a pass for some custom hardware — we’d like to split arrays
across hardware elements; this doesn’t work if consecutive writes to characters
get combined to a word.
> On Apr 16, 2019, 8:17 PM -0700, Tom Stellard <tstellar at
redhat.com>, wrote:
>
> > On 04/16/2019 11:38 AM, Neil Ryan via llvm-dev wrote:
> >
> > > LLVM's optimizer combines stores to consecutive characters
into a write of a single word. For instance, if I have char A[4] and I write
some static value to each element, these writes would be combined into a single
32-bit word write. I found this thread
<http://llvm.1065342.n5.nabble.com/disabling-combining-load-stores-in-optimizer-td37560.html>
from 2009 -- it seems like it wasn't possible then. Has anything changed
since?
> >
> > Why do you want to disable this optimization?
> >
> > -Tom
> >
> >
> >
> > > Neil
> > >
> > >
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > llvm-dev at lists.llvm.org
> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
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