Use of register 17 on x86

[Richard Henderson]

On Sun, Feb 18, 2001 at 07:59:29AM -0500, Richard Kenner wrote:
> There is a choice of using a SCRATCH with that register class or the
> explicit register 17.  If the former, gcse can move such computation, but
> we may try to move things using that class out of loops and have to do
> expensive moves in and out of CC.
> 
> Has any thought been given to this tradeoff?

Yes.  Moves into and out of flags are expensive enough that we
would prefer to avoid them at almost any cost.

> Also, why isn't x86 using CC0 anymore?  That would seem to be the most
> efficient, both in terms of generated code and compilation speed.

Because you can't get good representation of the architecture
that way.  It's is impossible to represent a comparison followed
by multiple cmove instructions.  Given the improvements in code
generation with the new backend, I think that gives lie to CC0
being more efficient in terms of generated code.


r~