From: Eelis van der Weegen <eelis@eelis.net>
Cc: libstdc++ <libstdc++@gcc.gnu.org>, gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [patch, libstdc++] std::shuffle: Generate two swap positions at a time if possible
Date: Tue, 03 May 2016 14:42:00 -0000 [thread overview]
Message-ID: <5728B8CC.7030405@eelis.net> (raw)
In-Reply-To: <CAH6eHdTnASg7A5USA_sTAHXPfUFmkrZvF27tZUJhj5ywwDVDag@mail.gmail.com>
[-- Attachment #1: Type: text/plain, Size: 985 bytes --]
Ah, thanks, I forgot to re-attach when I sent to include the libstdc++ list.
On 2016-05-03 14:38, Jonathan Wakely wrote:
> ENOPATCH
>
> On 1 May 2016 at 15:21, Eelis <eelis@eelis.net> wrote:
>> Sorry, forgot to include the libstdc++ list.
>>
>> On 2016-05-01 16:18, Eelis wrote:
>>>
>>> Hi,
>>>
>>> The attached patch optimizes std::shuffle for the very common case
>>> where the generator range is large enough that a single invocation
>>> can produce two swap positions.
>>>
>>> This reduces the runtime of the following testcase by 37% on my machine:
>>>
>>> int main()
>>> {
>>> std::mt19937 gen;
>>>
>>> std::vector<int> v;
>>> v.reserve(10000);
>>> for (int i = 0; i != 10000; ++i)
>>> {
>>> v.push_back(i);
>>> std::shuffle(v.begin(), v.end(), gen);
>>> }
>>>
>>> std::cout << v.front() << '\n';
>>> }
>>>
>>> Thoughts?
>>>
>>> Thanks,
>>>
>>> Eelis
>>>
>>
>>
[-- Attachment #2: double-step-shuffle.patch --]
[-- Type: text/x-patch, Size: 2440 bytes --]
Index: libstdc++-v3/include/bits/stl_algo.h
===================================================================
--- libstdc++-v3/include/bits/stl_algo.h (revision 235680)
+++ libstdc++-v3/include/bits/stl_algo.h (working copy)
@@ -3708,6 +3708,22 @@
#endif
#ifdef _GLIBCXX_USE_C99_STDINT_TR1
+
+ template<typename _IntType, typename _UniformRandomNumberGenerator>
+ inline _IntType
+ __generate_random_index_below(_IntType __bound, _UniformRandomNumberGenerator& __g)
+ {
+ const _IntType __urngrange = __g.max() - __g.min() + 1;
+ const _IntType __scaling = __urngrange / __bound;
+ const _IntType __past = __bound * __scaling;
+
+ for (;;)
+ {
+ const _IntType __r = _IntType(__g()) - __g.min();
+ if (__r < __past) return __r / __scaling;
+ }
+ }
+
/**
* @brief Shuffle the elements of a sequence using a uniform random
* number generator.
@@ -3740,6 +3756,40 @@
typedef typename std::make_unsigned<_DistanceType>::type __ud_type;
typedef typename std::uniform_int_distribution<__ud_type> __distr_type;
typedef typename __distr_type::param_type __p_type;
+
+ typedef typename std::remove_reference<_UniformRandomNumberGenerator>::type _Gen;
+ typedef typename std::common_type<typename _Gen::result_type, __ud_type>::type __uc_type;
+
+ const __uc_type __urngrange = _Gen::max() - _Gen::min() + 1;
+ const __uc_type __urange = __uc_type(__last - __first);
+
+ if (__urngrange / __urange >= __urange)
+ // I.e. (__urngrange >= __urange * __urange) but without wrap issues.
+ {
+ for (_RandomAccessIterator __i = __first + 1; __i != __last; )
+ {
+ const __uc_type __swap_range = __uc_type(__i - __first) + 1;
+
+ if (__i + 1 == __last)
+ {
+ const __uc_type __pos = __generate_random_index_below(__swap_range, __g);
+ std::iter_swap(__i, __first + __pos);
+ return;
+ }
+
+ // Use a single generator invocation to produce swap positions for
+ // both of the next two elements:
+
+ const __uc_type __comp_range = __swap_range * (__swap_range + 1);
+ const __uc_type __pospos = __generate_random_index_below(__comp_range, __g);
+
+ std::iter_swap(__i++, __first + (__pospos % __swap_range));
+ std::iter_swap(__i++, __first + (__pospos / __swap_range));
+ }
+
+ return;
+ }
+
__distr_type __d;
for (_RandomAccessIterator __i = __first + 1; __i != __last; ++__i)
next prev parent reply other threads:[~2016-05-03 14:42 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-01 14:18 Eelis
2016-05-01 14:30 ` Eelis
2016-05-03 12:39 ` Jonathan Wakely
2016-05-03 14:42 ` Eelis van der Weegen [this message]
2016-05-25 19:54 ` Eelis
2016-05-25 20:45 ` Eelis
2016-08-31 12:45 ` Jonathan Wakely
2016-09-01 15:14 ` Jonathan Wakely
2016-09-01 15:27 ` Marc Glisse
2016-09-01 15:35 ` Jonathan Wakely
2016-09-01 15:31 ` Eelis van der Weegen
2016-09-01 15:37 ` Jonathan Wakely
2016-09-02 18:20 ` Eelis van der Weegen
2016-09-02 18:53 ` Eelis
2016-09-02 19:27 ` Eelis
2016-10-14 19:42 ` Jonathan Wakely
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