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From: Tom de Vries <vries@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc r12-7799] [libatomic] Fix return value in libat_test_and_set Date: Thu, 24 Mar 2022 12:31:06 +0000 (GMT) [thread overview] Message-ID: <20220324123106.494273858C2C@sourceware.org> (raw) 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
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