[llvm-dev] Disable combining of loads and stores in instcombine

[Amara Emerson via llvm-dev]

> On Apr 17, 2019, at 9:35 AM, JF Bastien via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
>> 
>> On Apr 17, 2019, at 5:02 AM, Arsenault, Matthew via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> This won’t happen with volatile load/store
> 
> This is mostly true today, but AFAICT the LLVM memory model doesn’t actually offer this guarantee. It merely says that LLVM treats volatile like C / C++ treats volatile… which isn’t much of a guarantee because C / C++ volatile doesn’t normatively mean anything. Specifically, we cannot really honor this when volatile bitfields are used for which memory operations don’t exist:
> 
> struct {
>   volatile int a : 12;
>   volatile int b : 4;
> } s;
> 
> As things stand, we haven’t promised that we won’t combine adjacent volatile stores, and C / C++ certainly allow us to do so. I don’t think it would be a good idea to do so, but we certainly could.
Is this really true? The writes to two volatile variables are sequenced and can’t be re-ordered since they can have side effects, and so merging adjacent volatile stores would break this.
> 
> 
>>  -Matt 
>>  
>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org <mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
>> Reply-To: Neil Ryan <neilryan at cs.washington.edu <mailto:neilryan at cs.washington.edu>>
>> Date: Tuesday, April 16, 2019 at 9:01 PM
>> To: llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
>> Subject: [llvm-dev] Disable combining of loads and stores in instcombine
>>  
>>  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?          
>> 
>> Neil
>> _______________________________________________
>> 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 <https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> _______________________________________________
> 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 <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/20190417/7fcc1068/attachment.html>