Segher Boessenkool <seg...@kernel.crashing.org> writes: > On Mon, Mar 25, 2019 at 11:33:56PM +1100, Michael Ellerman wrote: >> Segher Boessenkool <seg...@kernel.crashing.org> writes: >> > On Fri, Mar 22, 2019 at 11:37:24PM +1100, Michael Ellerman wrote: >> >> + clrldi r6,r4,(64-12) // r6 = r4 & 0xfff >> > >> > You can just write >> > rlwinm r6,r4,0,0x0fff >> >> > if that is clearer? Or do you still want a comment with that :-) >> >> I don't think it's clearer doing a rotate of zero bits :) >> >> And yeah I'd probably still leave the comment, so I'm inclined to stick >> with the clrldi? > > I always have to think what the clrldi etc. do exactly, while with rlwinm > it is obvious. But yeah this may be different for other people who are > used to different idiom.
Interesting, I'm the opposite. You know ppc assembler better than me so I guess I just need to spend more time on it and embrace the zen of the rotate instructions. >> Would be nice if the assembler could support: >> >> andi r6, r4, 0x0fff >> >> And turn it into the rlwinm, or rldicl :) > > The extended mnemonics are *simple*, *one-to-one* mappings. It would still be simple and 1:1, but would only be valid for certain constants :) > Having "andi. 6,4,0x0f0f" a valid insn, but an extended mnemonic "andi > 6,4,0x0f0f" > that is not (and the other way around for say 0xff0000ff) would violate that. I agree that's a bit of a foot gun. I'm not sure it's vastly more hostile though than `andi. 6,4,0xffff` being valid but `andi. 6,4,0x1ffff` being not valid. The assembler could print a nice error saying you need to use a contiguous mask. And I mean how often do you andi. with a mask that isn't contiguous? > You could do some assembler macro, that can also expand to multiple insns > where that is useful. Also one for loading constants, etc. The downside > to that is you often do care how many insns are generated. > > Instead you could do a macro for only those cases that can be done with *one* > insn. But that then is pretty restricted in use, and people have to learn > what values are valid. > > I don't see a perfect solution. Yeah, I guess a new `andi` instruction is the only real answer :) cheers
