David Miller a écrit :
From: Eric Dumazet <dada1@xxxxxxxxxxxxx>
Date: Thu, 19 Apr 2007 16:14:23 +0200
On Wed, 18 Apr 2007 13:04:22 -0700 (PDT)
David Miller <davem@xxxxxxxxxxxxx> wrote:
Although I don't think gcc does anything fancy since we don't
use memcmp(). It's a tradeoff, we'd like to use unsigned long
comparisons when both objects are aligned correctly but we also
don't want it to use any more than one potentially mispredicted
branch.
Again, memcmp() *cannot* be optimized, because its semantic is to compare bytes.
memcpy() can take into account alignement if known at compile time, not memcmp()
http://lists.openwall.net/netdev/2007/03/13/31
I was prehaps thinking about strlen() where I know several
implementations work a word at a time even though it is
a byte-based operation:
--------------------
#define LO_MAGIC 0x01010101
#define HI_MAGIC 0x80808080
...
sethi %hi(HI_MAGIC), %o4
...
or %o4, %lo(HI_MAGIC), %o3
...
sethi %hi(LO_MAGIC), %o4
...
or %o4, %lo(LO_MAGIC), %o2
...
8:
ld [%o0], %o5
2:
sub %o5, %o2, %o4
andcc %o4, %o3, %g0
be,pt %icc, 8b
add %o0, 4, %o0
--------------------
I figured some similar trick could be done with strcmp() and
memcmp().
Hum, I was refering to IA64 (or the more spreaded x86 arches), that is litle
endian AFAIK.
On big endian machines, a compiler can indeed perform some word tricks for
memcmp() if it knows at compile time both pointers are word aligned.
PowerPc example (xlc compiler)
int func(const unsigned int *a, const unsigned int *b)
{
return memcmp(a, b, 6);
}
.func: # 0x00000000 (H.10.NO_SYMBOL)
l r5,0(r3)
l r0,0(r4)
cmp 0,r5,r0
bc BO_IF_NOT,CR0_EQ,__L2c
lhz r3,4(r3)
lhz r0,4(r4)
sf r0,r0,r3
sfze r3,r0
a r0,r3,r0
aze r3,r0
bcr BO_ALWAYS,CR0_LT
__L2c: # 0x0000002c (H.10.NO_SYMBOL+0x2c)
sf r0,r0,r5
sfze r3,r0
a r0,r3,r0
aze r3,r0
bcr BO_ALWAYS,CR0_LT
But to compare 6 bytes, known to be aligned to even addresses, current code is
just fine and portable. We *could* use arch/endian specific tricks to save one
or two cycles, but who really wants that ?
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