Re: [Patch,AVR]: PR50447: Tweak addhi3

[Denis Chertykov <chertykov at gmail dot com>]

2011/10/18 Georg-Johann Lay <avr@gjlay.de>:
> Denis Chertykov schrieb:
>> 2011/10/18 Georg-Johann Lay <avr@gjlay.de>:
>>> This patch do some tweaks to addhi3 like adding QI scratch register.
>>>
>>> The original *addhi3 insn is still there and located prior to new
>>> addhi3_clobber insn because addhi3 is special to reload (thanks Danis for this
>>> note) so that there is a version with and a version without scratch register.
>>>
>>> Patch passes without regressions.
>>>
>>
>> Which improvements added by this patch ?
>>
>> Denis.
>
> If the addhi3 is expanded early, the addition happens with QI scratch which
> avoids reload of constant if target register is in NO_LD. And reduce register
> pressure as only QI is needed and not reload of constant to HI.
>
> Otherwise, there might be sequences like
>
> ldi r31, 2 Â Â; *reload_inhi
> mov r12, r31
> clr r13
>
> add r14, r12 Â; *addhi3
> adc r15, r13
>
> which now will be
>
> ldi r31, 2 Â Â; addhi3_clobber
> add r14, r31
> adc r15, __zero_reg__
>
> Similar applies if the reload of the constant happens to LD regs:
>
> ldi r30, 2 Â Â; *movhi
> clr r31
>
> add r14, r12 Â; *addhi3
> adc r15, r13
>
> will become
>
> ldi r30, 2 Â Â; addhi3_clobber
> add r14, r30
> adc r15, __zero_reg__
>
> For *addhi3 insns the register pressure is not reduced but the insn sequence
> might be smarter if peep2 comes up with a QI scratch or if it detects a
> *reload_inhi insn just prior to the addition (and the reg that holds the
> reloaded constant dies after the addition).
>
> As *addhi3 is special to reload, there is still an "ordinary" add addhi insn
> without scratch. This is easier because, e.g. prologue and epilogue generation
> generate add insns (not by means of addhi3 expander but by explicit
> gan_rtx_PLUS). Yet the addhi3 expander factors out the situations when an
> addhi3 insn is to be generated via addhi3 expander late in the compilation process

Please provide any real world example.

Denis.