[libc++] Optimize basic_string::append(Iter, Iter) #169794
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You can test this locally with the following command:git-clang-format --diff origin/main HEAD --extensions ,cpp -- libcxx/include/string libcxx/test/benchmarks/containers/string.bench.cpp --diff_from_common_commit
View the diff from clang-format here.diff --git a/libcxx/test/benchmarks/containers/string.bench.cpp b/libcxx/test/benchmarks/containers/string.bench.cpp
index 68249ad89..8fb1d1a21 100644
--- a/libcxx/test/benchmarks/containers/string.bench.cpp
+++ b/libcxx/test/benchmarks/containers/string.bench.cpp
@@ -128,7 +128,7 @@ BENCHMARK(BM_StringResizeAndOverwrite);
template <class Container>
static void BM_StringAppend(benchmark::State& state) {
std::string orig_str = "This is some data so the string isn't empty";
- std::string str = orig_str;
+ std::string str = orig_str;
const char to_append_str[] = "This is a long string to make sure append does some work";
Container to_append;
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@llvm/pr-subscribers-libcxx Author: Nikolas Klauser (philnik777) ChangesFull diff: https://github.com/llvm/llvm-project/pull/169794.diff 2 Files Affected:
diff --git a/libcxx/include/string b/libcxx/include/string
index 2b3ba6d2d9b62..c0454ba2cb9f8 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1440,24 +1440,51 @@ public:
template <class _ForwardIterator, __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> = 0>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string&
append(_ForwardIterator __first, _ForwardIterator __last) {
- size_type __sz = size();
- size_type __cap = capacity();
- size_type __n = static_cast<size_type>(std::distance(__first, __last));
+ const size_type __size = size();
+ const size_type __cap = capacity();
+ const size_type __n = static_cast<size_type>(std::distance(__first, __last));
if (__n == 0)
return *this;
- if (__string_is_trivial_iterator_v<_ForwardIterator> && !__addr_in_range(*__first)) {
- if (__cap - __sz < __n)
- __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __sz, 0);
- __annotate_increase(__n);
- auto __end = __copy_non_overlapping_range(__first, __last, std::__to_address(__get_pointer() + __sz));
- traits_type::assign(*__end, value_type());
- __set_size(__sz + __n);
- return *this;
+ __annotate_delete();
+ auto __asan_guard = std::__make_scope_guard(__annotate_new_size(*this));
+
+ if (__n > __cap - __size) {
+ __long __buffer = __allocate_long_buffer(__alloc_, __get_amortized_growth_capacity(__size + __n));
+ __buffer.__size_ = __size + __n;
+ auto __guard = std::__make_exception_guard([&] {
+ __alloc_traits::deallocate(__alloc_, __buffer.__data_, __buffer.__cap_ * __endian_factor);
+ });
+ auto __end = __copy_non_overlapping_range(__first, __last, std::__to_address(__buffer.__data_) + __size);
+ traits_type::assign(*__end, _CharT());
+ traits_type::copy(std::__to_address(__buffer.__data_), data(), __size);
+ __guard.__complete();
+ __reset_internal_buffer(__buffer);
} else {
- const basic_string __temp(__first, __last, __alloc_);
- return append(__temp.data(), __temp.size());
+ _CharT* const __ptr = std::__to_address(__get_pointer());
+# ifndef _LIBCPP_CXX03_LANG
+ if constexpr (__libcpp_is_contiguous_iterator<_ForwardIterator>::value &&
+ is_same<value_type, __remove_cvref_t<decltype(*__first)>>::value) {
+ traits_type::move(__ptr + __size, std::__to_address(__first), __last - __first);
+ } else if constexpr (__has_bidirectional_iterator_category<_ForwardIterator>::value) {
+ auto __dest = __ptr + __size + __n;
+ while (__first != __last) {
+ --__last;
+ --__dest;
+ traits_type::assign(*__dest, *__last);
+ }
+ } else
+# endif
+ {
+ _CharT __first_val = *__first;
+ ++__first;
+ __copy_non_overlapping_range(__first, __last, __ptr + __size + 1);
+ traits_type::assign(__ptr[__size], __first_val);
+ }
+ traits_type::assign(__ptr[__size + __n], _CharT());
+ __set_size(__size + __n);
}
+ return *this;
}
# if _LIBCPP_STD_VER >= 23
diff --git a/libcxx/test/benchmarks/containers/string.bench.cpp b/libcxx/test/benchmarks/containers/string.bench.cpp
index 98216d22d0144..68249ad8963ed 100644
--- a/libcxx/test/benchmarks/containers/string.bench.cpp
+++ b/libcxx/test/benchmarks/containers/string.bench.cpp
@@ -11,7 +11,10 @@
#include <algorithm>
#include <cstdint>
#include <cstdlib>
+#include <deque>
+#include <iterator>
#include <new>
+#include <list>
#include <vector>
#include "../CartesianBenchmarks.h"
@@ -122,6 +125,26 @@ static void BM_StringResizeAndOverwrite(benchmark::State& state) {
}
BENCHMARK(BM_StringResizeAndOverwrite);
+template <class Container>
+static void BM_StringAppend(benchmark::State& state) {
+ std::string orig_str = "This is some data so the string isn't empty";
+ std::string str = orig_str;
+
+ const char to_append_str[] = "This is a long string to make sure append does some work";
+ Container to_append;
+ std::copy(std::begin(to_append_str), std::end(to_append_str), std::back_inserter(to_append));
+
+ for (auto _ : state) {
+ benchmark::DoNotOptimize(str);
+ str.append(to_append.begin(), to_append.end());
+ benchmark::DoNotOptimize(str);
+ str.resize(orig_str.size());
+ }
+}
+BENCHMARK(BM_StringAppend<std::vector<char>>);
+BENCHMARK(BM_StringAppend<std::deque<char>>);
+BENCHMARK(BM_StringAppend<std::list<char>>);
+
enum class Length { Empty, Small, Large, Huge };
struct AllLengths : EnumValuesAsTuple<AllLengths, Length, 4> {
static constexpr const char* Names[] = {"Empty", "Small", "Large", "Huge"};
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/libcxx-bot benchmark libcxx/test/benchmarks/containers/string.bench.cpp Benchmark results: |
ldionne
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Dec 2, 2025
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Can you please add an explanation of what the optimization is in the commit message? And also please include benchmark results.
