[LLVMdev] LLVM 2.2 Release Notes

[Dale Johannesen]

On Feb 10, 2008, at 11:26 PM, Chris Lattner wrote:

>> This is a matter of presentation, but some of the "GCC extensions"  
>> are
>> standard C99 (now, at least).  I noticed long long, C++-style  
>> comments
>> and designated initializers.
>>
>> I have plenty of complaints about the GCC documentation you're
>> pointing at, but this probably isn't the right forum  for that.  I do
>> think dropping "as fast as macros" from the "inline" description  
>> would
>> be a good idea, that's pure propaganda:  sometimes it's true and
>> sometimes it isn't.
>
> I completely agree with you here.  It looks like the GCC docs were
> written vs C90 not C99.  That whole section of the documentation was
> more useful when there were a lot of GCC features we didn't support.
> Now it looks like it can be ripped out and we can just keep a list of
> things that don't work in our own words.  Lets do this for the llvm
> 2.3 release notes though, after Monday.

ok.

>> The miscompilation of code containing both MMX vectors and long  
>> double
>> may be worth a mention, but it probably isn't going to bite very many
>> people.  (llvm does not insert EMMS everywhere it needs to, or from a
>> different viewpoint, uses MMX when the user didn't tell it to.)
>
> AFAIK, many commonly used GCC versions don't do this either.  If
> that's the case, I don't think it's worth mentioning, because most
> people will be concerned with "llvm regressions vs gcc" or something.
> I believe that if you explicitly use mmx intrinsics you have to insert
> emms yourself, and that the only other way to get mmx badness is
> through use of the gcc generic vector extension.  If this is true, I
> think we should be ok.

I think that is true, but, the situtation with llvm is rather worse  
than with gcc:  llvm uses MMX registers for mem-to-mem copies of 64- 
bit vectors, while gcc does not.  gcc attempts to restrict its use of  
MMX to the MMX builtins, and the loads and stores connected to them;  
we think it is possible for RA spilling to use MMX registers  
erroneously, but this is not common.