From: Andrew Haley <aph@redhat.com>
To: Cody Rigney <codyrigney92@gmail.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Compiler optimizing variables in inline assembly
Date: Fri, 21 Feb 2014 15:02:00 -0000 [thread overview]
Message-ID: <53076A99.1020709@redhat.com> (raw)
In-Reply-To: <CA+1=iYbAnZGdaVoFVuJnm+oQSCiHPTUOBmy8f7RycSAvu92i+w@mail.gmail.com>
On 02/21/2014 02:06 PM, Cody Rigney wrote:
> On Fri, Feb 21, 2014 at 4:53 AM, Andrew Haley <aph@redhat.com> wrote:
>>
>> You can use a memory operand as an output, as in "=m"(*a) or simply
>> add "memory" to the clobber list. And you must add all clobbered
>> registers to the clobber list. Then it should work.
>>
> I added all the registers and "memory" to clobber and it worked!
> Since I think it's more efficient to specify which memory is changing,
> I'm going to give the "=m" (*a) a shot. So even though you
> dereference the pointer, gcc will know the length of the memory(say 2
> bytes) that changes? Or do you have to specify each index of the
> memory(e.g. char a[2]; ..... asm ... "=m" (*a), "=m" (*(a+1)))?
It's hard for me to give a 100% answer to that one, but GCC has an
idea what memory is reachable from every pointer. So, this won't
clobber memory that's unreachable or has a different type from that
pointer. It probably doesn't matter.
Andrew.
P.S. Please don't top-post on GCC lists. :-)
next prev parent reply other threads:[~2014-02-21 15:02 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-19 19:05 Cody Rigney
2014-02-20 9:14 ` Andrew Haley
2014-02-20 19:30 ` Cody Rigney
2014-02-21 9:53 ` Andrew Haley
2014-02-21 14:06 ` Cody Rigney
2014-02-21 15:02 ` Andrew Haley [this message]
2014-02-21 15:20 ` Cody Rigney
2014-02-27 13:18 ` Cody Rigney
2014-02-27 14:03 ` Andrew Haley
2014-02-27 18:34 ` Cody Rigney
2014-02-21 9:54 ` David Brown
2014-02-21 9:55 ` David Brown
2014-02-20 9:54 ` David Brown
2014-02-20 19:39 ` Cody Rigney
2014-02-21 10:15 ` David Brown
2014-02-21 14:11 ` Cody Rigney
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