Magic-Mirror/llvm-capstone
1
0
Fork 0
mirror of https://github.com/capstone-engine/llvm-capstone.git synced 2024-11-23 22:00:10 +00:00
Code Issues Actions 2 Packages Projects Releases Wiki Activity

[libc++] Optimize string operator[] for known large inputs (#69500)

Browse Source 
If we know that index is larger than SSO size, we know that we can't be
in SSO case, and should access the pointer. This removes extra check
from operator[] for inputs known at compile time to be larger than SSO.
This commit is contained in:
Ilya Tocar 2023-10-26 13:09:20 -04:00 committed by GitHub
parent 3e79f4d2c2
commit 178a1fea57
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 27 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
libcxx/include/string
View File

@ -1198,11 +1198,17 @@ public:
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference operator[](size_type __pos) const _NOEXCEPT {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
return *(__get_long_pointer() + __pos);
}
return *(data() + __pos);
}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator[](size_type __pos) _NOEXCEPT {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
return *(__get_long_pointer() + __pos);
}
return *(__get_pointer() + __pos);
}

21
libcxx/test/std/strings/basic.string/string.access/index.pass.cpp
View File

@ -34,10 +34,31 @@ TEST_CONSTEXPR_CXX20 void test_string() {
assert(s2[0] == '\0');
}
// Same, but for the string that doesn't fit into SSO.
template <class S>
TEST_CONSTEXPR_CXX20 void test_string_long() {
S s("0123456789012345678901234567890123456789");
const S& cs = s;
ASSERT_SAME_TYPE(decltype(s[0]), typename S::reference);
ASSERT_SAME_TYPE(decltype(cs[0]), typename S::const_reference);
LIBCPP_ASSERT_NOEXCEPT(s[0]);
LIBCPP_ASSERT_NOEXCEPT(cs[0]);
for (typename S::size_type i = 0; i < cs.size(); ++i) {
assert(s[i] == static_cast<char>('0' + (i % 10)));
assert(cs[i] == s[i]);
}
assert(s[33] == static_cast<char>('0' + (33 % 10)));
assert(cs[34] == s[34]);
assert(cs[cs.size()] == '\0');
const S s2 = S();
assert(s2[0] == '\0');
}
TEST_CONSTEXPR_CXX20 bool test() {
test_string<std::string>();
#if TEST_STD_VER >= 11
test_string<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
test_string_long<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
#endif
return true;