RE: [PATCH, GCC, stage1] Fallback to copy-prop if constant-prop not possible

["Thomas Preud'homme" <thomas.preudhomme@arm.com>]

> From: Steven Bosscher [mailto:stevenb.gcc@gmail.com]
> Sent: Friday, March 20, 2015 8:14 PM
> 
> I put the cprop_reg_p check there instead of !HARD_REGISTER_P
> because
> I like to be able to quickly find all places where a similar check is
> performed. The check is whether the reg is something that copy
> propagation can handle, and that is what I added cprop_reg_p for.

Makes sense indeed. I didn't think about the meaning of it.

> (Note that cprop can _currently_ handle only pseudos but there is no
> reason why a limited set of hard regs can't be handled also, e.g. the
> flag registers like in targetm.fixed_condition_code_regs).
> 
> In this case, the result is that REG_P is checked twice.
> But then again, cprop_reg_p will be inlined and the double check
> optimized away.

True.

> 
> Anyway, I guess we've bikeshedded long enough over this patch as it is
> :-) Let's post a final form and declare it OK for stage1.

What about the cprop_reg_p that needs to be negated? Did I miss something
that makes it ok?

> 
> As for PSEUDO_REG_P: If it were up to me, I'd like to have in rtl.h:
> 
> static bool
> hard_register_p (rtx x)
> {
>   return (REG_P (x) && HARD_REGISTER_NUM_P (REGNO (x)));
> }
> 
> static bool
> pseudo_register_p (rtx x)
> {
>   return (REG_P (x) && !HARD_REGISTER_NUM_P (REGNO (x)));
> }
> 
> and do away with all the FIRST_PSEUDO_REGISTER tests. But I've
> proposed this in the past and there was opposition. Perhaps when we
> introduce a rtx_reg class...

Ok I'll try to dig up what was the reasons presented. Anyway, it would
be done in a separate patch so not a problem for this one.

FYI testing your patch with the one cprop_reg_p negated as said in my
previous email shows no regression on arm-none-eabi cross-compiler
targeting Cortex-M3. Testing for x86_64 is ongoing.

Best regards,

Thomas