Re: [PATCH] x86: use builtins to read eflags

From: Thomas Gleixner <tglx@linutronix.de>
To: Bill Wendling <morbo@google.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Juergen Gross <jgross@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	llvm@lists.linux.dev
Cc: Bill Wendling <morbo@google.com>
Subject: Re: [PATCH] x86: use builtins to read eflags
Date: Thu, 16 Dec 2021 01:57:01 +0100	[thread overview]
Message-ID: <87mtl1l63m.ffs@tglx> (raw)
In-Reply-To: <20211215211847.206208-1-morbo@google.com>

Bill,

On Wed, Dec 15 2021 at 13:18, Bill Wendling wrote:

please always CC the relevant mailing lists, i.e. this lacks a cc:
linux-kernel@vger.kernel.org

It's not that hard to figure that out because it's well documented.

> GCC and Clang both have builtins to read from and write to the
> EFLAGS register. This allows the compiler to determine the best way
> to generate the code, which can improve code generation.

Emphasis on *can*. Just claiming that this might improve things does not
cut it. Where is the prove?

IIRC, this was proposed before and the real reason was not better code
generation but to address the confusion of clang vs. the '=rm'
constraint which is still correct despite some clang folks having
different opinions.

So what has changed since then? AFAICT, nothing. So I consider this as
another attempt of "let's see whether it sticks".

Thanks,

        tglx