linux-kernel - Re: [discuss] [PATCH] x86-64: memset optimization

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Date:	Mon, 20 Aug 2007 10:03:46 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, discuss@...-64.org,
	linux-kernel@...r.kernel.org, jh@...e.cz
Subject: Re: [discuss] [PATCH] x86-64: memset optimization

 > > The problem is with the optimization flags: passing -Os causes the compiler
 > > to be stupid and not inline any memset/memcpy functions.
 > 
 > you get what you ask for.. if you don't want that then don't ask for
 > it ;)

Well, the compiler is really being dumb about -Os and in fact it's
giving bigger code, so I'm not really getting what I ask for.  

With my gcc at least (x86_64, gcc (GCC) 4.1.3 20070812 (prerelease)
(Ubuntu 4.1.2-15ubuntu2)) and Andi's example:

#include <string.h>

f(char x[6]) {
        memset(x, 1, 6);
}

compiling with -O2 gives

0000000000000000 <f>:
   0:	c7 07 01 01 01 01    	movl   $0x1010101,(%rdi)
   6:	66 c7 47 04 01 01    	movw   $0x101,0x4(%rdi)
   c:	c3                   	retq   

and compiling with -Os gives

0000000000000000 <f>:
   0:	48 83 ec 08          	sub    $0x8,%rsp
   4:	ba 06 00 00 00       	mov    $0x6,%edx
   9:	be 01 00 00 00       	mov    $0x1,%esi
   e:	e8 00 00 00 00       	callq  13 <f+0x13>
  13:	5a                   	pop    %rdx
  14:	c3                   	retq   

so the code gets bigger and worse in every way.

 - R.
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