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

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Date:   Sat, 16 Jul 2022 11:46:59 +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 Biesheuval <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>
Subject: Re: [PATCH] arm: lib: implement aeabi_uldivmod via div64_u64_rem

On Sat, Jul 16, 2022 at 2:16 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> Compilers frequently need to defer 64b division to a libcall with this
> symbol name. It essentially is div64_u64_rem, just with a different
> signature. Kernel developers know to call div64_u64_rem, but compilers
> don't.
>
> Link: https://lore.kernel.org/lkml/20220524004156.0000790e@garyguo.net/
> Suggested-by: Gary Guo <gary@...yguo.net>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

This has historically been strongly NAK'd, and I don't think that position
has changed in the meantime. A variable-argument 64-bit division is
really expensive, especially on 32-bit machines that lack a native
32-bit division instruction, and we don't want developers to accidentally
insert one in their driver code.

Explicitly calling one of the division helpers in linux/math64.h is the
established way for driver writers to declare that a particular division
cannot be turned into a cheaper operation and is never run in a
performance critical code path. The compiler of course cannot know
about either of those.

        Arnd