linux-kernel - Re: [patch 4/7] Immediate Values

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Date:	Wed, 19 Sep 2007 06:59:51 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@....de>, "H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [patch 4/7] Immediate Values - i386 Optimization

* Jeremy Fitzhardinge (jeremy@...p.org) wrote:
> Mathieu Desnoyers wrote:
> 
> > +#define immediate_read(name)						\
> > +	({								\
> > +		__typeof__(name##__immediate) value;			\
> > +		switch (sizeof(value)) {				\
> > +		case 1:							\
> > +			asm (	".section __immediate, \"a\", @progbits;\n\t" \
> > +					".long %1, (0f)+1, 1;\n\t"	\
> > +					".previous;\n\t"		\
> > +					"0:\n\t"			\
> > +					"mov %2,%0;\n\t"		\
> 
> Given that you're relying on the exact instruction that this mov
> generates, it might be better to explicitly put the opcodes in with
> .byte.  That way you're protected from the assembler deciding to
> generate some other form of the instruction (for whatever reason).  I
> guess substituting in different registers would be a pain.
> 

Good point. I thought it might come up, especially for 16 bits mov that
can be expressed under different forms, one of which has a prefix. I
would like to go for Peter's suggestion: putting the label _after_ the
instruction, since we know that we will be right after the immediate
value, but it has a drawback: we cannot insure correct alignment of the
immediate value in that case. But that would help not having to force
the register.

> Aside from that,  is there any reason not to just put $0 in there rather
> than use %2?
> 

Actually, no, since the initial value is written to the immediate value
references at early boot and at module load time. I originally thought
passing the referenced variable to it, but, as I recall, it brought
linker issues when the symbol was defined in another module. So yes,
just $0 is ok, I'll change that.

> 
> > +					".long %1, (0f)+1, 4;\n\t"	\
> > +					".previous;\n\t"		\
> > +					"1:\n\t"			\
> > +					".org (1b)+(3-((1b)%%4)), 0x90;\n\t" \
> >   
> Seems a little complex, but I couldn't come up with anything much better:
> 
> 	.org . + 3 - (. & 3), 0x90
> 
> You can use . rather than needing to define 1:, it doesn't need quite so
> many parens, and using &3 avoids the %% wart. 
> 

Yes, this one is tricky.. trying to align efficiently something on a 4
bytes address - 1 is not what gas is used to help doing.

> It's a pity that gas seems to generate plain 0x90 nops rather than
> long-nop forms here.  I thought it could do that.
> 

At least we will have at most 3 nops there.

Mathieu

>     J

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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