From mboxrd@z Thu Jan  1 00:00:00 1970
From: Edward Cree <ecree@solarflare.com>
Subject: Re: [PATCH v2 1/3] bpf: Use 1<<16 as ceiling for immediate alignment
 in verifier.
Date: Tue, 16 May 2017 13:37:42 +0100
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To: David Miller <davem@davemloft.net>, <daniel@iogearbox.net>
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On 15/05/17 17:04, David Miller wrote:
> If we use 1<<31, then sequences like:
>
> 	R1 = 0
> 	R1 <<= 2
>
> do silly things.
Hmm.  It might be a bit late for this, but I wonder if, instead of handling
 alignments as (1 << align), you could store them as -(1 << align), i.e.
 leading 1s followed by 'align' 0s.
Now the alignment of 0 is 0 (really 1 << 32), which doesn't change when
 left-shifted some more.  Shifts of other numbers' alignments also do the
 right thing, e.g. align(6) << 2 = (-2) << 2 = -8 = align(6 << 2).  Of
 course you do all this in unsigned, to make sure right shifts work.
This also makes other arithmetic simple to track; for instance, align(a + b)
 is at worst align(a) | align(b).  (Of course, this bound isn't tight.)
A number is 2^(n+1)-aligned if the 2^n bit of its alignment is cleared.
Considered as unsigned numbers, smaller values are stricter alignments.

-Ed