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Re: [RFC] Add asm constraint modifier to mark strict memory accesses

From: Jeff Law <law at redhat dot com>
To: Yury Gribov <y dot gribov at samsung dot com>, Jakub Jelinek <jakub at redhat dot com>, Richard Henderson <rth at redhat dot com>
Cc: gcc <gcc at gcc dot gnu dot org>, Konstantin Serebryany <konstantin dot s dot serebryany at gmail dot com>, Dmitry Vyukov <dvyukov at google dot com>, Marat Zakirov <m dot zakirov at samsung dot com>, Andrey Ryabinin <a dot ryabinin at samsung dot com>, Konstantin Khlebnikov <k dot khlebnikov at samsung dot com>
Date: Thu, 18 Sep 2014 12:15:43 -0600
Subject: Re: [RFC] Add asm constraint modifier to mark strict memory accesses
Authentication-results: sourceware.org; auth=none
References: <541ABD6E dot 3030904 at samsung dot com> <20140918111617 dot GF17454 at tucnak dot redhat dot com> <541ABFCB dot 7050107 at samsung dot com> <541ADFD8 dot 2050306 at redhat dot com> <541AEE4C dot 2050100 at samsung dot com>

On 09/18/14 08:38, Yury Gribov wrote:

On 09/18/2014 05:36 PM, Jeff Law wrote:

On 09/18/14 05:19, Yury Gribov wrote:


Would that modifier mean that the inline asm is unconditionally reading
resp. writing that memory? "m"/"=m" right now is always about might
read or might write, not must.


Yes, that's what I had in mind. Many inline asms (at least in kernel) do
read memory region unconditionally.

That's precisely what I'd expect such a modifier to mean.  Right now
memory modifiers are strictly "may" but I can see a use case for "must".

I think the question is will the kernel or glibc folks use that new
capability and if so, do we get a significant improvement in the amount
of checking we can do.    So I think both those groups need to be looped
into this conversation.


Right. Should I x-post or better send separate emails and then report
feedback on GCC list?

I think cross posting is fine. Most of us don't necessarily watch thekernel or glibc lists -- and in this case I think those cross listdiscussions could be extremely valuable.


Jeff

References:
- [RFC] Add asm constraint modifier to mark strict memory accesses
  - From: Yury Gribov
- Re: [RFC] Add asm constraint modifier to mark strict memory accesses
  - From: Jakub Jelinek
- Re: [RFC] Add asm constraint modifier to mark strict memory accesses
  - From: Yury Gribov
- Re: [RFC] Add asm constraint modifier to mark strict memory accesses
  - From: Jeff Law
- Re: [RFC] Add asm constraint modifier to mark strict memory accesses
  - From: Yury Gribov

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