linux-kernel - Re: [PATCH] arm: lib: implement aeabi_uldivmod via div64_u64

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Message-Id: <ddc3156d-5580-4ec1-a218-2cdf5c1e9a48@app.fastmail.com>
Date:   Tue, 11 Oct 2022 00:13:08 +0200
From:   "Arnd Bergmann" <arnd@...nel.org>
To:     "Nick Desaulniers" <ndesaulniers@...gle.com>
Cc:     clang-built-linux <llvm@...ts.linux.dev>,
        "Nathan Chancellor" <nathan@...nel.org>,
        "Miguel Ojeda" <ojeda@...nel.org>,
        "Ard Biesheuvel" <ardb@...nel.org>, "Gary Guo" <gary@...yguo.net>,
        "Russell King" <linux@...linux.org.uk>,
        "Linux ARM" <linux-arm-kernel@...ts.infradead.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Craig Topper" <craig.topper@...ive.com>,
        "Philip Reames" <listmail@...lipreames.com>, jh@...user.us
Subject: Re: [PATCH] arm: lib: implement aeabi_uldivmod via div64_u64_rem

On Mon, Oct 10, 2022, at 11:23 PM, Nick Desaulniers wrote:
> On Sat, Jul 16, 2022 at 2:47 AM Arnd Bergmann <arnd@...nel.org> wrote:
>> On Sat, Jul 16, 2022 at 2:16 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/nwfpe/softfloat.c#n2312
> Any creative ideas on how to avoid this? Perhaps putting the `aSig -=
> bSig;` in inline asm? Inserting a `barrier()` or empty asm statement
> into the loops also seems to work.

I was going to suggest adding a barrier() as well, should have
read on first ;-)

> I see this function seems to be based on Berkeley Softfloat
> http://www.jhauser.us/arithmetic/SoftFloat.html
> v2.  v3 looks like a total rewrite.  Looking at v3e, it looks like
> float64_rem() is now called f64_rem() and defined in f64_rem.c.  It
> doesn't look like there's anything from v3 that we could backport to
> the kernel's v2 to avoid this.
>
> Otherwise perhaps we just disable OABI_COMPAT for clang. Quite a few
> defconfigs explicitly enable FPE_NWFPE though.  Are there really a lot
> of OABI binaries still in use?

I'd do the minimal thing here, neither NWFPE nor OABI_COMPAT should
be too relevant here. Russell still uses some machines with old OABI
binaries, but I have not heard from anyone else using those in a
very long time.

Note that while gcc can still produce kernels that are OABI binaries,
gcc-4.6 marked the arm-linux-gnu target as obsolete and gcc-4.8
removed it, so any existing binaries were built with toolchains
predating that.

     Arnd