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

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Message-ID: <20070919110053.GB15500@Krystal>
Date:	Wed, 19 Sep 2007 07:00:53 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@....de>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [patch 4/7] Immediate Values - i386 Optimization

* H. Peter Anvin (hpa@...or.com) wrote:
> Jeremy Fitzhardinge 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.
> 
> Allowing different registers should be doable, but if so, one would have
> to put 0: at the *end* of the instruction and use (0f)-4 instead, since
> the non-%eax forms are one byte longer.
> 
> This also seems "safer", since an imm32 is always the last thing in the
> instruction.
> 

The idea is very interesting, but then, if I can't be sure of the size
of my instruction, how can I align the immediate value properly ?

> 	-hpa
> 

-- 
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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