http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51834
--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> 2012-04-19 15:06:58 UTC --- (In reply to comment #3) > (i++, i) + i is undefined. The sequence point only orders i++ and i inside > the > parens, but not the operands of +. The third example is not undefined. The example is not (i++, i) + i, but (i, i++, i) + i, which is different because there is a sequence point before and after the i++. Still, there seem to be disagreements on how to interpret the standard. There's a discussion "On sequence points and evaluation order" [1] in comp.std.c in 1995-12 (though that's a bit old), from which there are arguments to see the above expressions as UB. But "sequence points and evaluation order" [2] in comp.lang.c in 2006-09 and a message from Keith Thompson [3] in comp.std.c in 2010-10 both contradict it: they both say something like sin(x) + cos(x) has defined behavior even if sin() and cos() both modify errno (and that these functions can be implemented by a macro, as long as it has a sequence point). [1] http://groups.google.com/group/comp.std.c/browse_thread/thread/d133e9c51bef572b/0b6545278c23d37f [2] http://groups.google.com/group/comp.lang.c/browse_thread/thread/c4bc836b783b91be/d807a3ad7202b45b [3] http://groups.google.com/group/comp.std.c/msg/2dc8d2e8a0f4e572 What's strange is that GCC (4.4 to 4.7 at least) complains on (i ? (j |= 1, 0) : 0) | (i ? (j |= 1, 0) : 0); but not on (j |= 1, 0) | (j |= 1, 0); Contrary to GCC, I would say that the latter is UB (because from the root of the expression, one can evaluate both j |= 1 without getting a sequence point yet -- GCC should have output a warning, and that's bug 51562), but not the former (similar to the errno case). Here's a simple testcase I've used, with more tests: int i, j; static inline int set_flag (void) { j |= 1; return 0; } #define FOO (i ? (j |= 1, 0) : 0) #define BAR (i ? set_flag () : 0) void fct (void) { FOO || FOO; FOO | FOO; BAR | BAR; set_flag () + set_flag (); j = (++i, j) + (j, ++i); return; } GCC 4.7.0 warns only for "FOO | FOO;" (and I think that's incorrect, as said above).
