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Improve performance of til::equals (#19653)

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As explained in the added comment. Perf 📈.
This commit is contained in:
Leonard Hecker
2025-12-17 17:49:01 +01:00
committed by GitHub
parent e707c100e7
commit 5f6b814057
1 changed files with 20 additions and 7 deletions
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27
src/inc/til/string.h
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@@ -159,16 +159,29 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
// Just like std::wstring_view::operator==().
//
// At the time of writing wmemcmp() is not an intrinsic for MSVC,
// but the STL uses it to implement wide string comparisons.
// This produces 3x the assembly _per_ comparison and increases
// runtime by 2-3x for strings of medium length (16 characters)
// and 5x or more for long strings (128 characters or more).
// NOTE: If you pass a literal, ALWAYS pass it as the 2nd argument.
// This is because MSVC does not understand that `lhs.size() == rhs.size()` implies
// that a constant size on one side implies a constant size on the other side.
// As such the size argument to memcmp() must be the constant size, which is `rhs`.
//
// The STL uses wmemcmp() to implement wide string comparisons.
// Compared to memcmp() this results 3x the assembly per comparison,
// while running 2-3x slower for 16 chars and >5x slower for >128 chars.
// See: https://github.com/microsoft/STL/issues/2289
template<typename T, typename Traits>
bool equals(const std::basic_string_view<T, Traits>& lhs, const std::basic_string_view<T, Traits>& rhs) noexcept
constexpr bool equals(const std::basic_string_view<T, Traits>& lhs, const std::basic_string_view<T, Traits>& rhs) noexcept
{
return lhs.size() == rhs.size() && __builtin_memcmp(lhs.data(), rhs.data(), lhs.size() * sizeof(T)) == 0;
return lhs.size() == rhs.size() && __builtin_memcmp(lhs.data(), rhs.data(), rhs.size() * sizeof(T)) == 0;
}
constexpr bool equals(const std::string_view& lhs, const std::string_view& rhs) noexcept
{
return equals<>(lhs, rhs);
}
constexpr bool equals(const std::wstring_view& lhs, const std::wstring_view& rhs) noexcept
{
return equals<>(lhs, rhs);
}
// Just like _memicmp, but without annoying locales.