On 10/01/15 19:30, Paolo Bonzini wrote: > > > On 01/10/2015 19:07, Laszlo Ersek wrote: >>> In addition, C89 didn't say at all what the result was for signed data >>> types, so technically we could compile QEMU with -std=gnu89 (the default >>> until GCC5) and call it a day. >>> >>> Really the C standard should make this implementation-defined. >> >> Obligatory link: http://blog.regehr.org/archives/1180 > > Many ideas in there are good (e.g. mem*() being defined for invalid > argument and zero lengths, and of course item 7 which is the issue at > hand). In many cases it's also good to change undefined behavior to > unspecified values, however I think that goes too far. > > For example I'm okay with signed integer overflow being undefined > behavior, and I also disagree with "It is permissible to compute > out-of-bounds pointer values including performing pointer arithmetic on > the null pointer". Using uintptr_t is just fine. > > Also strict aliasing improves performance noticeably at least on some > kind of code. The relaxation of strict aliasing that GCC does with > unions would be a useful addition to the C standard, though.
What do you mean under "relaxation of strict aliasing that GCC does with unions"? I believe I know how unions affect this (although for details I'd obviously have to consult the standard :)), but what are the gcc specific parts? Thanks! Laszlo
