Re: Reload bug

[Jan Hubicka <jh@suse.cz>]

> > Hello Jan,
> > 
> > I've narrowed down the bug reported as PR optimization/10233, a regression 
> > from GCC 3.2 present on the 3.2 branch since 3.2.1 (but latent on all active 
> > branches), to this patch
> > 
> > Thu Oct 17 17:14:07 CEST 2002  Jan Hubicka  <jh@suse.cz>
> > 
> > 	PR opt/7630
> > 	* reload.c (reload_inner_reg_of_subreg): New argument output;
> > 	(push_reload): Update call.
> > 
> > 
> > We are trying to reload
> > 
> > 	(subreg:SI (reg/v:DI 29 rmm0 [68]) 4)
> > 
> > as the 'in' argument of push_reload.
> > 
> > Before your patch, the compiler correctly detected that the whole register 
> > needs to be reloaded because we can't access the high part of MMX regs.
> 
> How it did detected that?  I believe it is just pessimization masking a
> bug as currently there is no way how to realize that subreg:SI of MMX
> with offset 4 register is not valid (and with offset 0 is).
> 
> Thinking about it again, I think we can rely on HARD_REGNO_NREGS
> and compute how large the inidividular registers really are, but I don't
> think this is used anywhere in the code.
> 
> It should not get recognized earlier than in reload as we should not
> allocate value with subreg undoable in MMX register into MMX register
> (unless rest of insns dictates otherwise).  We've chatted about this
> with Richard

Actually I think the problem is just that the patch has never get into
3.2 line.
The patch is pretty involved:
	* combine.c (simplify_set): Reverse order of ragumetns to
	REG_CANNOT_CHANGE_MODE_P
	* df.c (df_def_record_1): Likewise.
	* recog.c (register_operand): Likewise.
	* simplify-rtx.c (simplify_subreg): Likewise.
	* hard-reg-set.h (REG_CANNOT_CHANGE_MODE_P): Update use of
	CANNOT_CHANGE_MODE_CLASS.
	* regclass.c (cannot_change_mode_set_regs, invalid_mode_change_p):
	Likewise.
	* reload.c (push_reload): Likewise.
	* alpha.h (CANNOT_CHANGE_MODE_CLASS): Update definition.
	* ia64.h (CANNOT_CHANGE_MODE_CLASS): Update definition.
	* mips.h (CANNOT_CHANGE_MODE_CLASS): Update definition.
	* mips-protos.h (mips_cannot_change_mode_class): Update prototype.
	* mips.c (mips_cannot_change_mode_class): Update.
	* pa64-regs.h (CANNOT_CHANGE_MODE_CLASS): Update definition.
	* rs6000.h (CANNOT_CHANGE_MODE_CLASS): Update definition.
	* s390.h (CANNOT_CHANGE_MODE_CLASS): Update definition.
	* sh.h (CANNOT_CHANGE_MODE_CLASS): Update definition.
	* sh-protos.h (sh_cannot_change_mode_class): Update prototype.
	* sh.c (sh_cannot_change_mode_class): Update.
	* i386.h (CANNOT_CHANGE_MODE_CLASS): New.
	* tm.texi (CANNOT_CHANGE_MODE_CLASS): Update documentation.
and I believe that the MMX suport in 3.2 is in so bad shape that I would
simply close the bugreport claiming that 3.3 fixes that.

Honza