Re: SPARC compiler optimisation

andrew@highland.oz.au (Andrew Morton)
Wed, 26 Feb 92 03:07:04 GMT

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Newsgroups: comp.compilers
From: andrew@highland.oz.au (Andrew Morton)
Keywords: sparc, optimize
Organization: Highland Logic, Moss Vale, Australia
Date: Wed, 26 Feb 92 03:07:04 GMT

> grunwald@foobar.cs.colorado.edu says:
>From comparison purposes, here's the code from gcc2, using
> gcc -O5 -funroll-loops foo.c


> ...
> ld [%o1-4000],%o0
> or %i0,%o0,%i0
> ld [%o1-4004],%o0
> or %i0,%o0,%i0
> ld [%o1-4008],%o0
> or %i0,%o0,%i0
> ld [%o1-4012],%o0
> or %i0,%o0,%i0
> ld [%o1-4016],%o0
> or %i0,%o0,%i0
> ld [%o1-4020],%o0
> or %i0,%o0,%i0
> ld [%o1-4024],%o0
> or %i0,%o0,%i0
> ld [%o1-4028],%o0


Forgive me if I'm wrong, but does this not incur a pipeline stall on
each 'or', waiting for the load to complete?


'twould be better thus:


ld [%o1-4000],%o0
ld [%o1-4004],%o2
ld [%o1-4008],%o3
ld [%o1-4012],%o4
ld [%o1-4016],%o5
ld [%o1-4020],%o6
ld [%o1-4024],%o7
ld [%o1-4028],%l0
or %o2,%o0,%o0
or %o3,%o0,%o0
or %o4,%o0,%o0
or %o5,%o0,%o0
or %o6,%o0,%o0
or %o7,%o0,%o0
or %l0,%o0,%o0


register hungry of course, but worthwhile if the loop is executed
enough times. The 'or's could be tweaked down to 4 + 2 + 1 as well,
using more registers.
--

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