https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97261
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |msebor at gcc dot gnu.org
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
const ab_t* ab = (ab_t*)(pb - 1);
note that this invokes undefined behavior as you are producing the address
of one-before the start of an object. You are also violating strict aliasing
rules with the access in g() which accesses a ab_t object but there's only
a double where you point to (well, there's nothing actually).
While the particular warning isn't very useful:
tst.c:21:17: warning: array subscript -1 is outside array bounds of 'double[1]'
[-Warray-bounds]
21 | const ab_t* ab = (ab_t*)(pb - 1);
| ^~
it does point to the offending statement. It probably should read
'warning: computing the address one element before the object 'double' invokes
undefined behavior'
or so. Mentioning double[1] when there's just a 'double' is likely also more
confusing than it helps. Maybe spelling out 'non-array object X' would be
better here.
All in all the bug for the testcase is invalid but I'm leaving it open to
eventually improve the actual diagnostic message.
