Re: [PATCH i386] Move CLOBBERED_REGS earlier in register class list

[Uros Bizjak <ubizjak@gmail.com>]

On Mon, May 11, 2015 at 6:25 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
>> >>> LEGACY_REGS, which causes IRA not to consider it separately for register
>> >>> allocation, even when it has lower cost than other classes.  This patch is
>> >>> useful to fix code generation problem that appears with no-PLT PIC tailcalls.
>> >>>
>> >>> Was there a specific reason for CLOBBERED_REGS class to be listed as late as
>> >>> it is?  On 32-bit this class contains only EAX, ECX, EDX.
>> >>
>> >> Uros moved CLOBBERED_REGS late in
>> >> https://gcc.gnu.org/ml/gcc-patches/2012-08/msg00796.html
>> >> which contains a rationale, too.
>> >>
>> >> I am adding Uros and Vladimir to CC just in case they missed the email :)
>> >> Honza
>> >
>> > Uh, I don't remember that far, but from the context of the referred
>> > patch, it looks like a "cleanup" of some sort. My atch matched 32bit
>> > to 64bit, but could be also in the opposite way. Let's try the patch
>> > and see what breaks.
>>
>> Ah, the reason was that 64bit targets have many more call-clobbered
>> registers. So, I tried to position CLOBBERED_REGS according to the
>> ascending number of registers in the register set. Maybe the most
>> clean solution is to split the class to CLOBBERED_REGS_32 and
>> CLOBBERED_REGS_64 classes and set the register constraint in
>> constraints.md depending on TARGET_64BIT.
>
> Is there something to be gained by doing such a split?
>
> It seems to me that trying clobbered regs for allocation earlier that others
> (thus, callee-saved) makes sense in general, at least for non-static functions,
> as callers in other TUs would have to save/restore those registers anyway.

OK, if the position of CLOBBERED_REGS doesn't affect other
functionality (for 32, 64 and MS_ABI targets), then just move the
definition to the better place. Please also add the comment explaining
the rationale to prevent possible "cleanup" attempts in the future.

Thanks,
Uros.