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

[Laszlo Ersek]

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