Re:Torvalds: Standards need to be questioned
by yoklov on Wednesday June 06, 2018 @01:29AM
(#17244561)
> Second, it is an error to call gcc's behavior "defined in documentation" as if that means it's okay to use. It's just a flag to support obsolete code. Long-term, the assumption that gcc will retain flags to support antique code is optimistic.
Type punning through a union works on GCC without -fno-strict-aliasing, and is documented in multiple places. Among others, it's suggested as an alternative to fix code broken under strict aliasing here: https://gcc.gnu.org/bugs/#nonbugs.
It's also relatively safe to assume that GCC will continue to be able to compile the Linux kernel for the indefinite future, since it would be very high profile, cause a ton of drama, and result in many people to moving away from GCC (including contributors). It seems very unlikely to be a smart decision.
(That's not to say lower profile projects than the linux kernel should rely on this. Using memcpy/memmove is just as fast and that much more difficult).
First, he's binding Linux to gcc forever. I've made that mistake and let me tell you, it's a mistake with surprisingly few benefits. A couple of asms and gccisms? Easily replaced.
Second, it is an error to call gcc's behavior "defined in documentation" as if that means it's okay to use. It's just a flag to support obsolete code. Long-term, the assumption that gcc will retain flags to support antique code is optimistic.
Third, "the standard is not workable in reality" is simply wrong. I work on low-level systems. I somehow manage to work with devices and networks on a day-to-day basis without relying on this.
Ultimately it comes down to this: Linus put his faith in something which was explicitly removed from C 30 years ago. The writing has been on the wall for about 10 years now[1]: Good optimizers are going to do away with this mis-feature. Time to start adapting to reality.
[1] http://blog.qt.io/blog/2011/06/10/type-punning-and-strict-al...