Messages in this thread | | | Date | Wed, 19 Sep 2007 07:08:46 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 4/7] Immediate Values - i386 Optimization |
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* Andi Kleen (andi@firstfloor.org) wrote: > Jeremy Fitzhardinge <jeremy@goop.org> writes: > > > > It's a pity that gas seems to generate plain 0x90 nops rather than > > long-nop forms here. I thought it could do that. > > .p2align does it. >
Sadly, p2align does not apply well to my context. I have to align on 4 bytes boundaries - 1 for the 4 bytes mov, so, if I would use p2align, I would end up aligning on 4 bytes with p2align and then add 3 bytes (worse case: adding 3 + 3 = 6 bytes of nops).
However, with the .org arithmetic, I can simply add then quantity of nops needed to make my alignment on 4 bytes - 1, so the worse case becomes adding 3 bytes.
The example is:
originally: address & 3 = 1 * p2align p2align adds 3 bytes to align on 4 bytes boundaries we add 3 bytes to align on the next 4 bytes - 1, so the immediate value within the instruction is aligned on 4 bytes boundaries
* org we add 2 bytes to be aligned on the next 4 bytes - 1.
And yes, it's a pity there is no way to produce the long-nops there. :(
Mathieu
> -Andi
-- 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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