Messages in this thread Patch in this message | | | Date | Mon, 24 Jul 2017 17:16:31 +0200 | From | Peter Zijlstra <> | Subject | [PATCH 1/3] lib/int_sqrt: Optimize small argument |
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The current int_sqrt() computation is sub-optimal for the case of small @x.
In this case, the compute loop:
while (m != 0) { b = y + m; y >>= 1;
if (x >= b) { x -= b; y += m; } m >>= 2; }
can be reduced to:
while (m > x) m >>= 2;
Because y==0, b==m and until x>=m y will remain 0.
And while this is computationally equivalent, it runs much faster because there's less code, in particular less branches.
cycles: branches: branch-misses:
OLD:
hot: 45.109444 +- 0.044117 44.333392 +- 0.002254 0.018723 +- 0.000593 cold: 187.737379 +- 0.156678 44.333407 +- 0.002254 6.272844 +- 0.004305
PRE:
hot: 67.937492 +- 0.064124 66.999535 +- 0.000488 0.066720 +- 0.001113 cold: 232.004379 +- 0.332811 66.999527 +- 0.000488 6.914634 +- 0.006568
POST:
hot: 43.633557 +- 0.034373 45.333132 +- 0.002277 0.023529 +- 0.000681 cold: 207.438411 +- 0.125840 45.333132 +- 0.002277 6.976486 +- 0.004219
Averages computed over all values <128k using a LFSR to generate order. Cold numbers have a LFSR based branch trace buffer 'confuser' ran between each int_sqrt() invocation.
Fixes: 30493cc9dddb ("lib/int_sqrt.c: optimize square root algorithm") Suggested-by: Anshul Garg <aksgarg1989@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- lib/int_sqrt.c | 3 +++ 1 file changed, 3 insertions(+)
--- a/lib/int_sqrt.c +++ b/lib/int_sqrt.c @@ -22,6 +22,9 @@ unsigned long int_sqrt(unsigned long x) return x; m = 1UL << (BITS_PER_LONG - 2); + while (m > x) + m >>= 2; + while (m != 0) { b = y + m; y >>= 1;
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