Here is an improved version of basic_string::find. The idea is to
split the string find in two parts:
1. search for the first match by using traits_type::find (this gets converted to memchr for x86)
2. see if there is a match (this gets converted to memcmp for x86)
Passes bootstrap on x86-64.
The patch results in good improvements on a synthetic test case I wrote using the google-benchmark.
following are the results.
Branch: master without patch
$ ./bin/string.libcxx.out
Run on (24 X 1899.12 MHz CPU s)
2016-12-06 16:41:55
***WARNING*** CPU scaling is enabled, the benchmark real time measurements may be noisy and will incur extra overhead.
Benchmark Time CPU Iterations
-----------------------------------------------------------------
BM_StringFindNoMatch/10 8 ns 8 ns 81880927
BM_StringFindNoMatch/64 52 ns 52 ns 13235018
BM_StringFindNoMatch/512 355 ns 355 ns 1962488
BM_StringFindNoMatch/4k 2769 ns 2772 ns 249090
BM_StringFindNoMatch/32k 22598 ns 22619 ns 30984
BM_StringFindNoMatch/128k 89745 ns 89830 ns 7996
BM_StringFindAllMatch/1 7 ns 7 ns 102893835
BM_StringFindAllMatch/8 9 ns 9 ns 75403364
BM_StringFindAllMatch/64 12 ns 12 ns 60766893
BM_StringFindAllMatch/512 31 ns 31 ns 23163999
BM_StringFindAllMatch/4k 141 ns 141 ns 4980386
BM_StringFindAllMatch/32k 1402 ns 1403 ns 483581
BM_StringFindAllMatch/128k 5604 ns 5609 ns 126123
BM_StringFindMatch1/1 44430 ns 44473 ns 15804
BM_StringFindMatch1/8 44315 ns 44357 ns 15741
BM_StringFindMatch1/64 44689 ns 44731 ns 15712
BM_StringFindMatch1/512 44247 ns 44290 ns 15724
BM_StringFindMatch1/4k 45010 ns 45053 ns 15678
BM_StringFindMatch1/32k 45717 ns 45761 ns 15278
BM_StringFindMatch2/1 44307 ns 44349 ns 15730
BM_StringFindMatch2/8 44631 ns 44674 ns 15721
BM_StringFindMatch2/64 44300 ns 44342 ns 15750
BM_StringFindMatch2/512 44239 ns 44281 ns 15713
BM_StringFindMatch2/4k 44886 ns 44928 ns 15787
Branch: master with patch
$ ./bin/string.libcxx.out
Run on (24 X 2892.28 MHz CPU s)
2016-12-06 18:51:38
***WARNING*** CPU scaling is enabled, the benchmark real time measurements may be noisy and will incur extra overhead.
Benchmark Time CPU Iterations
-----------------------------------------------------------------
BM_StringFindNoMatch/10 11 ns 11 ns 63049677
BM_StringFindNoMatch/64 12 ns 12 ns 57259381
BM_StringFindNoMatch/512 27 ns 27 ns 25495432
BM_StringFindNoMatch/4k 130 ns 130 ns 5301301
BM_StringFindNoMatch/32k 858 ns 859 ns 824048
BM_StringFindNoMatch/128k 4091 ns 4095 ns 171493
BM_StringFindAllMatch/1 14 ns 14 ns 53023977
BM_StringFindAllMatch/8 14 ns 14 ns 51516536
BM_StringFindAllMatch/64 17 ns 17 ns 40992668
BM_StringFindAllMatch/512 37 ns 37 ns 18503267
BM_StringFindAllMatch/4k 153 ns 153 ns 4494458
BM_StringFindAllMatch/32k 1460 ns 1461 ns 483380
BM_StringFindAllMatch/128k 5801 ns 5806 ns 120680
BM_StringFindMatch1/1 2062 ns 2064 ns 333144
BM_StringFindMatch1/8 2057 ns 2059 ns 335496
BM_StringFindMatch1/64 2083 ns 2085 ns 341469
BM_StringFindMatch1/512 2134 ns 2136 ns 336880
BM_StringFindMatch1/4k 2309 ns 2312 ns 308745
BM_StringFindMatch1/32k 3413 ns 3417 ns 208206
BM_StringFindMatch2/1 2053 ns 2055 ns 341523
BM_StringFindMatch2/8 2061 ns 2063 ns 343999
BM_StringFindMatch2/64 2075 ns 2077 ns 338479
BM_StringFindMatch2/512 2102 ns 2104 ns 332276
BM_StringFindMatch2/4k 2286 ns 2288 ns 300416
BM_StringFindMatch2/32k 3385 ns 3388 ns 204158
ChangeLog:
2016-12-07 Aditya Kumar <hiraditya@msn.com>
* include/bits/basic_string.tcc(find(const _CharT* __s, size_type
__pos, size_type __n) const)): Improve the algorithm
---
libstdc++-v3/include/bits/basic_string.tcc | 31 ++++++++++++++++++++++--------
1 file changed, 23 insertions(+), 8 deletions(-)
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index df1e8dd..7942ee6 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -1194,14 +1194,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if (__n == 0)
return __pos <= __size ? __pos : npos;
- if (__n <= __size)
- {
- for (; __pos <= __size - __n; ++__pos)
- if (traits_type::eq(__data[__pos], __s[0])
- && traits_type::compare(__data + __pos + 1,
- __s + 1, __n - 1) == 0)
- return __pos;
- }
+ if (__n > __size)
+ return npos;
+
+ const _CharT __elem0 = __s[0];
+ const _CharT* __first1 = __data;
+ const _CharT* __first2 = __s;
+ const _CharT* __last1 = __data + __size;
+ ptrdiff_t __len2 = __n - __pos;
+
+ while (true) {
+ ptrdiff_t __len1 = __last1 - __first1;
+ if (__len1 < __len2)
+ return npos;
+
+ // Find the first match.
+ __first1 = traits_type::find(__first1, __len1 - __len2 + 1, __elem0);
+ if (__first1 == 0)
+ return npos;
+ // Compare the full string when first match is found.
+ if (traits_type::compare(__first1, __first2, __len2) == 0)
+ return __first1 - __data;