[gcc r15-3586] libstdc++: Restore unrolling in std::find using pragma [PR116140]

[Alex Coplan]

https://gcc.gnu.org/g:3fd07d4f04f43816a038daf9b16c6d5bf2e96c9b

commit r15-3586-g3fd07d4f04f43816a038daf9b16c6d5bf2e96c9b
Author: Alex Coplan <alex.coplan@arm.com>
Date:   Fri Aug 2 09:56:07 2024 +0100

    libstdc++: Restore unrolling in std::find using pragma [PR116140]
    
    Together with the preparatory compiler patches, this patch restores
    unrolling in std::__find_if, but this time relying on the compiler to do
    it by using:
    
      #pragma GCC unroll 4
    
    which should restore the majority of the regression relative to the
    hand-unrolled version while still being vectorizable with WIP alignment
    peeling enhancements.
    
    On Neoverse V1 with LTO, this reduces the regression in xalancbmk (from
    SPEC CPU 2017) from 5.8% to 1.7% (restoring ~71% of the lost
    performance).
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/116140
            * include/bits/stl_algobase.h (std::__find_if): Add #pragma to
            request GCC to unroll the loop.

Diff:
---
 libstdc++-v3/include/bits/stl_algobase.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 27f6c377ad6f..f13662fc4482 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -2104,6 +2104,7 @@ _GLIBCXX_END_NAMESPACE_ALGO
     inline _Iterator
     __find_if(_Iterator __first, _Iterator __last, _Predicate __pred)
     {
+#pragma GCC unroll 4
       while (__first != __last && !__pred(__first))
 	++__first;
       return __first;