linux-kernel - Re: [RFC] Design for flag bit outputs from asms

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Message-ID: <5547D64B.6060708@zytor.com>
Date:	Mon, 04 May 2015 13:27:55 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Richard Henderson <rth@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vladimir Makarov <vmakarov@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC] Design for flag bit outputs from asms

On 05/04/2015 01:14 PM, H. Peter Anvin wrote:
>>
>> Therefore:
>>
>> (1) Each target defines a set of constraint strings,
>>
>>    E.g. for x86, wherein we're almost out of constraint letters,
>>
>>      ja   aux carry flag
>>      jc   carry flag
>>      jo   overflow flag
>>      jp   parity flag
>>      js   sign flag
>>      jz   zero flag
>>
> 
> 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.
>  There are 16 such conditions, 8 if we discard the inversions.
> 
> It is notable that the auxiliary carry flag has no Jcc/SETcc/CMOVcc
> instructions; it is only ever consumed by the DAA/DAS instructions which
> makes it pointless to try to model it in a compiler any more than, say, IF.
> 

OK, let me qualify that.  This is only necessary if it is impractical
for gcc to optimize boolean combinations of flags.  If such
optimizations are available then it doesn't matter and is probably
needlessly complex.  For example:

char foo(void)
{
	bool zf, sf, of;

	asm("xyzzy" : "=jz" (zf), "=js" (sf), "=jo" (of));

	return zf || (sf != of);
}

... should compile to ...

	xyzzy
	setng %al
	ret

	-hpa


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