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    SubjectRe: hammer: MAP_32BIT


    Ulrich Drepper wrote:
    > -----BEGIN PGP SIGNED MESSAGE-----
    > Hash: SHA1
    >
    > Timothy Miller wrote:
    >
    >
    >>If your program is capable of handling an address with more than 32
    >>bits, what point is there giving a hint? Either your program can handle
    >>64-bit pointers or it cannot. Any program flexible enough to handle
    >>either size dynamically would expend enough overhead checking that it
    >>would be worse than if it just made a hard choice.
    >
    >
    > Look at the x86-64 context switching code. If memory addressed by the
    > GDT entries has a 32-bit address it uses a different method than for
    > cases where the virtual address has more than 32 bits. This way of
    > handling GDT entries is faster according to ak. So, it's not a
    > correctness thing, it's a performance thing.
    >

    Alright. Sounds great. So my next question is this:

    Why does there ever need to be an explicit HINT that you would prefer a
    <32 bit address, when it's known a priori that <32 is better? Why
    doesn't the mapping code ALWAYS try to use 32-bit addresses before
    resorting to 64-bit?

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