On Fri, February 17, 2012 05:09, H. Peter Anvin wrote: > On 02/16/2012 07:27 PM, Indan Zupancic wrote: >> >> A JIT won't be able to merge accesses because it also has to merge other >> instructions and recognize when 64-bit operations are done with 32-bit >> instructions. I think that will be too hard for a JIT. >> > > Please Google "peephole optimizer". I have written one for uni. Like I said, I think it will be too hard for a BPF JIT because the pattern is too complex. Keep in mind that there is no 64-bit register where you can load the data to, everything is done on 32-bit values. So you have to recognize 32-bit code emulating 64-bit ops. I don't think anyone will add all the different patterns of doing that to the JIT, there are too many. The current JIT is networking-only and is very simplistic. It is a very long way to a sophisticated enough JIT that does such complex peephole optimisations. I'm not saying it's impossible in general, just that the kernel BPF JIT won't be able to do it. It's a lot easier to just add 64-bit support to BPF instead. Greetings, Indan -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
