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    SubjectRe: gcc -O2 vs gcc -Os performance
    Date
    In article <263740000.1044563891@[10.10.2.4]>,
    Martin J. Bligh <mbligh@aracnet.com> wrote:
    >>> All done with gcc-2.95.4 (Debian Woody). These machines (16x NUMA-Q) have
    >>> 700MHz P3 Xeons with 2Mb L2 cache ... -Os might fare better on celeron
    >>> with a puny cache if someone wants to try that out
    >>
    >> gcc 3.2 is a lot smarter about -Os and it makes a very big size
    >> difference according to the numbers the from the ACPI guys.
    >>
    >> Im not sure testing with a gcc from the last millenium is useful 8)
    >
    >Still no use.
    >/me throws gcc-3.2 in the trash can.
    >
    >2901299 vmlinux.O2
    >2667827 vmlinux.Os

    Well, Os is certainly smaller. One thing to look out for is that
    microbenchmarks for kernels are usually the _worst_ things to test with
    Os.

    That's since a large part of the premise of the -Os speed advantage is
    that it is better for icache (usually not an issue for microbenchmarks)
    and that it is better for load/startup times (generally not a huge issue
    for kernels, since the real startup costs of kernels tend to be entirely
    elsewhere).

    So I suspect -Os tends to be more appropriate for user-mode code, and
    especially code with low repeat rates. Possibly the "low repeat rate"
    thing ends up being true of certain kernel subsystems too.

    Think of it this way: if you win 10% in size, you're likely to map and
    load 10% less code pages at run-time. Which is not a big issue for
    traditional data-centric loads, but can be a _huge_ deal for things like
    GUI programs etc where there is often more code than data.

    Linus
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