Messages in this thread | | | Date | Sat, 29 Oct 2005 15:17:57 +0100 | From | Russell King <> | Subject | Re: [PATCH] include/linux/etherdevice.h, kernel 2.6.14 |
| |
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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |