----- Original Message -----
From: "Davide Libenzi" <davidel@xmailserver.org>
To: "Matthew Wilcox" <willy@debian.org>
Cc: "Patrick Mochel" <mochel@osdl.org>; <linux-kernel@vger.kernel.org>
Sent: Sunday, July 06, 2003 6:42 PM
Subject: Re: kobjects, sysfs and the driver model make my head hurt
> On Sun, 6 Jul 2003, Matthew Wilcox wrote:
>
> > Why on earth does it return the value of its argument?
>
> Maybe for the same reason 'strcpy' returns 'dest'. It allows you to use
> the function in a function parameter :
Another possible benefit (although I'm not sure we should care)
is that if the return variable is the same as the first argument,
the compiler can save an instruction or two on at least some archs.
Simple example:
char *tst(char *p, int i)
{
return p;
}
void tst2(char *p, int i)
{
*p = i;
p = tst(p,i);
p[1]=i;
}
void tst3(char *p, int i)
{
*p = i;
tst(p,i);
p[1]=i;
}
i386 ts2 saves 3 instructions compared to tst3
tst2:
pushl %ebp
movl %esp,%ebp
subl $20,%esp
pushl %ebx
movl 8(%ebp),%eax
movl 12(%ebp),%ebx
movb %bl,(%eax)
addl $-8,%esp
pushl %ebx
pushl %eax
call tst
movb %bl,1(%eax)
movl -24(%ebp),%ebx
leave
ret
.Lfe2:
.size tst2,.Lfe2-tst2
.align 4
.globl tst3
.type tst3,@function
tst3:
pushl %ebp
movl %esp,%ebp
subl $16,%esp
pushl %esi
pushl %ebx
movl 8(%ebp),%esi
movl 12(%ebp),%ebx
movb %bl,(%esi)
addl $-8,%esp
pushl %ebx
pushl %esi
call tst
movb %bl,1(%esi)
leal -24(%ebp),%esp
popl %ebx
popl %esi
leave
ret
.Lfe3:
On CRIS you save one register on stack instead of two
tst2:
Push $srp
subq 4,$sp
movem $r0,[$sp]
move.d $r10,$r9
move.d $r11,$r0
move.b $r11,[$r9]
Jsr tst
move.b $r0,[$r10+1]
movem [$sp+],$r0
Jump [$sp+]
.Lfe2:
.size tst2,.Lfe2-tst2
.align 1
.global tst3
.type tst3,@function
tst3:
Push $srp
subq 8,$sp
movem $r1,[$sp]
move.d $r10,$r0
move.d $r11,$r1
move.b $r11,[$r0+]
Jsr tst
move.b $r1,[$r0]
movem [$sp+],$r1
Jump [$sp+]
.Lfe3:
/Johan
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This archive was generated by hypermail 2b29 : Mon Jul 07 2003 - 22:00:28 EST