RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64

From: George Spelvin

Sent: 10 February 2016 14:44

...

quoted

I think the fastest loop is:
10:	adcq	0(%rdi,%rcx,8),%rax
	inc	%rcx
	jnz	10b
That loop looks like it will have no overhead on recent cpu.

Well, it should execute at 1 instruction/cycle.

I presume you do mean 1 adc/cycle.
If it doesn't unrolling once might help.

(No, a scaled offset doesn't take extra time.)

Maybe I'm remembering the 386 book.

To break that requires ADCX/ADOX:

10:	adcxq	0(%rdi,%rcx),%rax
	adoxq	8(%rdi,%rcx),%rdx
 	leaq	16(%rcx),%rcx
	jrcxz	11f
 	j	10b
11:

Getting 2 adc/cycle probably does require a little unrolling.
With luck the adcxq, adoxq and leaq will execute together.
The jrcxz is two clocks - so definitely needs a second adcoxq/adcxq pair.

Experiments would be needed to confirm guesses though.

	David