On 09/10/2013 03:02 PM, Linus Torvalds wrote: > On Tue, Sep 10, 2013 at 2:51 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote: >> On 09/10/2013 02:43 PM, Linus Torvalds wrote: >> >> Actually, the right thing here really is "er" (which I think you meant, >> but just to make it clear.) > > Yes, I was just answering the i-vs-e confusion. > >> "e" doesn't work on versions of gcc older than the first x86-64 release, >> but we don't care about that anymore. > > Indeed. > >> A final good question is if we should encapsulate the add/inc and >> sub/dec into a single function; one could easily do somethin glike: > > Yes. However, I would do that at a higher level than the one that > builds the actual functions. > > That said, there's a few cases where you might want to specify > add-vs-sub explicitly, but they are rather odd, namely the fact that > "-128" fits in a byte, but "128" does not. > > So it can be better to add 128 by doing a "subl $-128" than by doing > an "add $128". > > But we probably don't have any situation where we care about that > special value of "128". I've seen the trick, though. > Yes, and if __builtin_constant_p() we could even do it explicitly. Unfortunately I don't think gcc allows alternatives in asm() statements, unlike in its own pattern tables. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
