https://gcc.gnu.org/g:18aff7644ad1e44dc146d36a2b7e397977aa47ac
commit r15-6324-g18aff7644ad1e44dc146d36a2b7e397977aa47ac
Author: Tamar Christina <tamar.christ...@arm.com>
Date: Wed Dec 18 09:02:46 2024 +0000
libstdc++: Add inline keyword to _M_locate
In GCC 12 there was a ~40% regression in the performance of hashmap->find.
This regression came about accidentally:
Before GCC 12 the find function was small enough that IPA would inline it
even
though it wasn't marked inline. In GCC-12 an optimization was added to
perform
a linear search when the entries in the hashmap are small.
This increased the size of the function enough that IPA would no longer
inline.
Inlining had two benefits:
1. The return value is a reference. so it has to be returned and
dereferenced
even though the search loop may have already dereference it.
2. The pattern is a hard pattern to track for branch predictors. This
causes
a large number of branch misses if the value is immediately checked and
branched on. i.e. if (a != m.end()) which is a common pattern.
The patch fixes both these issues by adding the inline keyword to _M_locate
to allow the inliner to consider inlining again.
This and the other patches have been ran through serveral benchmarks where
the size, number of elements searched for and type (reference vs value) etc
were tested.
The change shows no statistical regression, but an average find improvement
of
~27% and a range between ~10-60% improvements. A selection of the results:
+-----------+--------------------+-------+----------+
| Group | Benchmark | Size | % Inline |
+-----------+--------------------+-------+----------+
| Find | unord<uint64_t | 11274 | 53.52% |
| Find | unord<uint64_t | 11254 | 47.98% |
| Find Mult | unord<uint64_t | 12 | 47.62% |
| Find Mult | unord<std::string | 12 | 44.94% |
| Find Mult | unord<std::string | 10 | 44.89% |
| Find Mult | unord<uint64_t | 11 | 40.90% |
| Find Mult | unord<uint64_t | 352 | 30.57% |
| Find | unord<uint64_t | 351 | 28.27% |
| Find Mult | unord<uint64_t | 342 | 26.80% |
| Find | unord<std::string | 12 | 25.66% |
| Find Mult | unord<std::string | 352 | 23.12% |
| Find | unord<std::string | 13 | 20.36% |
| Find Mult | unord<std::string | 355 | 19.23% |
| Find | unord<std::string | 353 | 18.59% |
| Find | unord<uint64_t | 350 | 15.43% |
| Find | unord<std::string | 11260 | 11.80% |
| Find | unord<std::string | 352 | 11.12% |
| Find | unord<std::string | 11262 | 9.97% |
+-----------+--------------------+-------+----------+
libstdc++-v3/ChangeLog:
* include/bits/hashtable.h: Inline _M_locate.
Diff:
---
libstdc++-v3/include/bits/hashtable.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/bits/hashtable.h
b/libstdc++-v3/include/bits/hashtable.h
index 2dc24985d58d..cd60fb58c758 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -2219,7 +2219,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typename _ExtractKey, typename _Equal,
typename _Hash, typename _RangeHash, typename _Unused,
typename _RehashPolicy, typename _Traits>
- auto
+ inline auto
_Hashtable<_Key, _Value, _Alloc, _ExtractKey, _Equal,
_Hash, _RangeHash, _Unused, _RehashPolicy, _Traits>::
_M_locate(const key_type& __k) const
