Re: [csl-arm] Fix iWMMXt call-clobbering

[Richard Earnshaw <rearnsha at gcc dot gnu dot org>]

On Tue, 2005-03-15 at 16:49, Daniel Jacobowitz wrote:
> The iWMMXt control registers are always clobbered; in fact, that's what the
> comment three lines up explains.  But that was implemented by clearing
> instead of setting call_used_regs.  It's set by default, and we need to
> leave it that way, because we don't have any prologue or epilogue code
> to save and restore it...
> 
> Tested by inspection on arm-linux with -march=iwmmxt, where without this fix
> expand_mult is miscompiled.  OK for csl-arm-branch/4.0/HEAD?

Not a comment on the patch, which I see paul has already approved, but a
comment on CONDITIONAL_REGISTER_USAGE on arm in general.

I don't think any of the changes made by CONDITIONAL_REGISTER_USAGE
change a register from call-saved to call-clobbered or vice versa.  In
pretty much all cases they either enable use of the register or disable
it.  Given this it would be much more sensible if FIXED_REGISTERS and
CALL_USED_REGISTERS encoded the call-saved/clobbered information and
that CONDITIONAL_REGISTER_USAGE only ever disables registers entirely
(when not available in the target compilation environment).  That makes
the code in C_R_U much more straight-forward (FPA regs aren't available,
iWMMX regs aren't available, etc) rather than faffing about trying to
decide which sub-classes of regs have to be changed to which state.

This isn't a completely trivial change though, so should probably only
be done on the trunk (csl-arm I'll leave up to whoever writes the code).

R.