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Re: Changing the default 32-bit x86 arch for Fedora 12



On Tue, Jun 16, 2009 at 09:33:22PM -0400, Orcan Ogetbil wrote:
> Now where does the "i686+SSE2" come into play? Does this SSE2 have any
> effect on those programs that do not contain SSE(2) related assembly code?
> Is this 1-2% improvement that you are mentioning only about these kind of
> programs (that do not contain assembly code)?

One advantage of SSE2 is that it can be used as a replacement for the
braindead x87 (floating point) instructions.  The x87 instructions are
architecturally stupid because they arrange the registers as a stack,
whereas what a compiler wants is a flat register file.

There was an experimental branch of the OCaml/i386 compiler which used
SSE2 as a replacement for x87 instructions, and it gained a 10-15%
increase in performance *on floating point benchmarks* [1] (ie. not
just on any old code, and not code which used specific hand-written
SSE2 optimizations).

(It's worth noting that SSE2 is always used on ocamlopt/x86_64)

Rich.

[1] http://caml.inria.fr/pub/ml-archives/caml-list/2009/05/781b091ad8006b117f8554014826665e.en.html

-- 
Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
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