From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:60962)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1ZKYBp-0006SL-IM
	for qemu-devel@nongnu.org; Wed, 29 Jul 2015 16:43:05 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1ZKYBg-0004Bu-8q
	for qemu-devel@nongnu.org; Wed, 29 Jul 2015 16:43:01 -0400
Received: from mail-wi0-f179.google.com ([209.85.212.179]:36809)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1ZKYBg-0004B2-3e
	for qemu-devel@nongnu.org; Wed, 29 Jul 2015 16:42:52 -0400
Received: by wicgb10 with SMTP id gb10so216844048wic.1
	for <qemu-devel@nongnu.org>; Wed, 29 Jul 2015 13:42:51 -0700 (PDT)
References: <1437755447-10537-1-git-send-email-aurelien@aurel32.net>
	<1437755447-10537-5-git-send-email-aurelien@aurel32.net>
	<87k2tjlz7a.fsf@linaro.org> <20150729162757.GB1655@aurel32.net>
From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
In-reply-to: <20150729162757.GB1655@aurel32.net>
Date: Wed, 29 Jul 2015 21:42:48 +0100
Message-ID: <87io92n19z.fsf@linaro.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Subject: Re: [Qemu-devel] [PATCH for-2.5 04/10] tcg/optimize: allow constant
	to have copies
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Aurelien Jarno <aurelien@aurel32.net>
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>


Aurelien Jarno <aurelien@aurel32.net> writes:

> On 2015-07-29 17:12, Alex Bennée wrote:
>> 
>> Aurelien Jarno <aurelien@aurel32.net> writes:
>> 
>> > Now that copies and constants are tracked separately, we can allow
>> > constant to have copies, deferring the choice to use a register or a
>> > constant to the register allocation pass. This prevent this kind of
>> > regular constant reloading:
>> >
>> > -OUT: [size=338]
>> > +OUT: [size=298]
>> >    mov    -0x4(%r14),%ebp
>> >    test   %ebp,%ebp
>> >    jne    0x7ffbe9cb0ed6
>> >    mov    $0x40002219f8,%rbp
>> >    mov    %rbp,(%r14)
>> > -  mov    $0x40002219f8,%rbp
>> >    mov    $0x4000221a20,%rbx
>> >    mov    %rbp,(%rbx)
>> >    mov    $0x4000000000,%rbp
>> >    mov    %rbp,(%r14)
<snip>
>> > --- a/tcg/optimize.c
>> > +++ b/tcg/optimize.c
>> > @@ -237,11 +237,6 @@ static void tcg_opt_gen_mov(TCGContext *s, TCGOp *op, TCGArg *args,
>> >          return;
>> >      }
>> >  
>> > -    if (temp_is_const(src)) {
>> > -        tcg_opt_gen_movi(s, op, args, dst, temps[src].val);
>> > -        return;
>> > -    }
>> > -
>> 
>> That looks suspicious, surely we only want to drop the move if we
>> already have the const somewhere else?
>
> That's actually the while point of this patchset, to avoid converting
> mov into moving for constant values and thus loosing the link between
> temps.

I get that, I guess I didn't follow the previous loading of the constant
value. Maybe showing the ops in the commit message will make it clearer.

>
> At this moment point of the code, the only way to know that the source
> temp is a constant is when it has been set using a movi before.

OK.


-- 
Alex Bennée