On 16/05/17 23:53, Alexei Starovoitov wrote: > following this line of thinking it feels that it should be possible > to get rid of 'aux_off' and 'aux_off_align' and simplify the code. > I mean we can always do > dst_reg->min_align = min(dst_reg->min_align, src_reg->min_align); > > and don't use 'off' as part of alignment checks at all. Problem with this approach, of course, is that (say) NET_IP_ALIGN + sizeof(ethhdr) = 16 is muchly aligned, whereas if you turn all constants into alignments you think you're only 2-byte aligned. I think you have to track exact offsets when you can, and only turn into an alignment when you introduce a variable. Of course it can still be fooled by e.g. 2 + (x << 2) + 14, which it will think is only 2-aligned when really it's 4-aligned, but unless you want to start tracking 'bits known to be 1' as well as 'bits known to be 0', I think you just accept that alignment tracking isn't commutative. The obvious cases (ihl << 2 and so) will work when written the obvious way, unless the compiler does something perverse. OTOH the 'track known 1s as well' might work in a nice generic way and cover all bases, I'll have to experiment a bit with that.
-Ed
