From 9d94997b5f9445afd4f2bccc5fa60ff7c4361ec1 Mon Sep 17 00:00:00 2001 From: Sunil K Pandey Date: Wed, 26 Jul 2023 08:34:05 -0700 Subject: [PATCH] x86_64: Optimize ffsll function code size. Ffsll function randomly regress by ~20%, depending on how code gets aligned in memory. Ffsll function code size is 17 bytes. Since default function alignment is 16 bytes, it can load on 16, 32, 48 or 64 bytes aligned memory. When ffsll function load at 16, 32 or 64 bytes aligned memory, entire code fits in single 64 bytes cache line. When ffsll function load at 48 bytes aligned memory, it splits in two cache line, hence random regression. Ffsll function size reduction from 17 bytes to 12 bytes ensures that it will always fit in single 64 bytes cache line. This patch fixes ffsll function random performance regression. Reviewed-by: Carlos O'Donell --- sysdeps/x86_64/ffsll.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/sysdeps/x86_64/ffsll.c b/sysdeps/x86_64/ffsll.c index 73cfc6d90e..28444284e6 100644 --- a/sysdeps/x86_64/ffsll.c +++ b/sysdeps/x86_64/ffsll.c @@ -26,13 +26,13 @@ int ffsll (long long int x) { long long int cnt; - long long int tmp; - asm ("bsfq %2,%0\n" /* Count low bits in X and store in %1. */ - "cmoveq %1,%0\n" /* If number was zero, use -1 as result. */ - : "=&r" (cnt), "=r" (tmp) : "rm" (x), "1" (-1)); + asm ("mov $-1,%k0\n" /* Initialize cnt to -1. */ + "bsf %1,%0\n" /* Count low bits in x and store in cnt. */ + "inc %k0\n" /* Increment cnt by 1. */ + : "=&r" (cnt) : "r" (x)); - return cnt + 1; + return cnt; } #ifndef __ILP32__ -- 2.43.5