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.
Tom de Vries
Jakub Jelinek
Jakub Jelinek
Richard Sandiford
John David Anglin
Richard Henderson