From: Segher Boessenkool <segher@kernel.crashing.org>
To: Jeff Law <law@redhat.com>
Cc: Jakub Jelinek <jakub@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: Tue, 13 Jan 2015 22:42:00 -0000 [thread overview]
Message-ID: <20150113222831.GA31575@gate.crashing.org> (raw)
In-Reply-To: <54B575D7.8030107@redhat.com>
On Tue, Jan 13, 2015 at 12:45:27PM -0700, Jeff Law wrote:
> On 01/13/15 09:38, Segher Boessenkool wrote:
> >On Tue, Jan 13, 2015 at 05:18:19PM +0100, Jakub Jelinek wrote:
> >>3) on request from Richard (which Segher on IRC argues against), "memory"
> >> clobber also prevents CSE;
> >
> >As extend.texi used to say:
> >
> >"
> >If your assembler instructions access memory in an unpredictable
> >fashion, add @samp{memory} to the list of clobbered registers. This
> >causes GCC to not keep memory values cached in registers across the
> >assembler instruction and not optimize stores or loads to that memory.
> >You also should add the @code{volatile} keyword if the memory
> >affected is not listed in the inputs or outputs of the @code{asm}, as
> >the @samp{memory} clobber does not count as a side-effect of the
> >@code{asm}.
> >"
> >
> >so a "memory" clobber in a non-volatile asm should not prevent CSE.
> My reading of that paragraph is somewhat different.
It seems so.
I read that as "GCC can delete a memory clobber if it wants to" (just
like it can delete any other clobber when it wants to).
The only difference between ASM_OPERANDS and any other RTL is that
recog is useless for ASM_OPERANDS, it cannot tell you if after you
modify the construct you are left with something valid; so the only
thing the compiler can change about an asm is to delete it whole.
So unlike most RTL, where the compiler is free to remove a clobber
if what is left is valid RTL, the only way to delete a clobber from
an asm is to delete the whole asm.
> The key here is the memory clobber affects optimization of instructions
> around the asm while the volatile specifier affects the optimization of
> the ASM itself.
Those are roughly the effects, yes. Writing unspecified stuff to
unspecified memory is a pretty heavy hammer ;-)
> A memory clobber must inhibit CSE of memory references on either side of
> the asm because the asm must be assumed to read or write memory in
> unpredictable ways.
I don't see how that follows. The asm itself can be CSEd; its clobber
then disappears in a puff of smoke.
> The volatile specifier tells the compiler that the asm itself must be
> preserved, even if dataflow shows the outputs as not used.
Yes.
Segher
WARNING: multiple messages have this Message-ID
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Jeff Law <law@redhat.com>
Cc: Jakub Jelinek <jakub@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: Wed, 14 Jan 2015 00:40:00 -0000 [thread overview]
Message-ID: <20150113222831.GA31575@gate.crashing.org> (raw)
Message-ID: <20150114004000.iTIn50mwEZJERfETDSXDjKVU9CugHvtqK-HgQwxF89s@z> (raw)
In-Reply-To: <54B575D7.8030107@redhat.com>
On Tue, Jan 13, 2015 at 12:45:27PM -0700, Jeff Law wrote:
> On 01/13/15 09:38, Segher Boessenkool wrote:
> >On Tue, Jan 13, 2015 at 05:18:19PM +0100, Jakub Jelinek wrote:
> >>3) on request from Richard (which Segher on IRC argues against), "memory"
> >> clobber also prevents CSE;
> >
> >As extend.texi used to say:
> >
> >"
> >If your assembler instructions access memory in an unpredictable
> >fashion, add @samp{memory} to the list of clobbered registers. This
> >causes GCC to not keep memory values cached in registers across the
> >assembler instruction and not optimize stores or loads to that memory.
> >You also should add the @code{volatile} keyword if the memory
> >affected is not listed in the inputs or outputs of the @code{asm}, as
> >the @samp{memory} clobber does not count as a side-effect of the
> >@code{asm}.
> >"
> >
> >so a "memory" clobber in a non-volatile asm should not prevent CSE.
> My reading of that paragraph is somewhat different.
It seems so.
I read that as "GCC can delete a memory clobber if it wants to" (just
like it can delete any other clobber when it wants to).
The only difference between ASM_OPERANDS and any other RTL is that
recog is useless for ASM_OPERANDS, it cannot tell you if after you
modify the construct you are left with something valid; so the only
thing the compiler can change about an asm is to delete it whole.
So unlike most RTL, where the compiler is free to remove a clobber
if what is left is valid RTL, the only way to delete a clobber from
an asm is to delete the whole asm.
> The key here is the memory clobber affects optimization of instructions
> around the asm while the volatile specifier affects the optimization of
> the ASM itself.
Those are roughly the effects, yes. Writing unspecified stuff to
unspecified memory is a pretty heavy hammer ;-)
> A memory clobber must inhibit CSE of memory references on either side of
> the asm because the asm must be assumed to read or write memory in
> unpredictable ways.
I don't see how that follows. The asm itself can be CSEd; its clobber
then disappears in a puff of smoke.
> The volatile specifier tells the compiler that the asm itself must be
> preserved, even if dataflow shows the outputs as not used.
Yes.
Segher
next prev parent reply other threads:[~2015-01-13 22:28 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
2015-01-13 22:42 ` Segher Boessenkool [this message]
2015-01-14 0:40 ` Segher Boessenkool
2015-01-14 7:12 ` Jeff Law
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