[PATCH] libstdc++: hashtable: No need to update before begin node in _M_remove_bucket_begin
Théo Papadopoulo
papadopoulo@gmail.com
Mon Jan 22 13:42:40 GMT 2024
On 1/22/24 14:29, Jonathan Wakely wrote:
> On Mon, 22 Jan 2024 at 13:22, Théo Papadopoulo <papadopoulo@gmail.com> wrote:
>> On 1/22/24 11:07, Jonathan Wakely wrote:
>>> On Thu, 18 Jan 2024 at 15:59, Théo Papadopoulo <papadopoulo@gmail.com> wrote:
>>>> Wouldn't it be clearer if written as:
>>>>
>>>> if (!__next_n) {
>>>> _M_buckets[__bkt] = nullptr;
>>>> } else if (__next_bkt != __bkt) {
>>>> _M_buckets[__next_bkt] = _M_buckets[__bkt];
>>>> _M_buckets[__bkt] = nullptr;
>>>> }
>>>>
>>>> I think this is strictly equivalent and clearer.
>>> But it does two branches, when the second one isn't necessary. If
>>> __next_bkt == __bkt the assignment is harmless.
>>>
>>> I think it's much clearer the way Huanghui Nie originally suggested.
>> But there are two branches in the original code as well (the ifs are
>> just nested)...
>> And they are sequential in both cases (with the !next_n on the faster path).
>> The condition are just a bit simpler in the proposed new form.
>
> Ah, yes, so you mean also replacing the earlier condition:
> if (!__next_n || __next_bkt != __bkt)
>
> I was only looking at the changed code not the earlier context.
> /facepalm
>
> Yes, looking at the full context I like your suggestion.
Similarly, with your response, I too understand better your answer !!!
>
>> Anyway, as a (the) maintainer, your opinion is the most important and if
>> you are more at ease
>> with one form than the other, so be it.
>>
>> Theo.
>>
>>
>>
>>>> Theo.
>>>>
>>>>
>>>> On 1/18/24 10:26, Huanghui Nie wrote:
>>>>
>>>> Yes, I have. I did a benchmark today.
>>>>
>>>> The conclusion is: the time consumption can be reduced by 0.4% ~ 1.2% when unordered_set erase(begin()), and 1.2% ~ 2.4% when erase(begin(), end()).
>>>>
>>>>
>>>> My test environment:
>>>>
>>>> CPU: Intel(R) Xeon(R) CPU E5-2680 v4 @ 2.40GHz, 2393.365 MHz, 56 CPUs
>>>>
>>>> MEM: 256G
>>>>
>>>> OS: CentOS-8.2
>>>>
>>>> g++: gcc version 8.3.1 20191121 (Red Hat 8.3.1-5) (GCC)
>>>>
>>>> Compile flags: -O3 -std=c++17
>>>>
>>>>
>>>> Test conclusion data (time taken to delete every 100 million elements):
>>>>
>>>> erase(begin()):
>>>>
>>>> |size of unordered_set |100 |1,000 |10,000 |100,000 |1,000,000|10,000,000|
>>>>
>>>> |base time consuming (ms)|3827.736|3807.725|3830.168|3807.373|3798.713 |3854.168 |
>>>>
>>>> |test time consuming (ms)|3783.406|3789.460|3791.146|3778.033|3783.494 |3808.137 |
>>>>
>>>> |Time-consuming reduction|1.16% |0.48% |1.02% |0.77% |0.40% |1.19% |
>>>>
>>>> erase(begin(),end()):
>>>>
>>>> |size of unordered_set |100 |1,000 |10,000 |100,000 |1,000,000|10,000,000|
>>>>
>>>> |base time consuming (ms)|2779.229|2768.550|2795.778|2767.385|2761.521 |2804.099 |
>>>>
>>>> |test time consuming (ms)|2712.759|2726.578|2752.224|2732.140|2718.953 |2739.727 |
>>>>
>>>> |Time-consuming reduction|2.39% |1.52% |1.56% |1.27% |1.54% |2.30% |
>>>>
>>>>
>>>> Please see the attachment for test code and detailed test result.
>>>>
>>>>
>>>> 2024年1月18日(木) 4:04 François Dumont <frs.dumont@gmail.com>:
>>>>> Hi
>>>>>
>>>>> Looks like a great finding to me, this is indeed a useless check, thanks!
>>>>>
>>>>> Have you any figures on the performance enhancement ? It might help to get proper approval as gcc is currently in dev stage 4 that is to say only bug fixes normally.
>>>>>
>>>>> François
>>>>>
>>>>> On 17/01/2024 09:11, Huanghui Nie wrote:
>>>>>
>>>>> Hi.
>>>>>
>>>>> When I implemented a hash table with reference to the C++ STL, I found that when the hash table in the C++ STL deletes elements, if the first element deleted is the begin element, the before begin node is repeatedly assigned. This creates unnecessary performance overhead.
>>>>>
>>>>>
>>>>> First, let’s see the code implementation:
>>>>>
>>>>> In _M_remove_bucket_begin, _M_before_begin._M_nxt is assigned when &_M_before_begin == _M_buckets[__bkt]. That also means _M_buckets[__bkt]->_M_nxt is assigned under some conditions.
