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| author | Torbjorn Granlund <tege@gmplib.org> | 2012-03-19 00:06:51 +0100 |
|---|---|---|
| committer | Torbjorn Granlund <tege@gmplib.org> | 2012-03-19 00:06:51 +0100 |
| commit | a96ddf53dd99f15f31327233b174769d686f4661 (patch) | |
| tree | 2f6b95031ed11a71133d6fed6b3ce783d5882fb4 /longlong.h | |
| parent | 693e7672173d377a00b26367696c148b89422fa5 (diff) | |
| download | gmp-a96ddf53dd99f15f31327233b174769d686f4661.tar.gz | |
(count_trailing_zeros): Write better pure C default variant.
Diffstat (limited to 'longlong.h')
| -rw-r--r-- | longlong.h | 31 |
1 files changed, 28 insertions, 3 deletions
diff --git a/longlong.h b/longlong.h index 5b65c1500..e83206011 100644 --- a/longlong.h +++ b/longlong.h @@ -2050,6 +2050,7 @@ __GMP_DECLSPEC UWtype __MPN(udiv_w_sdiv) (UWtype *, UWtype, UWtype, UWtype); /* This version gives a well-defined value for zero. */ #define COUNT_LEADING_ZEROS_0 (W_TYPE_SIZE - 1) #define COUNT_LEADING_ZEROS_NEED_CLZ_TAB +#define COUNT_LEADING_ZEROS_SLOW #endif /* clz_tab needed by mpn/x86/pentium/mod_1.asm in a fat binary */ @@ -2062,9 +2063,9 @@ extern const unsigned char __GMP_DECLSPEC __clz_tab[128]; #endif #if !defined (count_trailing_zeros) -/* Define count_trailing_zeros using count_leading_zeros. The latter might be - defined in asm, but if it is not, the C version above is good enough. */ -#define count_trailing_zeros(count, x) \ +#if !defined (COUNT_LEADING_ZEROS_SLOW) +/* Define count_trailing_zeros using an asm count_leading_zeros. */ +#define count_trailing_zeros(count, x) \ do { \ UWtype __ctz_x = (x); \ UWtype __ctz_c; \ @@ -2072,6 +2073,30 @@ extern const unsigned char __GMP_DECLSPEC __clz_tab[128]; count_leading_zeros (__ctz_c, __ctz_x & -__ctz_x); \ (count) = W_TYPE_SIZE - 1 - __ctz_c; \ } while (0) +#else +/* Define count_trailing_zeros in plain C, assuming small counts are common. + We use clz_tab without ado, since the C count_leading_zeros above will have + pulled it in. */ +#define count_trailing_zeros(count, x) \ + do { \ + UWtype __ctz_x = (x); \ + int __ctz_c; \ + \ + if (LIKELY ((__ctz_x & 0x7f) != 0)) \ + (count) = __clz_tab[__ctz_x & -__ctz_x] - 2; \ + else \ + { \ + for (__ctz_c = 5; __ctz_c <= W_TYPE_SIZE; __ctz_c += 7) \ + { \ + __ctz_x >>= 7; \ + if (LIKELY ((__ctz_x & 0x7f) != 0)) \ + break; \ + } \ + \ + (count) = __ctz_c + __clz_tab[__ctz_x & -__ctz_x]; \ + } \ + } while (0) +#endif #endif #ifndef UDIV_NEEDS_NORMALIZATION |