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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	David Woodhouse <dwmw2@infradead.org>,
	Arnd Bergmann <arnd@arndb.de>, Dmitry Golovin <dima@golovin.in>,
	Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] x86: support i386 with Clang
Date: Mon, 11 May 2020 11:12:32 -0700	[thread overview]
Message-ID: <CAHk-=wif=_ZomkWJAmQRCUAMHQ72V3NEQ-OteiPE56K7KoSjbQ@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdmspKUknbzDn9kY2jMgkFw=Ktvst0ZtwambDOfybqJGWw@mail.gmail.com>

On Mon, May 11, 2020 at 10:24 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Bumping for comment+review.
>
> On Mon, May 4, 2020 at 4:03 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > -                   : "qi" ((pto_T__)(val)));           \
> > +                   : "qi" ((unsigned char)(unsigned long)(val))); \

I find the extra casts really annoying and illogical.

I kind of see why they happen: just casting to 'unsigned char' can
cause warnings, when this case isn't even taken, and the sizeof(var)
is something else (and you cast a pointer to a char without going
through the 'unsigned long' first.

But that doesn't make this change any more sensible or readable.

Would using "__builtin_choose_expr()" be able to avoid this whole issue?

Because I'd rather re-write the whole thing with a different model
that doesn't have this issue, than make the code look insane.

And it does look insane, because that whole thing makes for a very
natural question: "why do we cast to unsigned long and then to char,
when we just checked that the size of the type is 1"?

                 Linus

  parent reply	other threads:[~2020-05-11 18:12 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-04 23:03 [PATCH] x86: support i386 with Clang Nick Desaulniers
2020-05-11 17:23 ` Nick Desaulniers
2020-05-11 18:09   ` Brian Gerst
2020-05-11 18:46     ` Nick Desaulniers
2020-05-11 19:34       ` Brian Gerst
2020-05-11 20:18         ` Nick Desaulniers
2020-05-11 22:54         ` Brian Gerst
2020-05-11 18:12   ` Linus Torvalds [this message]
2020-05-11 18:24     ` Linus Torvalds
2020-05-11 18:36       ` Linus Torvalds
2020-05-11 19:52       ` Nick Desaulniers
2020-05-11 20:01         ` Linus Torvalds
2020-05-11 20:03           ` David Woodhouse
2020-05-12 20:35             ` Nick Desaulniers

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