Re: Comparison of GCC-4.6.1 and LLVM-2.9 on x86/x86-64 targets

[Vladimir Makarov <vmakarov at redhat dot com>]

On 09/07/2011 11:28 AM, Duncan Sands wrote:
> Hi Vladimir, thanks for doing this.
>
> > The above said about compilation speed is true when GCC front-end is
> > used for LLVM.
>
> It's not clear to me which GCC front-end you mean.  There is llvm-gcc
> (based on gcc-4.2) and the dragonegg plugin (the 2.9 version works with
> gcc-4.5; the development version works also with gcc-4.6).  Can you
> please clarify.

This is not dragonegg.  It was llvm-gcc4.2-2.9-x86_64-linux.
>   By the way, some highly unscientific experiments I did
> suggest that the GCC tree optimizers are (almost) as fast as the LLVM IR
> optimizers while doing a better job;
That is probably not only tree optimizers.  RTL optimizers seem not slow 
as well.

> while at -O3 the LLVM code generators
> are significantly faster than the GCC code generators and do a comparable
> and sometimes better job.  Unfortunately I haven't had time to do a 
> serious
> study, so this might just be an accident of the benchmarks I looked at 
> and
> the options I happened to use rather than anything meaningful.
I frequently see that on small benchmarks some optimizations (I mostly 
work on RA) look better than on bigger benchmarks.  It would be 
interesting to run SPEC2006 for comparison because it contains even 
bigger benchmarks.  Unfortunately, it will take 2 weeks on my machine to 
run SPEC2006 for the same # of option sets.