From: Richard Sandiford <rdsandiford@googlemail.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Jakub Jelinek <jakub@redhat.com>,
Richard Henderson <rth@redhat.com>,
Richard Biener <richard.guenther@gmail.com>,
Jeff Law <law@redhat.com>, Richard Biener <rguenther@suse.de>,
Eric Botcazou <ebotcazou@adacore.com>,
gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Reenable CSE of non-volatile inline asm (PR rtl-optimization/63637)
Date: Sat, 24 Jan 2015 14:39:00 -0000 [thread overview]
Message-ID: <87lhkse6o9.fsf@googlemail.com> (raw)
In-Reply-To: <20150124011858.GA31255@gate.crashing.org> (Segher Boessenkool's message of "Fri, 23 Jan 2015 19:18:58 -0600")
Segher Boessenkool <segher@kernel.crashing.org> writes:
> On Fri, Jan 23, 2015 at 10:48:50PM +0100, Jakub Jelinek wrote:
>> On Fri, Jan 23, 2015 at 03:39:40PM -0600, Segher Boessenkool wrote:
>> > I understand that argument. But it is not what GCC actually does, nor
>> > what I think it should do. Consider this program:
>> >
>> > --- 8< ---
>> > int main(void)
>> > {
>> > int x[100], y[100];
>> >
>> > x[31] = 42;
>> >
>> > asm("# eww %0" : "=m"(y[4]) : : "memory");
>> >
>> > return 0;
>> > }
>> > --- 8< ---
>>
>> Here x isn't addressable, so it is certainly fine to DSE it.
>> x shouldn't be considered memory.
>> If the address of x escaped, either to the assembly or to some global var
>> etc., then it probably shouldn't be removed.
>
> But GCC does consider it memory. If you look at the (tree) dump files
> you see both arrays are clobbered after the asm. Tree DCE removes the
> store to x[31] nevertheless.
>
> If the address of x escapes then of course the store to x[31] should
> not be removed, irrespective of whether the clobber implies a read
> or not.
Just tried some other examples out of curiosity. In:
int main(void)
{
int x[100], y[100];
asm volatile("# foo" :: "r"(x));
x[31] = 42;
asm("# eww %0" : "=m"(y[4]) : : "memory");
return 0;
}
"x[31]" can only validly escape to the second asm. In this case the
assignment is kept, as it is with:
int main(void)
{
int x[100], y;
asm volatile("# foo" :: "r"(x));
x[31] = 42;
asm("# eww %0" : "=r"(y) : : "memory");
return y;
}
But remove the clobber and it goes away:
int main(void)
{
int x[100], y;
asm volatile("# foo" :: "r"(x));
x[31] = 42;
asm("# eww %0" : "=r"(y));
return y;
}
So it looks like these four cases (including yours) are handled correctly.
Thanks,
Richard
next prev parent reply other threads:[~2015-01-24 11:53 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-13 16:22 Jakub Jelinek
2015-01-13 17:06 ` Segher Boessenkool
2015-01-13 20:02 ` Jeff Law
2015-01-13 20:29 ` Jakub Jelinek
2015-01-13 22:28 ` Jeff Law
2015-01-14 3:44 ` Segher Boessenkool
2015-01-14 6:52 ` Jeff Law
2015-01-14 15:40 ` Segher Boessenkool
2015-01-15 6:46 ` Jeff Law
2015-01-15 7:54 ` Richard Biener
2015-01-15 8:40 ` Jakub Jelinek
2015-01-15 8:43 ` Richard Biener
2015-01-15 9:50 ` Jakub Jelinek
2015-01-15 18:22 ` Jeff Law
2015-01-23 21:39 ` Richard Henderson
2015-01-23 22:53 ` Segher Boessenkool
2015-01-23 23:12 ` Jakub Jelinek
2015-01-24 7:23 ` Segher Boessenkool
2015-01-24 14:39 ` Richard Sandiford [this message]
2015-01-13 22:42 ` Segher Boessenkool
2015-01-14 0:40 ` Segher Boessenkool
2015-01-14 7:12 ` Jeff Law
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