Apple will use LLVM

[drago01]

On Thu, Feb 16, 2012 at 5:12 PM, Bill Nottingham <notting at redhat.com> wrote:
> Vladimir Makarov (vmakarov at redhat.com) said:
>> >Since I was a bit (intentionally) curt and dismissive in my other
>> >response in this thread, I'll add some anecdata here.  I have actually
>> >tried building xserver with clang and running the standard set of
>> >microbenchmarks.  I found one relevant path where the clang build was
>> >~15% faster [1].  Something like 60% of the rest were within ±3%.  For
>> >everything else clang was uniformly worse by usually about 5%.
>> >
>> The another usual mistake when people compare speed of GCC and LLVM
>> is to use -O2 for the both compilers.  But the true is that -O1 of
>> GCC is -O2 of LLVM with the point of code generation quality.  The
>> compiler speed of GCC with -O1 is the same as for LLVM with -O2.
>> You can find the latest comparison of LLVM and GCC on
>> http://vmakarov.fedorapeople.org/spec/ (see 2011 comparison at the
>> bottom of the left frame).
>
> Speaking of potential magic bullets... is there any reason
> we don't enable auto-vectorization by default (with -O3, or with the
> assorted -f/-m flags?)
>
> - Is it not stable enough?
> - Does it not take effect often enough?
> - Is it not done generically enough that we would run into instruction set
>  problems? (I belive on x86_64 we can assume SSE2. x86 is a mess, obvs.)

Yes SSE2 is part of the x86_64 abi, but last time I asked Jakub said
that it generates larger code which might end up slower due to not
being as cache efficient.