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authorDavid Howells <dhowells@redhat.com>2012-02-09 15:48:20 +0000
committerLinus Torvalds <torvalds@linux-foundation.org>2012-02-09 10:09:30 -0800
commit690d137f448d4c4da9001871e6569d5636f116c7 (patch)
treea6ccb2a00ce2e1fd2f3d8facabd0242e61431704 /lib/kstrtox.c
parentd65b4e98d7ea3038b767b70fe8be959b2913f16d (diff)
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 <dhowells@redhat.com> [ 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 <torvalds@linux-foundation.org>
Diffstat (limited to 'lib/kstrtox.c')
-rw-r--r--lib/kstrtox.c18
1 files changed, 13 insertions, 5 deletions
diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index 7a94c8f14e29..b1dd3e7d88cb 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;

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