linux-kernel - Re: [PATCH v2 4/4] __arch_xprod64(): make __always

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Message-Id: <6fa4d8af-0f9e-4525-adf6-8c3c3d059b2f@app.fastmail.com>
Date: Mon, 08 Jul 2024 07:21:02 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Nicolas Pitre" <nico@...xnic.net>
Cc: "Russell King" <linux@...linux.org.uk>,
 Linux-Arch <linux-arch@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] __arch_xprod64(): make __always_inline when optimizing for
 performance

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