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Re: help interpreting gcc 4.1.1 optimisation bug
andrew@walrond.org writes:
> On Tue, Jun 13, 2006 at 12:01:39PM +0100, Andrew Haley wrote:
> >
> > All you've got here is an inline asm version of
> >
> > inline void longcpy(long* _dst, long* _src, unsigned _numwords)
> > {
> > __builtin_memcpy (_dst, _src, _numwords * sizeof (long));
> > }
> >
> > which gcc will optimize if it can.
> >
> > These days, "rep movs" is not as advantageous as it once was, and you
> > may get better performance by allowing gcc to choose how to do memory
> > copies.
>
> Actually, I knew this, but I was using longcpy as a bellwether
> of many more complex inline-asm functions in a c++ big integer
> library.
OK. I'm used to seeing random bits of inline asm that might have made
sense Once Upon A Time but now only serve to slow down a program!
> I've just finished a trawl through the entire library, fixing a Good
> Many Things which I now know (thanks to you guys) could really confuse
> the optimiser.
That's great.
Andrew.