From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gdb-return-21131-listarch-gdb=sources.redhat.com@sources.redhat.com>
Received: (qmail 22747 invoked by alias); 4 Apr 2005 19:35:03 -0000
Mailing-List: contact gdb-help@sources.redhat.com; run by ezmlm
Precedence: bulk
List-Subscribe: <mailto:gdb-subscribe@sources.redhat.com>
List-Archive: <http://sources.redhat.com/ml/gdb/>
List-Post: <mailto:gdb@sources.redhat.com>
List-Help: <mailto:gdb-help@sources.redhat.com>, <http://sources.redhat.com/ml/#faqs>
Sender: gdb-owner@sources.redhat.com
Received: (qmail 22671 invoked from network); 4 Apr 2005 19:34:57 -0000
Received: from unknown (HELO legolas.inter.net.il) (192.114.186.24)
  by sourceware.org with SMTP; 4 Apr 2005 19:34:57 -0000
Received: from zaretski (IGLD-80-230-67-97.inter.net.il [80.230.67.97])
	by legolas.inter.net.il (MOS 3.5.6-GR)
	with ESMTP id EBQ08468 (AUTH halo1);
	Mon, 4 Apr 2005 22:34:30 +0300 (IDT)
Date: Mon, 04 Apr 2005 19:35:00 -0000
From: "Eli Zaretskii" <eliz@gnu.org>
To: gdb@sources.redhat.com, Reiner.Steib@gmx.de
Message-ID: <01c5394c$Blat.v2.4$e4580a80@zahav.net.il>
Content-Transfer-Encoding: 7BIT
Content-Type: text/plain; charset=ISO-8859-1
In-reply-to: <20050404133743.GA32163@nevyn.them.org> (message from Daniel
	Jacobowitz on Mon, 4 Apr 2005 09:37:44 -0400)
Subject: Re: Variable "foo" is not available
Reply-to: Eli Zaretskii <eliz@gnu.org>
References: <v9mzsisb72.fsf@marauder.physik.uni-ulm.de> <20050401171947.GA19058@nevyn.them.org> <01c53768$Blat.v2.4$d52008a0@zahav.net.il> <20050402142639.GA27550@nevyn.them.org> <01c537af$Blat.v2.4$c36667c0@zahav.net.il> <20050402184023.GA20247@nevyn.them.org> <01c537c6$Blat.v2.4$427763a0@zahav.net.il> <20050402210541.GA16758@nevyn.them.org> <01c538d4$Blat.v2.4$b261c020@zahav.net.il> <20050404133743.GA32163@nevyn.them.org>
X-SW-Source: 2005-04/txt/msg00034.txt.bz2

> Date: Mon, 4 Apr 2005 09:37:44 -0400
> From: Daniel Jacobowitz <drow@false.org>
> Cc: gdb@sources.redhat.com, Reiner.Steib@gmx.de
> 
> int foo();
> int foo2 (int *);
> int bar(int a)
> {
>   foo ();
>   a += 3;
>   foo2 (&a);
>   return a + foo();
> }
> 
>    0:   55                      push   %ebp
>    1:   89 e5                   mov    %esp,%ebp
>    3:   83 ec 08                sub    $0x8,%esp
>    6:   e8 fc ff ff ff          call   7 <bar+0x7>
>    b:   83 45 08 03             addl   $0x3,0x8(%ebp)
>    f:   8d 45 08                lea    0x8(%ebp),%eax
>   12:   89 04 24                mov    %eax,(%esp)
>   15:   e8 fc ff ff ff          call   16 <bar+0x16>
>   1a:   e8 fc ff ff ff          call   1b <bar+0x1b>
>   1f:   8b 55 08                mov    0x8(%ebp),%edx
>   22:   89 ec                   mov    %ebp,%esp
>   24:   5d                      pop    %ebp
>   25:   01 d0                   add    %edx,%eax
>   27:   c3                      ret
> 
> See the instruction at 0xb?

But this kind of code is only possible if the compiler examines all
the callers of `bar' and finds that none of them uses the value of
`bar's argument after `bar' returns.  So such code is probably only
possible in practice with static functions, right?  Or am I missing
something?

Also, is such optimizations really worth it?  I mean, the more
traditional code will mov the argument into a register and do the math
there; is adding to a memory location really faster than a mov and a
register-based add?

> GCC won't reuse the slot for an unrelated variable at present. 
> However, in the future, it would be a valid optimization.

Again, only if the compiler has enough information about the callers,
right?