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    SubjectRe: [patch 2/7] enable unit-at-a-time optimisations for gcc4
    Sam Ravnborg <> writes:

    First I must object to the thread/patch title. x86-64 always used
    unit-at-a-time when available. Other architectures used the compiler
    default which is on in newer gccs. Only a popular legacy architecture
    didn't. It should reflect that.

    > On Fri, Jan 06, 2006 at 12:18:42PM -0500, Jeff Garzik wrote:
    > >
    > > ACK, with a note: gcc also supports limited program-at-a-time -- you
    > > pass multiple .c files on the same command line, and specify a single
    > > output on the command line.
    > >
    > > It would be nice to update kbuild to do this for single directory
    > > modules....
    > How much will it gain?

    You just get cross inlining between .c files. Nothing more.
    Since the kernel doesn't use -O3 only functions marked inline would
    be considered for this. And the fraction of functions in .c
    files that are marked inline, but not static is probably very small.

    The feature also has some drawbacks - last time I checked it
    was still quite green (as in bananas). First it causes gcc
    to eat a lot more memory because it has to hold completely directories
    worth of source in memory. This might slow things down if setups
    that didn't swap before start doing this now.

    I suspect it'll also run slower with this because it has some algorithms
    that scale with the size of the input source and some of the
    directories in the kernel can be quite big (e.g. i'm not
    sure letting a optimizer lose on all of xfs at the same
    time is a good idea)

    And gcc is really picky about type compatibility between source files
    with program-at-a-time. If any types of the same symbols are
    incompatible even in minor ways you get an ICE. That's technically
    illegal, but tends to happen often in practice (e.g. when people
    use extern) It might end up being quite a lot of work to clean this up.

    I wouldn't bother implementing this right now - it's probably not
    worth it for the inlining.

    If gcc ever makes this feature usable and fixes the problems and
    actually learns more optimizations using it that could be

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