Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo