Messages in this thread | | | Date | Thu, 20 Jul 2017 17:27:49 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] lib/int_sqrt.c: Optimize square root function |
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On Thu, Jul 20, 2017 at 01:24:49PM +0200, Peter Zijlstra wrote: > ~/tmp$ gcc -o sqrt sqrt.c -lm -O2 -DLOOPS=10000000 -DNEW=1 -DFLS=1 -DANSHUL=1 ; perf stat --repeat 10 -e cycles:u -e instructions:u ./sqrt > > Performance counter stats for './sqrt' (10 runs): > > 328,415,775 cycles:u ( +- 0.15% ) > 1,138,579,704 instructions:u # 3.47 insn per cycle ( +- 0.00% ) > > 0.088703205 seconds time elapsed
> static __always_inline unsigned long fls(unsigned long word) > { > asm("rep; bsr %1,%0" > : "=r" (word) > : "rm" (word)); > return BITS_PER_LONG - 1 - word; > }
That is actually "lzcnt", if I used the regular fls implementation:
static __always_inline unsigned long __fls(unsigned long word) { asm("bsr %1,%0" : "=r" (word) : "rm" (word)); return word; }
It ends up slightly more expensive:
~/tmp$ gcc -o sqrt sqrt.c -lm -O2 -DLOOPS=10000000 -DNEW=1 -DFLS=1 -DANSHUL=1 ; perf stat --repeat 10 -e cycles:u -e instructions:u ./sqrt
Performance counter stats for './sqrt' (10 runs):
384,842,215 cycles:u ( +- 0.08% ) 1,118,579,712 instructions:u # 2.91 insn per cycle ( +- 0.00% )
0.103018001 seconds time elapsed
Still loads cheaper than pretty much any other combination.
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