[Patch, AVR]: Fix PR46779

Denis Chertykov chertykov@gmail.com
Mon Jun 13 22:37:00 GMT 2011


2011/6/13 Georg-Johann Lay <avr@gjlay.de>:
>
> So you think is is pointless/discouraged to give a more realistic
> description of AVR addressing be means of MODE_CODE_BASE_REG_CLASS (instead
> of BASE_REG_CLASS) resp. REGNO_MODE_CODE_OK_FOR_BASE_P?
>
>> Look carefully at `out_movqi_r_mr'.
>> There are even two fake addressing modes:
>> 1. [Y + infinite-dslacement];
>> 2. [X + (0...63)].
>
> Yes, I know. The first is introduced by avr_legitimate_address_p and the
> second appears to be artifact of LEGITIMIZE_RELOAD_ADDRESS.
>
> The changes are basically MODE_CODE_BASE_REG_CLASS (introduced in 4.2) and a
> rewrite of avr_legitimate_address_p. The changes aim at a better addressing
> for X and to minimize fake addresses.
>
>> I have spent a many hours (days, months) to debug GCC (especially avr port
>> and reload) for right addressing modes.
>> I have stopped on this code.
>> AVR have a limited memory addressing and GCC can't handle it in native
>> form.
>> Because of that I have supported a fake adddressing modes.
>
> I assume the code is from prior to 4.2 when REGNO_MODE_CODE_OK_FOR_BASE_P
> and MODE_CODE_BASE_REG_CLASS had not been available so that supporting X
> required some hacking.
> All that would still be fine; however the new register allocator leads to
> code that noone would accept. Accessing a structure through a pointer is not
> uncommon, not even on AVR. So if Z is used for, say accessing flash, X
> appears to be the best register.
>
> The shortcoming in GCC is that there is no way to give costs of addressing
> (TARGET_ADDRESS_COST does different things).
>
> So take a look what avr-gcc compiles here:
>  http://gcc.gnu.org/bugzilla/attachment.cgi?id=22242
> I saw similar complains in forums on the web.
>
>> (Richard Henderson have a different opinion: GCC can, AVR port can't)
>
> What does he mean with that?
>
>> IMHO that three limited pointer registers is not enough for C compiler.
>> Even more with frame pointer it's only two and X is a very limited.
>
> The current implementation has several oddities like
>
> * allowing SUBREGs in avr-legitimate_address_p
> * changing BASE_REG_CLASS on the fly (by means of reload_completed)
>
> isn't that supposed to cause trouble?

You can try to remove all oddities and check results.
Definitely something changed in GCC core since I wrote addressing code.

>>>
>>> * "zero_extendqidi2"
>>> * "zero_extendhidi2"
>>> * "zero_extendsidi2"
>>>
>>> These are "orphan" insns: they deal with DI without having movdi
>>> support so I removed them.
>>
>> This seems strange for me.
>
> As far as I know, to support a mode a respective mov insn is needed, which
> is not the case for DI. I don't know the exact rationale behind that
> (reloading?), just read is on gcc list by Ian Taylor (and also that it is
> stronly discouraged to have more than one mov insn per mode).
>
> So if the requirement to have mov insn is dropped and without the burden to
> implement movdi, it would be rather easy to implement adddi3 and subdi3 for
> avr...

Personally, I don't like to maintain 64bits integers in AVR port, but
may be somebody use it.

Denis.



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