Re: [PATCH] arch:um: Only disable SSE on clang to work around old GCC bugs

From: David Gow <davidgow@google.com>
To: Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	 Johannes Berg <johannes@sipsolutions.net>
Cc: linux-um@lists.infradead.org, x86@kernel.org,
	llvm@lists.linux.dev,  rust-for-linux@vger.kernel.org,
	kunit-dev@googlegroups.com,  linux-kernel@vger.kernel.org,
	SeongJae Park <sj@kernel.org>,
	 Roberto Sassu <roberto.sassu@huaweicloud.com>,
	Miguel Ojeda <ojeda@kernel.org>,
	 Nathan Chancellor <nathan@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	 Thomas Gleixner <tglx@linutronix.de>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH] arch:um: Only disable SSE on clang to work around old GCC bugs
Date: Fri, 31 Mar 2023 16:25:44 +0800	[thread overview]
Message-ID: <CABVgOS=pr=jXq4qRrKaCy47xczDaFXFKCPanVvaZ+MfQmtwK8A@mail.gmail.com> (raw)
In-Reply-To: <20230318041555.4192172-1-davidgow@google.com>

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On Sat, 18 Mar 2023 at 12:16, David Gow <davidgow@google.com> wrote:
>
> As part of the Rust support for UML, we disable SSE (and similar flags)
> to match the normal x86 builds. This both makes sense (we ideally want a
> similar configuration to x86), and works around a crash bug with SSE
> generation under Rust with LLVM.
>
> However, this breaks compiling stdlib.h under gcc < 11, as the x86_64
> ABI requires floating-point return values be stored in an SSE register.
> gcc 11 fixes this by only doing register allocation when a function is
> actually used, and since we never use atof(), it shouldn't be a problem:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652
>
> Nevertheless, only disable SSE on clang setups, as that's a simple way
> of working around everyone's bugs.
>
> Fixes: 884981867947 ("rust: arch/um: Disable FP/SIMD instruction to match x86")
> Reported-by: Roberto Sassu <roberto.sassu@huaweicloud.com>
> Link: https://lore.kernel.org/linux-um/6df2ecef9011d85654a82acd607fdcbc93ad593c.camel@huaweicloud.com/
> Tested-by: Roberto Sassu <roberto.sassu@huaweicloud.com>
> Tested-by: SeongJae Park <sj@kernel.org>
> Signed-off-by: David Gow <davidgow@google.com>
> ---

Any chance we could get this picked up as a fix for 6.3? The UML build
is still broken with older gcc versions.

Cheers,
-- David

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