From 690d137f448d4c4da9001871e6569d5636f116c7 Mon Sep 17 00:00:00 2001 From: David Howells Date: Thu, 9 Feb 2012 15:48:20 +0000 Subject: Reduce the number of expensive division instructions done by _parse_integer() _parse_integer() does one or two division instructions (which are slow) per digit parsed to perform the overflow check. Furthermore, these are particularly expensive examples of division instruction as the number of clock cycles required to complete them may go up with the position of the most significant set bit in the dividend: if (*res > div_u64(ULLONG_MAX - val, base)) which is as maximal as possible. Worse, on 32-bit arches, more than one of these division instructions may be required per digit. So, assuming we don't support a base of more than 16, skip the check if the top nibble of the result is not set at this point. Signed-off-by: David Howells [ Changed it to not dereference the pointer all the time - even if the compiler can and does optimize it away, the code just looks cleaner. And edited the top nybble test slightly to make the code generated on x86-64 better in the loop - test against a hoisted constant instead of shifting and testing the result ] Signed-off-by: Linus Torvalds --- lib/kstrtox.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) (limited to 'lib/kstrtox.c') diff --git a/lib/kstrtox.c b/lib/kstrtox.c index 7a94c8f14e2..b1dd3e7d88c 100644 --- a/lib/kstrtox.c +++ b/lib/kstrtox.c @@ -44,12 +44,13 @@ const char *_parse_integer_fixup_radix(const char *s, unsigned int *base) * * Don't you dare use this function. */ -unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *res) +unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *p) { + unsigned long long res; unsigned int rv; int overflow; - *res = 0; + res = 0; rv = 0; overflow = 0; while (*s) { @@ -64,12 +65,19 @@ unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long if (val >= base) break; - if (*res > div_u64(ULLONG_MAX - val, base)) - overflow = 1; - *res = *res * base + val; + /* + * Check for overflow only if we are within range of + * it in the max base we support (16) + */ + if (unlikely(res & (~0ull << 60))) { + if (res > div_u64(ULLONG_MAX - val, base)) + overflow = 1; + } + res = res * base + val; rv++; s++; } + *p = res; if (overflow) rv |= KSTRTOX_OVERFLOW; return rv; -- cgit v1.2.3-18-g5258