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From: Richard Henderson <rth@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.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, 04 May 2015 13:57:13 -0700	[thread overview]
Message-ID: <5547DD29.2020805@redhat.com> (raw)
In-Reply-To: <CA+55aFz9jW_ii58pMtmVipEmDWfWak0eL47oH_nik4G97Jo72w@mail.gmail.com>

On 05/04/2015 01:45 PM, Linus Torvalds wrote:
> On Mon, May 4, 2015 at 1:33 PM, Richard Henderson <rth@redhat.com> wrote:
>>
>> A fair point.  Though honestly, I was hoping that this feature would mostly be
>> used for conditions that are "weird" -- that is, not normally describable by
>> arithmetic at all.  Otherwise, why are you using inline asm for it?
> 
> I could easily imagine using some of the combinations for atomic operations.
> 
> For example, doing a "lock decl", and wanting to see if the result is
> negative or zero. Sure, it would be possible to set *two* booleans (ZF
> and SF), but there's a contiional for "BE"..

Sure.

I'd be more inclined to support these compound conditionals directly, rather
than try to get the compiler to recognize them after the fact.

Indeed, I believe we have a near complete set of them in the x86 backend
already.  It'd just be a matter of selecting the spellings for the constraints.


r~

  reply	other threads:[~2015-05-04 20:57 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 [this message]
2015-05-04 21:23                     ` H. Peter Anvin
2015-05-04 20:35               ` Linus Torvalds
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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