[gcc r15-1879] libstdc++: Fix memchr path in std::ranges::find for non-common range [PR115799]

[Jonathan Wakely via Gcc-cvs]

https://gcc.gnu.org/g:762ee55d369a3257997092ef7853cf9dd4d5cc4f

commit r15-1879-g762ee55d369a3257997092ef7853cf9dd4d5cc4f
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Jul 5 18:58:00 2024 +0100

    libstdc++: Fix memchr path in std::ranges::find for non-common range 
[PR115799]
    
    The memchr optimization introduced in r15-1857 needs to advance the
    start iterator instead of returning the sentinel.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/115799
            * include/bits/ranges_util.h (__find_fn): Return iterator
            instead of sentinel.
            * testsuite/25_algorithms/find/constrained.cc: Check non-common
            contiguous sized range of char.

Diff:
---
 libstdc++-v3/include/bits/ranges_util.h               | 19 +++++++++----------
 .../testsuite/25_algorithms/find/constrained.cc       | 10 ++++++++++
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/libstdc++-v3/include/bits/ranges_util.h 
b/libstdc++-v3/include/bits/ranges_util.h
index 186acae4f70..a1f42875b11 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -501,17 +501,16 @@ namespace ranges
              if constexpr (contiguous_iterator<_Iter>)
                if (!is_constant_evaluated())
                  {
-                   if (static_cast<iter_value_t<_Iter>>(__value) != __value)
-                     return __last;
-
+                   using _Vt = iter_value_t<_Iter>;
                    auto __n = __last - __first;
-                   if (__n > 0)
-                     {
-                       const int __ival = static_cast<int>(__value);
-                       const void* __p0 = std::to_address(__first);
-                       if (auto __p1 = __builtin_memchr(__p0, __ival, __n))
-                         __n = (const char*)__p1 - (const char*)__p0;
-                     }
+                   if (static_cast<_Vt>(__value) == __value) [[likely]]
+                     if (__n > 0)
+                       {
+                         const int __ival = static_cast<int>(__value);
+                         const void* __p0 = std::to_address(__first);
+                         if (auto __p1 = __builtin_memchr(__p0, __ival, __n))
+                           __n = (const char*)__p1 - (const char*)__p0;
+                       }
                    return __first + __n;
                  }
 
diff --git a/libstdc++-v3/testsuite/25_algorithms/find/constrained.cc 
b/libstdc++-v3/testsuite/25_algorithms/find/constrained.cc
index e94751fcf89..7357a40bcc4 100644
--- a/libstdc++-v3/testsuite/25_algorithms/find/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/find/constrained.cc
@@ -66,9 +66,19 @@ test02()
   static_assert(ranges::find(y, 5, &Y::j) == y+3);
 }
 
+void
+test_pr115799()
+{
+  const char str[3] = { 'a', 'b', 'c' };
+  __gnu_test::test_contiguous_sized_range<const char> r(str);
+  VERIFY(std::ranges::find(r, 'a') == std::ranges::begin(r));
+  VERIFY(std::ranges::find(r, 'a'+255) == std::ranges::end(r));
+}
+
 int
 main()
 {
   test01();
   test02();
+  test_pr115799();
 }