Optimizer does not completely remove leading noop statements

[fenner]

When creating a simple filter for ICMP type 42, starting at the IPv4 layer:

filtertest IPv4 'icmp[icmptype] = 42'

we end up with a dangling instruction at the start:

(000) ld       #0x0
(001) ldb      [9]
(002) jeq      #0x1             jt 3    jf 9
(003) ldh      [6]
(004) jset     #0x1fff          jt 9    jf 5
...

This loads a constant 0, and then obviously overwrites it with the IP protocol number ([9]).

The pre-optimized code, from filtertest -O IPv4 'icmp[icmptype] = 42', makes it a little more clear what could be happening:

(000) ld       #0x0
(001) jeq      #0x0             jt 2    jf 23
(002) ld       #0x0
(003) jeq      #0x0             jt 4    jf 23
(004) ldb      [9]

My guess is it's trying to generate code to look at a link type, and since there is no link type, it's just generating "load 0, if 0, load 0, if 0". Unfortunately the optimizer fails to completely optimize this away.

I've tried this with 5bfcce9 since I know there was some recent optimizer work.

Unsurprisingly, the same happens with DLT_IPv6 - filtertest IPv6 'icmp6[icmptype] = 160'

[infrastation]

The origin of the first ld, which is a no-op, is from the icmp keyword, which implies IPv4. The latter means that the generator for DLTs that support more than one address family produces a check for the protocol family:

$ filtertest -O RAW 'icmp'
(000) ldb      [0]                                  ; IPv4
(001) and      #0xf0                                ; IPv4
(002) jeq      #0x40            jt 3	jf 6        ; IPV4
(003) ldb      [9]
(004) jeq      #0x1             jt 5	jf 6
(005) ret      #262144
(006) ret      #0

For DLTs that are IPv4-only the generator does not omit the check, but replaces it with the result of gen_true():

$ filtertest -O IPV4 'icmp'
(000) ld       #0x0
(001) jeq      #0x0             jt 2	jf 5        ; TRUE
(002) ldb      [9]
(003) jeq      #0x1             jt 4	jf 5
(004) ret      #262144
(005) ret      #0

Then, as you note, the optimizer removes the always-true jeq, which makes the ld a no-op, but does not remove the ld:

$ filtertest IPV4 'icmp'
(000) ld       #0x0                                 ; no-op
(001) ldb      [9]
(002) jeq      #0x1             jt 3	jf 4
(003) ret      #262144
(004) ret      #0

This is a specific case of #1022, so I am closing this as a duplicate and adding this case into the latter issue.