On Thu, May 21, 2015 at 1:38 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote: > On 05/20/2015 02:21 PM, Denys Vlasenko wrote: >> So what we need is to put something like ".p2align 64,,7" >> before every function. >> >> ( >> Why 7? >> >> defconfig vmlinux (w/o FRAME_POINTER) has 42141 functions. >> 6923 of them have 1st insn 5 or more bytes long, >> 5841 of them have 1st insn 6 or more bytes long, >> 5095 of them have 1st insn 7 or more bytes long, >> 786 of them have 1st insn 8 or more bytes long, >> 548 of them have 1st insn 9 or more bytes long, >> 375 of them have 1st insn 10 or more bytes long, >> 73 of them have 1st insn 11 or more bytes long, >> one of them has 1st insn 12 bytes long: >> this "heroic" instruction is in local_touch_nmi() >> 65 48 c7 05 44 3c 00 7f 00 00 00 00 >> movq $0x0,%gs:0x7f003c44(%rip) >> >> Thus ensuring that at least seven first bytes do not cross >> 64-byte boundary would cover >98% of all functions. >> ) >> >> gcc can't do that right now. With -falign-functions=N, >> it emits ".p2align next_power_of_2(N),,N-1" >> >> We need to make it just a tiny bit smarter. >> >>> We'd need toolchain help to do saner alignment. >> >> Yep. >> I'm going to create a gcc BZ with a feature request, >> unless you disagree with my musings above. > > The BZ is here: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66240 ...and now this BZ has a working patch, which implements e.g. -falign-functions=64,7 -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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