-march=x86-64 -mtune=generic ; generic is i686!

Question

I just wanted to add that the arch packages and makepkg.conf on arch64 uses c(xx)flags that state to use x86-64 instruction sets, but tune for i686!
i686 means e.g. half the registers and no sse/sse2 support enabled, besides the half bandwith of the registers!
I hope I understand the gcc docs right when they say that "Moreover, specifying -march=cpu-type implies -mtune=cpu-type.", as then the -mtune flag is just redundant, but i'm not entirelly sure that I understand that right, since e.g. Slackware32 and Zenwalk uses i484 and i686 tuning and VectorLinux uses i586 and i686 tuning, so maybe the gcc statement means that -march only implies -mtune, if not -mtune is added specifically in addition to the -march ?

Answer 1

OK, Thanks!
Sorry, but if I can just come with one final closing argument; I know it's the same as i've said before, but I will just cut it out in cardboard now, or so to speak...
Under the -mtune option there's listed a bunch of cpu-types and cpu-families,but they are all called cpu-type in the man, no matter if they are a cpu-family: "The choices for cpu-type are"...
Now under generic it's stated that if you don't know what cpu end users are using, then this should be used, but keep in mind that all the available choises that can be entered after -mtune are called cpu-type in the man, including e.g. i585, i686 and X86-64, even though they are cpu-families and hence imho generic isn't right for our choise where we do know the cpu-familly!
In short, they are saying that if you don't can use ANY of the available -mtune cpu-type/familly valyes, then generic is the way to go!
Now if we still use it, then it's stated to be for common i686 and x86-64 cpu's + under i686 it's stated to be the same as generic.
This means that if using -mtune generic without an -march, then the code will be using i386 instruction set, but tuned for best schedulling on common i686/x86-64 cpu's.
But, if additionally adding an -march in addition to the -mtune, then you override the i386 instruction set code with another value, like e.g. i686 or whatever, but nowhere does it state that you imposses or losses the restriction on the -mtune value, which is for scheduling and not instruction set and abi!...
-mtune are for processor scedulling without taking instruction sets into equation, whereas -march are specifically for instruction sets and abi, and i cannot find any place where it's stated that -mtune uses the -march value as minimum cpu schedulling-type. Instead there says that -march equals -mtune, but we then override that with adding another specific -mtune value!
I would like that we ditch -mtune, since that equals the same -mtune as our -march!.
Now if i thought that generic in our case would equal different x86-64 cpu's and no i686 cpu's,  i would also think that it would be fine, but i just don't think that based on the spec(man).
Sorry for blabbing on, but i just wanted to explain myself so as to ppl dosen't just think that i'm an idiot, but that i've actually given it some thought also...
I'll shut up now, as i haven't got any new arguments...
Last edited by mhertz (2010-11-30 09:22:35)