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

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Date:	Tue, 18 Sep 2007 15:12:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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

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.

	-hpa

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