From: Linus Torvalds <torvalds@linux-foundation.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Richard Henderson <rth@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Vladimir Makarov <vmakarov@redhat.com>,
Jakub Jelinek <jakub@redhat.com>, Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Borislav Petkov <bp@alien8.de>,
"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: [RFC] Design for flag bit outputs from asms
Date: Mon, 4 May 2015 13:35:59 -0700 [thread overview]
Message-ID: <CA+55aFzCuu=n0SFiVyxv7ETxFTYZCBW-0S_RX=WZnwzORh4-7g@mail.gmail.com> (raw)
In-Reply-To: <5547D30B.2020507@zytor.com>
On Mon, May 4, 2015 at 1:14 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>
> I would argue that for x86 what you actually want is to model the
> *conditions* that are available on the flags, not the flags themselves.
Yes. Otherwise it would be a nightmare to try to describe simple
conditions like "le", which a rather complicated combination of three
of the actual flag bits:
((SF ^^ OF) || ZF) = 1
which would just be ridiculously painful for (a) the user to describe
and (b) fior the compiler to recognize once described.
Now, I do admit that most of the cases where you'd use inline asm with
condition codes would probably fall into just simple "test ZF or CF".
But I could certainly imagine other cases.
Linus
next prev parent reply other threads:[~2015-05-04 20:36 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-01 15:16 [PATCH] x86: Optimize variable_test_bit() Peter Zijlstra
2015-05-01 16:03 ` Linus Torvalds
2015-05-01 16:16 ` Peter Zijlstra
2015-05-01 16:29 ` Peter Zijlstra
2015-05-01 16:18 ` Peter Zijlstra
2015-05-01 16:33 ` Jakub Jelinek
2015-05-01 16:45 ` Linus Torvalds
2015-05-01 16:46 ` Peter Zijlstra
2015-05-01 17:17 ` Ingo Molnar
2015-05-01 19:02 ` Vladimir Makarov
2015-05-01 20:49 ` Linus Torvalds
2015-05-01 22:22 ` Vladimir Makarov
2015-05-02 12:39 ` Peter Zijlstra
2015-05-04 15:37 ` Richard Henderson
2015-05-04 19:33 ` [RFC] Design for flag bit outputs from asms Richard Henderson
2015-05-04 20:14 ` H. Peter Anvin
2015-05-04 20:27 ` H. Peter Anvin
2015-05-04 20:33 ` Richard Henderson
2015-05-04 20:45 ` Linus Torvalds
2015-05-04 20:57 ` Richard Henderson
2015-05-04 21:23 ` H. Peter Anvin
2015-05-04 20:35 ` Linus Torvalds [this message]
2015-05-04 20:42 ` H. Peter Anvin
2015-05-05 9:01 ` Gabriel Paubert
2015-05-05 13:50 ` Segher Boessenkool
2015-05-05 15:37 ` Linus Torvalds
2015-05-05 16:10 ` Segher Boessenkool
2015-05-02 12:43 ` [PATCH] x86: Optimize variable_test_bit() Peter Zijlstra
2015-05-04 18:07 ` Vladimir Makarov
2015-05-04 20:14 ` H. Peter Anvin
2015-05-04 13:42 ` Peter Zijlstra
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