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Add optimization to basic_string::assign for compile-time known constant values. 

Summary:
This change optimizes the assign() methods for string where either the contents or lengths are compile time known constants. For small strings (< min_cap) we can execute the assignment entirely inline. For strings up to 128 bytes we allow the compiler to efficiently inline the copy operation after we call the offline __resize<>() method. Short / long branches are taken at the call site for better branch prediction and allowing FDO optimizations.

Benchmarks (unstable / google perflab):
```
name                                                old time/op             new time/op             delta
BM_StringAssignAsciiz_Empty_Opaque                  5.69ns ± 7%             5.97ns ± 7%     ~             (p=0.056 n=5+5)
BM_StringAssignAsciiz_Empty_Transparent             5.39ns ± 7%             0.79ns ± 8%  -85.36%          (p=0.008 n=5+5)
BM_StringAssignAsciiz_Small_Opaque                  11.2ns ± 5%             11.0ns ± 6%     ~             (p=0.548 n=5+5)
BM_StringAssignAsciiz_Small_Transparent             10.1ns ± 7%              1.0ns ± 8%  -89.76%          (p=0.008 n=5+5)
BM_StringAssignAsciiz_Large_Opaque                  23.5ns ± 7%             23.8ns ± 7%     ~             (p=0.841 n=5+5)
BM_StringAssignAsciiz_Large_Transparent             21.4ns ± 7%             12.7ns ± 7%  -40.83%          (p=0.008 n=5+5)
BM_StringAssignAsciiz_Huge_Opaque                    336ns ± 4%              327ns ± 7%     ~             (p=0.421 n=5+5)
BM_StringAssignAsciiz_Huge_Transparent               331ns ± 5%              324ns ± 7%     ~             (p=0.548 n=5+5)
BM_StringAssignAsciizMix_Opaque                     13.6ns ±10%             13.7ns ± 9%     ~             (p=0.690 n=5+5)
BM_StringAssignAsciizMix_Transparent                12.9ns ± 8%              3.6ns ± 8%  -71.82%          (p=0.008 n=5+5)
```

Reviewers: EricWF, #libc!

Subscribers: jfb, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D82221
This commit is contained in:
Martijn Vels 2020-06-19 14:24:03 -04:00
parent c84a952dc7
commit 2bad222680
3 changed files with 47 additions and 2408 deletions
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4
libcxx/include/__string
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@ -152,7 +152,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*, size_type)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_of(value_type const*, size_type, size_type) const) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, size_type, value_type)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*, size_type)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::__assign_external(value_type const*, size_type)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::__assign_external(value_type const*)) \
_Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::reserve(size_type)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*, size_type)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(basic_string const&, size_type, size_type)) \
@ -176,7 +177,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
_Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(value_type const*) const) \
_Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*) const) \
_Func(_LIBCPP_FUNC_VIS _CharType& basic_string<_CharType>::at(size_type)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*)) \
_Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type const*, size_type, size_type) const) \
_Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, basic_string const&, size_type, size_type) const) \
_Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*, size_type) const) \

62
libcxx/include/string
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@ -1659,6 +1659,19 @@ private:
_NOEXCEPT
{}
basic_string& __assign_external(const value_type* __s);
basic_string& __assign_external(const value_type* __s, size_type __n);
// Assigns the value in __s, guaranteed to be __n < __min_cap in length.
inline basic_string& __assign_short(const value_type* __s, size_type __n) {
pointer __p = __is_long()
? (__set_long_size(__n), __get_long_pointer())
: (__set_short_size(__n), __get_short_pointer());
traits_type::move(_VSTD::__to_address(__p), __s, __n);
traits_type::assign(__p[__n], value_type());
return *this;
}
_LIBCPP_INLINE_VISIBILITY void __invalidate_all_iterators();
_LIBCPP_INLINE_VISIBILITY void __invalidate_iterators_past(size_type);
@ -2267,26 +2280,32 @@ basic_string<_CharT, _Traits, _Allocator>::__assign_no_alias(
return *this;
}
template <class _CharT, class _Traits, class _Allocator>
basic_string<_CharT, _Traits, _Allocator>&
basic_string<_CharT, _Traits, _Allocator>::__assign_external(
const value_type* __s, size_type __n) {
size_type __cap = capacity();
if (__cap >= __n) {
value_type* __p = _VSTD::__to_address(__get_pointer());
traits_type::move(__p, __s, __n);
traits_type::assign(__p[__n], value_type());
__set_size(__n);
__invalidate_iterators_past(__n);
} else {
size_type __sz = size();
__grow_by_and_replace(__cap, __n - __cap, __sz, 0, __sz, __n, __s);
}
return *this;
}
template <class _CharT, class _Traits, class _Allocator>
basic_string<_CharT, _Traits, _Allocator>&
basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s, size_type __n)
{
_LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
size_type __cap = capacity();
if (__cap >= __n)
{
value_type* __p = _VSTD::__to_address(__get_pointer());
traits_type::move(__p, __s, __n);
traits_type::assign(__p[__n], value_type());
__set_size(__n);
__invalidate_iterators_past(__n);
}
else
{
size_type __sz = size();
__grow_by_and_replace(__cap, __n - __cap, __sz, 0, __sz, __n, __s);
}
return *this;
return (_LIBCPP_BUILTIN_CONSTANT_P(__n) && __n < __min_cap)
? __assign_short(__s, __n)
: __assign_external(__s, __n);
}
template <class _CharT, class _Traits, class _Allocator>
@ -2471,14 +2490,23 @@ basic_string<_CharT, _Traits, _Allocator>::assign(const _Tp & __t, size_type __p
}
template <class _CharT, class _Traits, class _Allocator>
basic_string<_CharT, _Traits, _Allocator>&
basic_string<_CharT, _Traits, _Allocator>::__assign_external(const value_type* __s) {
return __assign_external(__s, traits_type::length(__s));
}
template <class _CharT, class _Traits, class _Allocator>
basic_string<_CharT, _Traits, _Allocator>&
basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s)
{
_LIBCPP_ASSERT(__s != nullptr, "string::assign received nullptr");
return assign(__s, traits_type::length(__s));
return _LIBCPP_BUILTIN_CONSTANT_P(*__s)
? (traits_type::length(__s) < __min_cap
? __assign_short(__s, traits_type::length(__s))
: __assign_external(__s, traits_type::length(__s)))
: __assign_external(__s);
}
// append
template <class _CharT, class _Traits, class _Allocator>

2389
libcxx/lib/abi/x86_64-apple-darwin.v2.abilist
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