[gcc r12-7799] [libatomic] Fix return value in libat_test_and_set

[gcc r12-7799] [libatomic] Fix return value in libat_test_and_set

[Tom de Vries]

https://gcc.gnu.org/g:11fb784ac592567dbcb7874c27e67ee0feb8fbf0

commit r12-7799-g11fb784ac592567dbcb7874c27e67ee0feb8fbf0
Author: Tom de Vries <tdevries@suse.de>
Date:   Wed Mar 23 16:37:45 2022 +0100

    [libatomic] Fix return value in libat_test_and_set
    
    On nvptx (using a Quadro K2000 with driver 470.103.01) I ran into this:
    ...
    FAIL: gcc.dg/atomic/stdatomic-flag-2.c -O1 execution test
    ...
    which mimimized to:
    ...
      #include <stdatomic.h>
      atomic_flag a = ATOMIC_FLAG_INIT;
      int main () {
        if ((atomic_flag_test_and_set) (&a))
          __builtin_abort ();
        return 0;
      }
    ...
    
    The atomic_flag_test_and_set is implemented using __atomic_test_and_set_1,
    which corresponds to the "word-sized compare-and-swap loop" version of
    libat_test_and_set in libatomic/tas_n.c.
    
    The semantics of a test-and-set is that the return value is "true if and only
    if the previous contents were 'set'".
    
    But the code uses:
    ...
      return woldval != 0;
    ...
    which means it doesn't look only at the byte that was either set or not set,
    but at the entire word.
    
    Fix this by using instead:
    ...
      return (woldval & ((UTYPE) ~(UTYPE) 0 << shift)) != 0;
    ...
    
    Tested on nvptx.
    
    libatomic/ChangeLog:
    
    2022-03-24  Tom de Vries  <tdevries@suse.de>
    
            PR target/105011
            * tas_n.c (libat_test_and_set): Fix return value.

Diff:
---
 libatomic/tas_n.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libatomic/tas_n.c b/libatomic/tas_n.c
index d0d8c283b49..524312e7d8d 100644
--- a/libatomic/tas_n.c
+++ b/libatomic/tas_n.c
@@ -73,7 +73,7 @@ SIZE(libat_test_and_set) (UTYPE *mptr, int smodel)
 				     __ATOMIC_RELAXED, __ATOMIC_RELAXED));
 
   post_barrier (smodel);
-  return woldval != 0;
+  return (woldval & ((UTYPE) ~(UTYPE) 0 << shift)) != 0;
 }
 
 #define DONE 1