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| __long __buffer = __allocate_long_buffer(__alloc_, __get_amortized_growth_capacity(__size + __n)); | ||
| __buffer.__size_ = __size + __n; |
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Semantically, that's what we have:
Suggested change
| __long __buffer = __allocate_long_buffer(__alloc_, __get_amortized_growth_capacity(__size + __n)); | |
| __buffer.__size_ = __size + __n; | |
| const size_type __new_size = __size + __n; | |
| const size_type __new_capacity = __get_amortized_growth_capacity(__new_size); | |
| __long __buffer = __allocate_long_buffer(__alloc_, __new_size, __new_capacity); |
It might not be necessary to actually name __new_size, but I think it would make sense for the API to allow us to write this.
The other way I can see to achieve the same result without losing the fact that we're doing amortized growth at the call site would be:
struct __amortized_growth_strategy_t {};
__long __buffer = __allocate_long_buffer(__alloc_, __size + __n, __amortized_growth_strategy_t{});and then
_LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX20 __long
__allocate_long_buffer(_Allocator& __alloc, size_type __size, __amortized_growth_strategy_t) {
_LIBCPP_ASSERT_INTERNAL(!__fits_in_sso(__size),
"Trying to allocate long buffer for a size what would fit into the small buffer");
size_type const __capacity = has-amortized-growth-strategy ? __get_amortized_growth_capacity(__size) : __size;
auto __buffer = std::__allocate_at_least(__alloc, __align_allocation_size(__capacity));
if (__libcpp_is_constant_evaluated()) {
for (size_type __i = 0; __i != __buffer.count; ++__i)
std::__construct_at(std::addressof(__buffer.ptr[__i]));
}
return __long(__buffer, __size);
}| __long __buffer = __allocate_long_buffer(__alloc_, __get_amortized_growth_capacity(__size + __n)); | ||
| __buffer.__size_ = __size + __n; | ||
| auto __guard = std::__make_exception_guard([&] { | ||
| __alloc_traits::deallocate(__alloc_, __buffer.__data_, __buffer.__cap_ * __endian_factor); |
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I think we should add a member function to the long buffer representation that gives you the correct capacity. I'd like to do that refactoring before we land this patch.
| } else { | ||
| const basic_string __temp(__first, __last, __alloc_); | ||
| return append(__temp.data(), __temp.size()); | ||
| _CharT* const __ptr = std::__to_address(__get_pointer()); |
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_CharT* const __ptr = std::__to_address(__get_pointer());
# ifndef _LIBCPP_CXX03_LANG
if constexpr (__libcpp_is_contiguous_iterator<_ForwardIterator>::value &&
is_same<value_type, __remove_cvref_t<decltype(*__first)>>::value) {
traits_type::move(__ptr + __size, std::__to_address(__first), __last - __first);
} else if constexpr (__has_bidirectional_iterator_category<_ForwardIterator>::value) {
auto __dest = __ptr + __size + __n;
while (__first != __last) {
--__last;
--__dest;
traits_type::assign(*__dest, *__last);
}
} else {
_CharT __first_val = *__first;
++__first;
__copy_non_overlapping_range(__first, __last, __ptr + __size + 1);
traits_type::assign(__ptr[__size], __first_val);
}
# else
_CharT __first_val = *__first;
++__first;
__copy_non_overlapping_range(__first, __last, __ptr + __size + 1);
traits_type::assign(__ptr[__size], __first_val);
# endif
traits_type::assign(__ptr[__size + __n], _CharT());
__set_size(__size + __n);IMO this is a lot easier to read because the interaction between the preprocessor and if constexpr is a lot more straightforward. There's a tiny bit more duplication, but IMO it increases readability over status quo.
| traits_type::move(__ptr + __size, std::__to_address(__first), __last - __first); | ||
| } else if constexpr (__has_bidirectional_iterator_category<_ForwardIterator>::value) { | ||
| auto __dest = __ptr + __size + __n; | ||
| while (__first != __last) { |
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This is non-trivial. Can you please add a comment explaining why you go from the back?
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| template <class _ForwardIterator, __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> = 0> | ||
| _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& | ||
| append(_ForwardIterator __first, _ForwardIterator __last) { |
Member
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At the moment, we are not optimizing append(char8_t*, char8_t*) into a memmove, but we could. We should figure out how to do that, perhaps in __copy_non_overlapping_range.
| std::string str = orig_str; | ||
|
|
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| const char to_append_str[] = "This is a long string to make sure append does some work"; | ||
| Container to_append; |
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Suggested change
| Container to_append; | |
| Container to_append(std::begin(to_append_str), std::end(to_append_str)); |
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| template <class Container> | ||
| static void BM_StringAppend(benchmark::State& state) { | ||
| std::string orig_str = "This is some data so the string isn't empty"; |
Member
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Suggested change
| std::string orig_str = "This is some data so the string isn't empty"; | |
| std::string const orig_str = "This is some data so the string isn't empty"; |
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