On Mon, Jul 8, 2024, at 03:21, Nicolas Pitre wrote:
> On Sun, 7 Jul 2024, Arnd Bergmann wrote:
>> On Sun, Jul 7, 2024, at 21:14, Nicolas Pitre wrote:
>
> Oh, most likely yes. The non-constant base has to go through the whole
> one-bit-at-a-time division loop whereas the constant base with
> __div64_const32 results in 4 64-bits multiply and add. Moving
> __arch_xprod_64() out of line adds the argument shuffling overhead and
> it can't skip overflow handling, but still.
>
> Here's some numbers. With latest patches using __always_inline:
>
> test_div64: Starting 64bit/32bit division and modulo test
> test_div64: Completed 64bit/32bit division and modulo test, 0.048285584s elapsed
>
> Latest patches but __always_inline left out:
>
> test_div64: Starting 64bit/32bit division and modulo test
> test_div64: Completed 64bit/32bit division and modulo test, 0.053023584s elapsed
>
> Forcing both constant and non-constant base through the same path:
>
> test_div64: Starting 64bit/32bit division and modulo test
> test_div64: Completed 64bit/32bit division and modulo test, 0.103263776s elapsed
>
> It is worth noting that test_div64 does half the test with non constant
> divisors already so the impact is greater than what those numbers show.
>
> And for what it is worth, those numbers were obtained using QEMU. The
> gcc version is 14.1.0.
Right, so with the numbers in qemu matching your explanation,
that seems reasonable to assume it will behave the same way
across a wide range of physical CPUs.
Arnd
