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    SubjectRe: Is gcc thread-unsafe?
    On Wed, 24 Oct 2007 21:29:56 -0700
    "David Schwartz" <davids@webmaster.com> wrote:

    >
    > > Well that's exactly right. For threaded programs (and maybe even
    > > real-world non-threaded ones in general), you don't want to be
    > > even _reading_ global variables if you don't need to. Cache misses
    > > and cacheline bouncing could easily cause performance to completely
    > > tank in some cases while only gaining a cycle or two in
    > > microbenchmarks for doing these funny x86 predication things.
    >
    > For some CPUs, replacing an conditional branch with a conditional
    > move is a *huge* win because it cannot be mispredicted.

    please name one...
    Hint: It's not one made by either Intel or AMD in the last 4 years...


    > In general,
    > compilers should optimize for unshared data since that's much more
    > common in typical code. Even for shared data, the usual case is that
    > you are going to access the data few times, so pulling the cache line
    > to the CPU is essentially free since it will happen eventually.

    it's not about pulling it to the CPU, it's pulling it *out* of all the
    other cpus AS WELL. (and writing it back to memory, taking away memory
    bandwidth)


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