Dimitry Andric added the comment: Note that gcc has documented accessing union members in this way as an "implementation defined" feature: https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit-fields-implementation.html#Structures-unions-enumerations-and-bit-fields-implementation
They specifically mention an example that is very similar to the dtoa pattern as being allowed under *their* implementation, here: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Type-punning However, whether that means that it is generally allowed by all standards is unfortunately still rather vague. I have seen more than one complaint that the standards committees should make this very explicit, instead of weaseling it into post-release footnotes. That said, I'd like to reply to some other posters here. Eric V. Smith writes: > At most, I think Mark's idea to use -fno-strict-aliasing only on dtoa.c and > nowhere else would be the better approach. Indeed, this is exactly the solution I chose for FreeBSD. Just the one file that needs it is compiled with -fno-strict-aliasing. The caveat is that this might not work when link-time optimization is in effect. Serhiy Storchaka writes: > Could we use Clang specific pragma in dtoa.c rather than a compiler option? Unfortunately, as far as I know, clang still does not support function-level optimization pragmas. Maybe it was implemented recently, but then you would still have to have a workaround for older versions. STINNER Victor writes: > If we decide to go for the -fno-strict-aliasing only for dtoa.c, I suggest to > use it also for GCC. GCC might decide to also optimize dtoa.c further in the > future. Theoretically they could, but as I pointed out above, they explicitly document this as a feature of their union implementation. I estimate the probability of them dropping this in the future to be near zero. > I don't think that the flag has a major impact on performance if it's > restricted to dtoa.c, and it would simplify the build system to only have > "per compiler" flags. (Ex: Does ICC also "miscompile" dtoa?) Disabling strict aliasing for just that file, or even just the one function it applies to (by splitting it off to a separate file, for instance) should not result in different assembly output, unless the compiler is very old and/or dumb. > FreeBSD uses the following syntax to only add the flag on a specific C file. > Does it work with GNU and BSD make? (is it a "portable" syntax?) > > CFLAGS.gdtoa_${src}+=-fno-strict-aliasing No, this is specifically a feature of BSD's bsd.sys.mk infrastructure. It's most likely not compatible with GNU make. ---------- nosy: +dim _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue30124> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
