can't use verify in parser

[jnfoster]

If I try to invoke verify (a built-in function whose type is currently defined in core.p4), in the parser using the bmv2 backend,

#include <v1model.p4>

struct h { }

struct m { }

parser MyParser(packet_in b, out h hdr, inout m meta, inout standard_metadata_t std) {
  state start {
    verify(true, error.NoError);
    transition accept;
  }
}

control MyVerifyChecksum(in h hdr, inout m meta) {
  apply {}
}
control MyIngress(inout h hdr, inout m meta, inout standard_metadata_t std) {
  apply { }
}
control MyEgress(inout h hdr, inout m meta, inout standard_metadata_t std) {
  apply { }
}

control MyComputeChecksum(inout h hdr, inout m meta) {
  apply {}
}
control MyDeparser(packet_out b, in h hdr) {
  apply { }
}

V1Switch(MyParser(), MyVerifyChecksum(), MyIngress(), MyEgress(), MyComputeChecksum(), MyDeparser()) main;

I get an error in the JSON converter:

vagrant@vagrant-ubuntu-trusty-64:~$ p4c-bm2-ss --p4-16 min.p4
min.p4(9): error: MethodCallStatement: not supported in parser on this target
    verify(true, error.NoError);
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^

The culprit seems to be that the implementation of JsonConverter::convertParserStatement is incomplete:
https://github.com/p4lang/p4c/blob/master/backends/bmv2/jsonconverter.cpp#L2111

[mihaibudiu]

As far as I know the behavioral simulator does not support implementing a verify function.
In particular, there is no way to specify errors in the user program.

[mihaibudiu]

Actually, since the error is invisible anywhere in v1model.p4, we can probably implement verify by just a conditional transition to reject.

[jnfoster]

At lunch today, Chris suggested that verify would be compiled down into
TCAM matches and then special handing for the transition to reject. Is this
what you mean?

-N

On Tue, Nov 8, 2016 at 4:56 PM, Mihai Budiu notifications@github.com
wrote:

Actually, since the error is invisible anywhere, we can probably implement
verify by just a conditional transition to reject.

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[jnfoster]

I'm getting a confusing warning message:

warning: SelectCase: unreachable case

The generated JSON looks like this:

{
  "program" : "141.p4",
  "header_types" : [
    {
      "name" : "standard_metadata_t",
      "id" : 1,
      "fields" : [
        ["ingress_port", 9, false],
        ["egress_spec", 9, false],
        ["egress_port", 9, false],
        ["clone_spec", 32, false],
        ["instance_type", 32, false],
        ["drop", 1, false],
        ["recirculate_port", 16, false],
        ["packet_length", 32, false],
        ["_padding", 4, false]
      ]
    }
  ],
  "headers" : [
    {
      "name" : "standard_metadata",
      "id" : 0,
      "header_type" : "standard_metadata_t",
      "metadata" : true
    }
  ],
  "header_stacks" : [],
  "field_lists" : [],
  "parsers" : [
    {
      "name" : "parser",
      "id" : 0,
      "init_state" : "start",
      "parse_states" : [
        {
          "name" : "start",
          "id" : 0,
          "parser_ops" : [],
          "transitions" : [
            {
              "value" : "default",
              "mask" : null,
              "next_state" : "start_join"
            }
          ],
          "transition_key" : []
        },
        {
          "name" : "start_join",
          "id" : 1,
          "parser_ops" : [],
          "transitions" : [
            {
              "value" : "default",
              "mask" : null,
              "next_state" : null
            }
          ],
          "transition_key" : []
        }
      ]
    }
  ],
  "deparsers" : [
    {
      "name" : "deparser",
      "id" : 0,
      "order" : []
    }
  ],
  "meter_arrays" : [],
  "counter_arrays" : [],
  "register_arrays" : [],
  "calculations" : [],
  "learn_lists" : [],
  "actions" : [
    {
      "name" : ".drop",
      "id" : 0,
      "runtime_data" : [],
      "primitives" : [
        {
          "op" : "drop",
          "parameters" : []
        }
      ]
    }
  ],
  "pipelines" : [
    {
      "name" : "ingress",
      "id" : 0,
      "init_table" : "MyIngress.exit",
      "tables" : [
        {
          "name" : "MyIngress.exit",
          "id" : 0,
          "key" : [
            {
              "match_type" : "exact",
              "target" : ["standard_metadata", "drop"]
            }
          ],
          "match_type" : "exact",
          "type" : "simple",
          "max_size" : 1,
          "with_counters" : false,
          "support_timeout" : false,
          "direct_meters" : null,
          "actions" : [".drop"],
          "action_ids" : [0],
          "next_tables" : {
            ".drop" : null
          }
        }
      ],
      "conditionals" : []
    },
    {
      "name" : "egress",
      "id" : 1,
      "init_table" : "MyEgress.exit",
      "tables" : [
        {
          "name" : "MyEgress.exit",
          "id" : 1,
          "key" : [
            {
              "match_type" : "exact",
              "target" : ["standard_metadata", "drop"]
            }
          ],
          "match_type" : "exact",
          "type" : "simple",
          "max_size" : 1,
          "with_counters" : false,
          "support_timeout" : false,
          "direct_meters" : null,
          "actions" : [".drop"],
          "action_ids" : [0],
          "next_tables" : {
            ".drop" : null
          }
        }
      ],
      "conditionals" : []
    }
  ],
  "checksums" : [],
  "force_arith" : [
    ["standard_metadata", "ingress_port"],
    ["standard_metadata", "egress_spec"],
    ["standard_metadata", "egress_port"],
    ["standard_metadata", "clone_spec"],
    ["standard_metadata", "instance_type"],
    ["standard_metadata", "drop"],
    ["standard_metadata", "recirculate_port"],
    ["standard_metadata", "packet_length"]
  ]
}

[mihaibudiu]

Yes, we should remove the warning.
This happens because the verify compiles into a select statement where the 'false' label is unreachable.

[mihaibudiu]

BTW: to see what this compiles into you can use the --top4 flag supplying a pass name.
To see all pass names run it first with -v.

[jnfoster]

Right. What's confusing is that my program didn't contain any select statements, and the parser in the generated JSON doesn't have it either. So it's purely an intermediate construct that is later compiled down...

  "parsers" : [
    {
      "name" : "parser",
      "id" : 0,
      "init_state" : "start",
      "parse_states" : [
        {
          "name" : "start",
          "id" : 0,
          "parser_ops" : [],
          "transitions" : [
            {
              "value" : "default",
              "mask" : null,
              "next_state" : "start_join"
            }
          ],
          "transition_key" : []
        },
        {
          "name" : "start_join",
          "id" : 1,
          "parser_ops" : [],
          "transitions" : [
            {
              "value" : "default",
              "mask" : null,
              "next_state" : null
            }
          ],
          "transition_key" : []
        }
      ]
    }
  ]

[mihaibudiu]

I have just pushed a commit to remove this warning; it's now on top of the existing pull request.

[jnfoster]

Looks good.