Messages in this thread | | | Date | Wed, 19 Sep 2007 07:00:53 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 4/7] Immediate Values - i386 Optimization |
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* H. Peter Anvin (hpa@zytor.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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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