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    SubjectRe: [GIT PULL] optimize 64-by-32 ddivision for constant divisors on 32-bit machines
    Date
    On Monday 16 November 2015 20:20:38 you wrote:
    > Arnd,
    >
    > Please pull the following branch:
    >
    > git://git.linaro.org/people/nicolas.pitre/linux div64
    >
    > This contains those patches I've initially posted here:
    >
    > https://lkml.org/lkml/2015/11/2/715
    >
    > Only changes to those posted patches are cosmetic improvements such as
    > the use of ilog2() replacing the custom __div64_ffs(). Exposure in
    > linux-next would be a good thing.
    >
    > I also included fixes for a couple do_div() misuses that an allyesconfig
    > build turned up after switching ARM to the generic do_div() code.
    > Those patches have been posted separately and addressed to relevant
    > maintainers. They are included here until/unless those maintainers
    > include those patches in their tree.
    >
    > Original cover letter:
    >
    > This is a generalization of the optimization I produced for ARM a decade
    > ago to turn constant divisors into a multiplication by the divisor
    > reciprocal. Turns out that after all those years gcc is still not
    > optimizing things on its own for that case.
    >
    > This has important performance benefits as discussed in this thread:
    >
    > https://lkml.org/lkml/2015/10/28/851
    >
    > This series brings the formerly ARM-only optimization to all 32-bit
    > architectures using C code by default. The possibility for the actual
    > multiplication to be implemented in assembly is provided in order to get
    > optimal code. The ARM version can be used as an example implementation
    > for other interested architectures to implement.

    I'm now getting a build regressing with the attached randconfig configuration,
    when compiling drivers/net/wireless/iwlegacy/common.o:

    drivers/built-in.o: In function `il_send_rxon_timing':
    :(.text+0xbbac80): undefined reference to `__aeabi_uldivmod'
    :(.text+0xbbac9c): undefined reference to `__aeabi_uldivmod'
    :(.text+0xbbacdc): undefined reference to `__aeabi_uldivmod'
    :(.text+0xbbadc8): undefined reference to `__aeabi_uldivmod'
    :(.text+0xbbadf8): undefined reference to `__aeabi_uldivmod'
    :(.text+0xbbae3c): more undefined references to `__aeabi_uldivmod' follow
    drivers/built-in.o: In function `il_send_rxon_timing':
    :(.text+0xbbb11c): undefined reference to `____ilog2_NaN'

    I've verified that this goes away if I turn off CONFIG_PROFILE_ALL_BRANCHES,
    and it only shows up with gcc-5.0 through gcc-5.2.1, but not 4.9.3.

    Aside from those, I have not been able to reduce the failure scenario.

    Also, I have only tested on ARM32, no idea if this shows up elsewhere.

    Arnd[unhandled content-type:application/gzip]
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