Eric Lee / smarc-fsl-linux-kernel

26 Apr, 2007

1 commit

08 Dec, 2006

1 commit

  • fa4adc614   [ARM] 3611/4: optimize do_div() when divisor is constant ... Browse Code »

    On ARM all divisions have to be performed "manually". For 64-bit
    divisions that may take more than a hundred cycles in many cases.

    With 32-bit divisions gcc already use the recyprocal of constant
    divisors to perform a multiplication, but not with 64-bit divisions.

    Since the kernel is increasingly relying upon 64-bit divisions it is
    worth optimizing at least those cases where the divisor is a constant.
    This is what this patch does using plain C code that gets optimized away
    at compile time.

    For example, despite the amount of added C code, do_div(x, 10000) now
    produces the following assembly code (where x is assigned to r0-r1):

    adr r4, .L0
    ldmia r4, {r4-r5}
    umull r2, r3, r4, r0
    mov r2, #0
    umlal r3, r2, r5, r0
    umlal r3, r2, r4, r1
    mov r3, #0
    umlal r2, r3, r5, r1
    mov r0, r2, lsr #11
    orr r0, r0, r3, lsl #21
    mov r1, r3, lsr #11
    ...
    .L0:
    .word 948328779
    .word 879609302

    which is the fastest that can be done for any value of x in that case,
    many times faster than the __do_div64 code (except for the small x value
    space for which the result ends up being zero or a single bit).

    The fact that this code is generated inline produces a tiny increase in
    .text size, but not significant compared to the needed code around each
    __do_div64 call site this code is replacing.

    The algorithm used has been validated on a 16-bit scale for all possible
    values, and then recodified for 64-bit values. Furthermore I've been
    running it with the final BUG_ON() uncommented for over two months now
    with no problem.

    Note that this new code is compiled with gcc versions 4.0 or later.
    Earlier gcc versions proved themselves too problematic and only the
    original code is used with them.

    Signed-off-by: Nicolas Pitre
    Signed-off-by: Russell King

    Nicolas Pitre
     

17 Apr, 2005

1 commit

  • 1da177e4c   Linux-2.6.12-rc2 ... Browse Code »

    Initial git repository build. I'm not bothering with the full history,
    even though we have it. We can create a separate "historical" git
    archive of that later if we want to, and in the meantime it's about
    3.2GB when imported into git - space that would just make the early
    git days unnecessarily complicated, when we don't have a lot of good
    infrastructure for it.

    Let it rip!

    Linus Torvalds