Eric Lee / smarc-ti-linux-kernel

27 May, 2012

1 commit

  • 36126f8f2   word-at-a-time: make the interfaces truly generic ... Browse Code »

    This changes the interfaces in to be a bit more
    complicated, but a lot more generic.

    In particular, it allows us to really do the operations efficiently on
    both little-endian and big-endian machines, pretty much regardless of
    machine details. For example, if you can rely on a fast population
    count instruction on your architecture, this will allow you to make your
    optimized file with that.

    NOTE! The "generic" version in include/asm-generic/word-at-a-time.h is
    not truly generic, it actually only works on big-endian. Why? Because
    on little-endian the generic algorithms are wasteful, since you can
    inevitably do better. The x86 implementation is an example of that.

    (The only truly non-generic part of the asm-generic implementation is
    the "find_zero()" function, and you could make a little-endian version
    of it. And if the Kbuild infrastructure allowed us to pick a particular
    header file, that would be lovely)

    The functions are as follows:

    - WORD_AT_A_TIME_CONSTANTS: specific constants that the algorithm
    uses.

    - has_zero(): take a word, and determine if it has a zero byte in it.
    It gets the word, the pointer to the constant pool, and a pointer to
    an intermediate "data" field it can set.

    This is the "quick-and-dirty" zero tester: it's what is run inside
    the hot loops.

    - "prep_zero_mask()": take the word, the data that has_zero() produced,
    and the constant pool, and generate an *exact* mask of which byte had
    the first zero. This is run directly *outside* the loop, and allows
    the "has_zero()" function to answer the "is there a zero byte"
    question without necessarily getting exactly *which* byte is the
    first one to contain a zero.

    If you do multiple byte lookups concurrently (eg "hash_name()", which
    looks for both NUL and '/' bytes), after you've done the prep_zero_mask()
    phase, the result of those can be or'ed together to get the "either
    or" case.

    - The result from "prep_zero_mask()" can then be fed into "find_zero()"
    (to find the byte offset of the first byte that was zero) or into
    "zero_bytemask()" (to find the bytemask of the bytes preceding the
    zero byte).

    The existence of zero_bytemask() is optional, and is not necessary
    for the normal string routines. But dentry name hashing needs it, so
    if you enable DENTRY_WORD_AT_A_TIME you need to expose it.

    This changes the generic strncpy_from_user() function and the dentry
    hashing functions to use these modified word-at-a-time interfaces. This
    gets us back to the optimized state of the x86 strncpy that we lost in
    the previous commit when moving over to the generic version.

    Signed-off-by: Linus Torvalds

    Linus Torvalds
     

25 May, 2012

1 commit