This is the mail archive of the
mailing list for the GCC project.
Re: [RFC] Add asm constraint modifier to mark strict memory accesses
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Jeff Law <law at redhat dot com>,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 16:32:40 +0200
- 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>
On September 18, 2014 3:36:24 PM CEST, Jeff Law <firstname.lastname@example.org> wrote:
>On 09/18/14 05:19, Yury Gribov wrote:
>>> Would that modifier mean that the inline asm is unconditionally
>>> 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)
>> 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
>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
>into this conversation.
> From an implementation standpoint, are you thinking a different
>modifier (my first choice)? That wouldn't allow us to say something
>like the first X bytes of this memory region are written and the
>remaining Y bytes may be written, but I suspect that's not a use case
>we're likely to care about.
It would also enable us to do more DSE as the asm stmt is then known to kill a specific part of memory. Maybe we even want to constrain the effective type of the memory accesses so we can do TBAA against inline asms?