Re: [PATCH] ARM: NWFPE: avoid compiler-generated __aeabi_uldivmod

From: "Arnd Bergmann" <arnd@kernel.org>
To: "Nick Desaulniers" <ndesaulniers@google.com>,
	"Russell King" <linux@armlinux.org.uk>
Cc: "Tom Rix" <trix@redhat.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	"Miguel Ojeda" <ojeda@kernel.org>,
	"Ard Biesheuvel" <ardb@kernel.org>, "Gary Guo" <gary@garyguo.net>,
	"Craig Topper" <craig.topper@sifive.com>,
	"Philip Reames" <listmail@philipreames.com>,
	jh@jhauser.us, "Nathan Chancellor" <nathan@kernel.org>
Subject: Re: [PATCH] ARM: NWFPE: avoid compiler-generated __aeabi_uldivmod
Date: Tue, 11 Oct 2022 13:01:22 +0200	[thread overview]
Message-ID: <679c0129-ca59-47a4-aa83-f79ec166951a@app.fastmail.com> (raw)
In-Reply-To: <20221010225342.3903590-1-ndesaulniers@google.com>

On Tue, Oct 11, 2022, at 12:53 AM, Nick Desaulniers wrote:
> clang-15's ability to elide loops completely became more aggressive when
> it can deduce how a variable is being updated in a loop. Counting down
> one variable by an increment of another can be replaced by a modulo
> operation.
>
> For 64b variables on 32b ARM EABI targets, this can result in the
> compiler generating calls to __aeabi_uldivmod, which it does for a do
> while loop in float64_rem().
>
> For the kernel, we'd generally prefer that developers not open code 64b
> division via binary / operators and instead use the more explicit
> helpers from div64.h. On arm-linux-gnuabi targets, failure to do so can
> result in linkage failures due to undefined references to
> __aeabi_uldivmod().
>
> While developers can avoid open coding divisions on 64b variables, the
> compiler doesn't know that the Linux kernel has a partial implementation
> of a compiler runtime (--rtlib) to enforce this convention.
>
> It's also undecidable for the compiler whether the code in question
> would be faster to execute the loop vs elide it and do the 64b division.
>
> While I actively avoid using the internal -mllvm command line flags, I
> think we get better code than using barrier() here, which will force
> reloads+spills in the loop for all toolchains.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1666
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Works for me,

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

I would have been fine with disallowing NWFPE for clang, or with
adding a barrier in the loop as well, i.e. any approach that
doesn't cause invalid behavior or a maintenance burden, given that
there is probably nobody that actually needs nwfpe on a clang built
kernel.

    Arnd

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