[gcc r12-7105] libstdc++: Add comment to acinclude.m4

[gcc r12-7105] libstdc++: Add comment to acinclude.m4

[Jonathan Wakely]

https://gcc.gnu.org/g:8dbb60b8dfc642a81f1c41fde11767150e8f91a5

commit r12-7105-g8dbb60b8dfc642a81f1c41fde11767150e8f91a5
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Feb 8 13:41:33 2022 +0000

    libstdc++: Add comment to acinclude.m4
    
    libstdc++-v3/ChangeLog:
    
            * acinclude.m4 (GLIBCXX_ENABLE_LOCK_POLICY): Add comment about
            checking for CAS on correct word size.

Diff:
---
 libstdc++-v3/acinclude.m4 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 7cc52f4db96..f53461c85a5 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -3609,6 +3609,8 @@ AC_DEFUN([GLIBCXX_ENABLE_LOCK_POLICY], [
     ac_save_CXXFLAGS="$CXXFLAGS"
 
     dnl Why do we care about 2-byte CAS on targets with 4-byte _Atomic_word?!
+    dnl Why don't we check 8-byte CAS for sparc64, where _Atomic_word is long?!
+    dnl New targets should only check for CAS for the _Atomic_word type.
     AC_TRY_COMPILE([
     #if ! defined __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2
     # error "No 2-byte compare-and-swap"