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 -- 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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