>>>>>
>>>>> _M_remove_bucket_begin is called by _M_erase and _M_extract_node:
>>>>>
>>>>> Case _M_erase a range: _M_remove_bucket_begin is called in a for loop when __is_bucket_begin is true. And if __is_bucket_begin is true and &_M_before_begin == _M_buckets[__bkt], __prev_n must be &_M_before_begin. __prev_n->_M_nxt is always assigned in _M_erase. That means _M_before_begin._M_nxt is always assigned, if _M_remove_bucket_begin is called and &_M_before_begin == _M_buckets[__bkt]. So there’s no need to assign _M_before_begin._M_nxt in _M_remove_bucket_begin.
>>>>> Other cases: _M_remove_bucket_begin is called when __prev_n == _M_buckets[__bkt]. And __prev_n->_M_nxt is always assigned in _M_erase and _M_before_begin. That means _M_buckets[__bkt]->_M_nxt is always assigned. So there's no need to assign _M_buckets[__bkt]->_M_nxt in _M_remove_bucket_begin.
>>>>>
>>>>> In summary, there’s no need to check &_M_before_begin == _M_buckets[__bkt] and assign _M_before_begin._M_nxt in _M_remove_bucket_begin.
>>>>>
>>>>>
>>>>> Then let’s see the responsibility of each method:
>>>>>
>>>>> The hash table in the C++ STL is composed of hash buckets and a node list. The update of the node list is responsible for _M_erase and _M_extract_node method. _M_remove_bucket_begin method only needs to update the hash buckets. The update of _M_before_begin belongs to the update of the node list. So _M_remove_bucket_begin doesn’t need to update _M_before_begin.
>>>>>
>>>>>
>>>>> Existing tests listed below cover this change:
>>>>>
>>>>> 23_containers/unordered_set/allocator/copy.cc
>>>>>
>>>>> 23_containers/unordered_set/allocator/copy_assign.cc
>>>>>
>>>>> 23_containers/unordered_set/allocator/move.cc
>>>>>
>>>>> 23_containers/unordered_set/allocator/move_assign.cc
>>>>>
>>>>> 23_containers/unordered_set/allocator/swap.cc
>>>>>
>>>>> 23_containers/unordered_set/erase/1.cc
>>>>>
>>>>> 23_containers/unordered_set/erase/24061-set.cc
>>>>>
>>>>> 23_containers/unordered_set/modifiers/extract.cc
>>>>>
>>>>> 23_containers/unordered_set/operations/count.cc
>>>>>
>>>>> 23_containers/unordered_set/requirements/exception/basic.cc
>>>>>
>>>>> 23_containers/unordered_map/allocator/copy.cc
>>>>>
>>>>> 23_containers/unordered_map/allocator/copy_assign.cc
>>>>>
>>>>> 23_containers/unordered_map/allocator/move.cc
>>>>>
>>>>> 23_containers/unordered_map/allocator/move_assign.cc
>>>>>
>>>>> 23_containers/unordered_map/allocator/swap.cc
>>>>>
>>>>> 23_containers/unordered_map/erase/1.cc
>>>>>
>>>>> 23_containers/unordered_map/erase/24061-map.cc
>>>>>
>>>>> 23_containers/unordered_map/modifiers/extract.cc
>>>>>
>>>>> 23_containers/unordered_map/modifiers/move_assign.cc
>>>>>
>>>>> 23_containers/unordered_map/operations/count.cc
>>>>>
>>>>> 23_containers/unordered_map/requirements/exception/basic.cc
>>>>>
>>>>>
>>>>> Regression tested on x86_64-pc-linux-gnu. Is it OK to commit?
>>>>>
>>>>>
>>>>> ---
>>>>>
>>>>> ChangeLog:
>>>>>
>>>>>
>>>>> libstdc++: hashtable: No need to update before begin node in _M_remove_bucket_begin
>>>>>
>>>>>
>>>>> 2024-01-16 Huanghui Nie <nnnjkk@gmail.com>
>>>>>
>>>>>
>>>>> gcc/
>>>>>
>>>>> * libstdc++-v3/include/bits/hashtable.h
>>>>>
>>>>>
>>>>> ---
>>>>>
>>>>>
>>>>> diff --git a/libstdc++-v3/include/bits/hashtable.h b/libstdc++-v3/include/bits/hashtable.h
>>>>>
>>>>> index b48610036fa..6056639e663 100644
>>>>>
>>>>> --- a/libstdc++-v3/include/bits/hashtable.h
>>>>>
>>>>> +++ b/libstdc++-v3/include/bits/hashtable.h
>>>>>
>>>>> @@ -872,13 +872,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>>>>
>>>>> if (!__next_n || __next_bkt != __bkt)
>>>>>
>>>>> {
>>>>>
>>>>> // Bucket is now empty
>>>>>
>>>>> - // First update next bucket if any
>>>>>
>>>>> + // Update next bucket if any
>>>>>
>>>>> if (__next_n)
>>>>>
>>>>> _M_buckets[__next_bkt] = _M_buckets[__bkt];
>>>>>
>>>>>
>>>>>
>>>>> - // Second update before begin node if necessary
>>>>>
>>>>> - if (&_M_before_begin == _M_buckets[__bkt])
>>>>>
>>>>> - _M_before_begin._M_nxt = __next_n;
>>>>>
>>>>> _M_buckets[__bkt] = nullptr;
>>>>>
>>>>> }
>>>>>
>>>>> }
>>>>>
>>>>>
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