On 02/16/2012 10:02 AM, Adam Jackson wrote:
On Thu, 2012-02-16 at 10:22 +0530, Rahul Sundaram wrote:
On 02/16/2012 10:08 AM, Genes MailLists wrote:
Not to mention that the kernel devs use gcc to compile the kernel -
and it most certainly puts a lot of pressure on the compiler. I suspect
unless linus drops gcc as well, we'll at a minimum need to keep it to
build the kernel itself.
Not quite. LLVM can be used to build the kernel and what Linus does
doesn't matter as much (kernel is important but only one component) as
showing that Fedora on the whole will actually benefit from moving to
LLVM. For that, LLVM has to so much better than GCC and someone has to
do the work within Fedora to show that it is the case.
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).
GCC has a big community of very dedicated people. LLVM has no such
community. So IMHO GCC will be more high quality compiler than LLVM
until LLVM gets such community.
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