git: 4c4a29267cbd - main - libcxx: add comment explaining why umtx is only used for 64bits

The branch main has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=4c4a29267cbdd05471322e03bfd5eff8eb68e750

commit 4c4a29267cbdd05471322e03bfd5eff8eb68e750
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2023-01-24 15:36:24 +0000
Commit:     Konstantin Belousov <kib@FreeBSD.org>
CommitDate: 2023-01-25 01:19:42 +0000

    libcxx: add comment explaining why umtx is only used for 64bits
    
    Requested by:   imp
    Reviewed by:    arichardson, emaste
    Sponsored by:   The FreeBSD Foundation
    MFC after:      1 week
    Differential revision:  https://reviews.freebsd.org/D38178
---
 contrib/llvm-project/libcxx/src/atomic.cpp | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/contrib/llvm-project/libcxx/src/atomic.cpp b/contrib/llvm-project/libcxx/src/atomic.cpp
index 4a2e466bd145..f90d28c2e569 100644
--- a/contrib/llvm-project/libcxx/src/atomic.cpp
+++ b/contrib/llvm-project/libcxx/src/atomic.cpp
@@ -78,6 +78,11 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
 }
 
 #elif defined(__FreeBSD__) && __SIZEOF_LONG__ == 8
+/*
+ * Since __cxx_contention_t is int64_t even on 32bit FreeBSD
+ * platforms, we have to use umtx ops that work on the long type, and
+ * limit its use to architectures where long and int64_t are synonyms.
+ */
 
 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
                                               __cxx_contention_t __val)