On Sat, Oct 29, 2005 at 04:10:46PM +0200, Michal Srajer wrote:
> Description: Very small optimization patch for include/linux/etherdevice.h in 2.6.14 kernel.
How is this an optimisation?
typedef unsigned char u8;
static int is_zero_ether_addr1(const u8 *addr)
{
return !(addr[0] | addr[1] | addr[2] | addr[3] | addr[4] | addr[5]);
}
static int is_zero_ether_addr2(const u8 *addr)
{
return !(addr[0] || addr[1] || addr[2] || addr[3] || addr[4] || addr[5]);
}
produces on x86:
is_zero_ether_addr1:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %edx
movb 1(%edx), %al
orb (%edx), %al
orb 2(%edx), %al
orb 3(%edx), %al
orb 4(%edx), %al
orb 5(%edx), %al
sete %al
movzbl %al, %eax
leave
ret
is_zero_ether_addr2:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %edx
xorl %eax, %eax
cmpb $0, (%edx)
jne .L3
cmpb $0, 1(%edx)
jne .L3
cmpb $0, 2(%edx)
jne .L3
cmpb $0, 3(%edx)
jne .L3
cmpb $0, 4(%edx)
jne .L3
cmpb $0, 5(%edx)
jne .L3
movl $1, %eax
.L3:
leave
ret
and on ARM:
is_zero_ether_addr1:
ldrb r1, [r0, #1] @ zero_extendqisi2
ldrb r3, [r0, #0] @ zero_extendqisi2
ldrb r2, [r0, #2] @ zero_extendqisi2
orr r3, r3, r1
ldrb r1, [r0, #3] @ zero_extendqisi2
orr r2, r2, r3
ldrb r3, [r0, #4] @ zero_extendqisi2
orr r1, r1, r2
ldrb r2, [r0, #5] @ zero_extendqisi2
orr r3, r3, r1
orrs r2, r2, r3
movne r0, #0
moveq r0, #1
mov pc, lr
is_zero_ether_addr2:
ldrb r3, [r0, #0] @ zero_extendqisi2
mov r2, #0
cmp r3, r2
bne .L3
ldrb r3, [r0, #1] @ zero_extendqisi2
cmp r3, r2
bne .L3
ldrb r3, [r0, #2] @ zero_extendqisi2
cmp r3, r2
bne .L3
ldrb r3, [r0, #3] @ zero_extendqisi2
cmp r3, r2
bne .L3
ldrb r3, [r0, #4] @ zero_extendqisi2
cmp r3, r2
bne .L3
ldrb r3, [r0, #5] @ zero_extendqisi2
cmp r3, r2
movne r2, #0
moveq r2, #1
.L3:
mov r0, r2
mov pc, lr
The former looks far more optimised in both cases. In fact, the
latter on ARM is many times less efficient due to the LDR result
delays being incurred for every test.
--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core
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