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Re: [Path,AVR]: Improve loading of 32-bit constants
2011/7/6 Georg-Johann Lay <avr@gjlay.de>:
> Denis Chertykov wrote:
>> 2011/7/6 Georg-Johann Lay <avr@gjlay.de>:
>>> For loading a 32-bit constant in a register, there is room for
>>> improvement:
>>>
>>> * SF can be handled the same way as SI and therefore the patch
>>> Âadds a peep2 to produce a *reload_insf analogon to *reload_insi.
>>>
>>> * If the destination register overlaps NO_LD_REGS, values already
>>> Âloaded into some other byte can be reused by a simple MOV.
>>> ÂThis is helpful then moving values like, e.g. -2, -100 etc. because
>>> Âall high bytes are 0xff.
>>>
>>> * 0.0f can be directly moved to memory.
>>>
>>> * The mov insns contain "!d" constraint. I see no reason to make "d"
>>> Âexpensive and discourage use of d-regs. ÂA "*d" to hide is better
>>> Âbecause it does it neither puts additional pressure on "d" nor
>>> Âdiscourages "d".
>>>
>>
>> I would like to have a real code examples.
>>
>> Denis.
>
> Hi Denis.
>
> Attached you find a small C file and the asm that is generated by new
> and old versions (-Os -mmcu=atmega88 -S -dp).
>
> I took away some regs as potential clobbers (or -fno-peephole2) to
> show the effect of high register pressure. ÂBit even if a clobber was
> available you can see that the new version is smarter in reusing
> values, e.g. note the loading of -1L to r22-r25.
I have asked about example of *d instead of !d.
Just svn GCC with *d vs svn GCC !d.
Denis.