2011/6/14 Georg-Johann Lay <a...@gjlay.de>: > Denis Chertykov schrieb: >> 2011/6/13 Georg-Johann Lay <a...@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. >> >> >> Denis. > > For your interest, here is a patch that shows the changes in > addressing mode.
Generally, the patch seems as a "right thing". I like it. How about a regression testing and code quality. Denis.
