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Thread: GCC To Receive Automatic Parallelization Support

  1. #11
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    Quote Originally Posted by MPF View Post
    Hey ! I'm not sure about it, but it seems like it will be the first compiler to support this killer-feature !

    Am I wrong ?
    Yes. Intel, IBM, Sun, and probably others have supported it for a while. And it's not a killer feature, I don't think it's that great personally.

  2. #12
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    Quote Originally Posted by Yuma View Post
    Yes. Intel, IBM, Sun, and probably others have supported it for a while. And it's not a killer feature, I don't think it's that great personally.
    The way I read that mailing list message, GCC4.3 already has some very basic loop auto-parallelization code as well. This is about integrating a new model which will allow it to become much more complex and useful.

  3. #13
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    Quote Originally Posted by FunkyRider View Post
    What happens when a Gentoo user wants to change his CPU+Motherboard?
    Me another happy Gentoo camper. And such things are non-issue unless you have over-optimized for a specific machine. For example, I have compiled my 64-bit Gentoo with cflags "march=core2" (including mcore2 in kernel), created an image and transferred it to my older athlon64x2 3800+ and it runs flawlessly. Sure it may be a bit suboptimal for my athlon, but since most of my machines are core2, I decided to go with it.

    (Btw, I was expecting a few issues with advanced instruction sets like sssse3 and sse4aaaaa111 being absent from my athlon, but so far all is good. Including mplayer and stuff. Its a different experience from my first experience when I once compiled with march=pentium3 and it gave problems with a p2 class celeron. The second time, I was happily able to run march=pentium4 gentoo on all my machines ranging from my athlon, sempron to core duo and core2 duo. This time, I first used march=nocona, and when it went good, I was brave to try march=core2 on this older athlon.)

    Also, as a side note, it has been mentioned many times that Gentoo is not just for ricers. I use it mainly because of its clean and elegant management. (On that note, other distros are yet to come up with something like 'eselect opengl' which can for example make switching graphics easy.) Portage is a gem like APT. Its sad that as mentioned in other forum posts too, that new portage features like updating by git and package sets get no coverage while codenames for ubuntu and fedora are debated.

    And the other thing about Gentoo is its stability and system consistency. There are no missing libraries / conflicts on a good gentoo system. Sure all distros can do that, but what requires a effort in them is natural in gentoo. The amount of dev packages one needs to install when occasionally compiling even a small 3rd party package is sometimes nagging in other distros.

    And one last thing. In my experience, for most packages, there is no difference in speed between debian i386 packages and super optimized gentoo packages. In fact, mostly debian feels much lighter.

    Hope I didn't hijack this thread .

  4. #14
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    Quote Originally Posted by hdas View Post
    (Btw, I was expecting a few issues with advanced instruction sets like sssse3 and sse4aaaaa111 being absent from my athlon, but so far all is good. Including mplayer and stuff.
    march=core2 won't include anything past SSE2 by default. You have to add -mssse3, -mssse4.1, and -mssse4.2 to get the special instructions that won't work on your A64. I think the standard instructions on those chips are identical, unless you get into stuff like special hardware VM support.

    Really, SSE2+ support is only going to affect a few applications anyway, and they've probably got special flags and optimizations setup within the ebuild or program. I'm pretty sure MPlayer, for example, contains lots of manual assembly code and detects what CPU you have at runtime and picks it's fastest paths available.
    Last edited by smitty3268; 03-11-2009 at 02:59 AM.

  5. #15
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    Premature optimization is the root of all evil.
    You gentoo users don't test whether it actually gives you any gain...

  6. #16
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    Overly generalised. Just about all gentoo-ers know that you should not over-optimize.

    I use both Gentoo and Ubuntu, and for my dev-stations I always go for gentoo, since I don't get that library hell that is any other distribution...

  7. #17
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    Default How parallel

    Does "Parallel" means code generation that utilize multiple threads (TFA talks about "better performance on multi-core systems", so I assume multiple threads must be involved here) or better detecting loops that can be computed in parallel using SIMD instructions?

    If first case is true, does compiler care about cost of creating/killing threads on specific OS?

  8. #18
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    Also, as a side note, it has been mentioned many times that Gentoo is not just for ricers.
    +1

    a distribution that allows you to dynamically alter package dependencies (while resolving dependencies) according to your needs is not something you come across everyday. its package manager also transparently handles binary, from-source or development snapshot (from git,svn,hg,darcs,cvs, etc) packages in the same manner.

  9. #19
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    Quote Originally Posted by smitty3268 View Post
    march=core2 won't include anything past SSE2 by default. You have to add -mssse3, -mssse4.1, and -mssse4.2 to get the special instructions that won't work on your A64. I think the standard instructions on those chips are identical, unless you get into stuff like special hardware VM support.

    Really, SSE2+ support is only going to affect a few applications anyway, and they've probably got special flags and optimizations setup within the ebuild or program. I'm pretty sure MPlayer, for example, contains lots of manual assembly code and detects what CPU you have at runtime and picks it's fastest paths available.
    That was precisely the thing I was counting on, that by and large march=core2 implies generic x86_64 along with some sse stuff (just like march=pentium4 is apparently equivalent to march=i686 plus mmmx, msse, msse2). What I was unsure was to what extent. My athlon64 has upto sse3 (pni) and afaik, all core2 chips have ssse3 (at least my merom t5270 and penryn p8700 have). Anyway, looks like I am good . Although what bothers me is that if these optimizations play around with cache workings and if it affects performance by a lot. (For example, when choosing processor-family in kernel config, between generic x86-64 and core2, some of the parameters changed are CONFIG_X86_L1_CACHE_BYTES, CONFIG_X86_INTERNODE_CACHE_BYTES, CONFIG_X86_L1_CACHE_SHIFT. Interestingly, changing between core2 and k8, the only difference in the config is CONFIG_X86_P6_NOP.)

  10. #20
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    Quote Originally Posted by FunkyRider View Post
    What happens when a Gentoo user wants to change his CPU+Motherboard?
    I have switched from a P4 to a Core2Quad and I setup both boxes with Gentoo.
    The only tricky thing is to find the correct options for the kernel. Compiling a kernel with only the options you need is always a nightmare.
    The rest of installation is really easy, and, with a Core2Quad, really fast...

    I hope seeing soon the GCC4.4 on Gentoo, which is one of the rare distro to be able to handle that optimization, as other generic distro are compiled for generic i586...

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