[libc++] Remove a few unnecessary branches from basic_string::find

[philnik777]

I've recently looked at the assembly for basic_string::find() and realized that there were a few branches I didn't expect. It turns out that we check for null before calling __constexpr_memchr in some cases, which the compiler doesn't optimize away. This is a really uncommon case though, so I'm not convinced it makes a ton of sense to optimize for that.
The second case is where __pos >= __sz. There, we can instead check __pos > __sz, which the optimizer is able to remove if __pos == 0, which is also a quite common case (basic_string::find(CharT), without an explicit size parameter).

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

I've recently looked at the assembly for basic_string::find() and realized that there were a few branches I didn't expect. It turns out that we check for null before calling __constexpr_memchr in some cases, which the compiler doesn't optimize away. This is a really uncommon case though, so I'm not convinced it makes a ton of sense to optimize for that.
The second case is where __pos >= __sz. There, we can instead check __pos > __sz, which the optimizer is able to remove if __pos == 0, which is also a quite common case (basic_string::find(CharT), without an explicit size parameter).

---

Full diff: https://github.com/llvm/llvm-project/pull/137266.diff

1 Files Affected:

• (modified) libcxx/include/__string/char_traits.h (+1-5)

diff --git a/libcxx/include/__string/char_traits.h b/libcxx/include/__string/char_traits.h
index 60ec186ead826..86c92477cbfeb 100644
--- a/libcxx/include/__string/char_traits.h
+++ b/libcxx/include/__string/char_traits.h
@@ -132,8 +132,6 @@ struct char_traits<char> {
 
   static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
   find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT {
-    if (__n == 0)
-      return nullptr;
     return std::__constexpr_memchr(__s, __a, __n);
   }
 
@@ -250,8 +248,6 @@ struct char_traits<wchar_t> : __char_traits_base<wchar_t, wint_t, static_cast<wi
 
   static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
   find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT {
-    if (__n == 0)
-      return nullptr;
     return std::__constexpr_wmemchr(__s, __a, __n);
   }
 };
@@ -352,7 +348,7 @@ inline _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t char_traits<char32_t>::length(const
 template <class _CharT, class _SizeT, class _Traits, _SizeT __npos>
 inline _SizeT _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI
 __str_find(const _CharT* __p, _SizeT __sz, _CharT __c, _SizeT __pos) _NOEXCEPT {
-  if (__pos >= __sz)
+  if (__pos > __sz)
     return __npos;
   const _CharT* __r = _Traits::find(__p + __pos, __sz - __pos, __c);
   if (__r == nullptr)