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Re: ppc64 floating point usage [was Re: PPC64 Compiler bug !!]
On Fri, Jun 13, 2003 at 06:38:37PM -0400, Michael Meissner wrote:
>
> Ummm, one of the earlier messages stated this furor came about because a driver
> writer did not use the -msoft-float option. If they are going to neglect using
> a switch that has been used by the Linux kernel for at least 4 years for all of
> its PowerPC builds (for precisely this reason BTW), I can't really imagine them
> using any new switch.
Exactly.
> Note, at least a few years ago, getting USER space code to use floating point
> registers to speed up structure moves (since using the FP registers is the only
> way to move 64 bits at a time in a 32 bit environment) was a big deal. I am
> worried that if we change the compiler, we may be be slowing down these
> programs.
Which is why my patch just affects 64 bit code, and only Linux. Linux
32 bit probably wants the same patch because contrary to what you say
above, using a fp reg for structure copies might be very slow. You can
cause an exception due to the lazy fp save/restore used by Linux.
However, since the 32 bit case is a little more controversial, I
restricted my patch to 64 bit Linux.
--
Alan Modra
IBM OzLabs - Linux Technology Centre