[P4-dev] Where should the operations on arbitrary-precision integers be declared?

Antonin Bas antonin at barefootnetworks.com
Mon Jun 20 16:17:10 EDT 2016


What is the latest commit in your behavioral-model clone? Because I added
modulo support this morning...

On Mon, Jun 20, 2016 at 1:12 PM, Javier <richardqa at gmail.com> wrote:

> Hi Antonin,
>
> Thank you so much for the reply,
>
> I tried to replicate the code simple_router.p4 with p4v1.1 using the ideas
> that you comment: "http://pastebin.com/hukUDq4e"
>
> As you see, I added one new action "myaction" and one new table "c".
>
> From command.txt, I just add one entry:
>
> table_add c myaction 0 => 5 3 0
>
> Now, in the action ""set_nhop", I changed this line:
>
> +    ipv4.ttl = ipv4.ttl - 1;
> -     ipv4.ttl = ipv4.ttl - routing_metadata.f1;
>
> Now, I execute run_demo.sh and all ok, but when I do ping between h1 and
> h2  I should get this value 62? (64 - 5mod3), However i didn't get to ping
> between them (no reply).
> If I change the operator '%' to '*' operator It works  64 - 3*5=49 when
> ping h1,h2.
>
> Could you tell me whether am I doing something bad please?
>
>
> Thank you in advance,
> Javier
>
>
>
>
> 2016-06-20 15:39 GMT-03:00 Antonin Bas <antonin at barefootnetworks.com>:
>
>> Hi,
>>
>>
>> I just noticed that modulo and divide where not supported by the P4
>> frontend, so I made the change. In P4v1.1, using modulo would look like
>> this:
>> action my_action(in int<32> a, in int<32> b) {
>>     modify_field(*h1.f1*, a % b);  // a >= 0, b > 0
>>     // or *h1.f1* = a % b;
>> }
>> The second syntax is supported by the p4-hlir compiler, but I don't think
>> it is described in the official spec.
>> This action can then be referenced by a table.
>> Really now that I have made the change in the frontend, using "modulo" is
>> no different from using "add" or "substract". See
>> https://github.com/p4lang/tutorials/blob/master/p4v1_1/simple_router/p4src/simple_router.p4#L129
>> for a P4v1.1 program.
>>
>> Thanks,
>>
>> Antonin
>>
>> On Mon, Jun 20, 2016 at 4:53 AM, Christian Esteve Rothenberg <
>> chesteve at dca.fee.unicamp.br> wrote:
>>
>>> Thanks Antonin for your comprehensive reply!
>>>
>>> Are you aware of any publicly available P4 code example using the
>>> modulo operation?
>>>
>>> Just a code snippet would suffice...
>>>
>>> thanks,
>>> Christian
>>>
>>> On Fri, Jun 17, 2016 at 5:05 PM, Antonin Bas
>>> <antonin at barefootnetworks.com> wrote:
>>> > Hi Javier,
>>> >
>>> > In P4v1.1, the only variables you have are fields (packet fields and
>>> > metadata fields), and they have global scope. These variables all have
>>> a
>>> > type which includes their bitwidth (with the exception of the very
>>> specific
>>> > variable-length field case). For example, the ethernet header can be
>>> > declared as follows:
>>> >
>>> > header_type ethernet_t {
>>> >     fields {
>>> >         bit<48> dstAddr;
>>> >         bit<48> srcAddr;
>>> >         bit<16> etherType;
>>> >     }
>>> > }
>>> >
>>> >
>>> > Notice that all three fields are bitstrings. One can also declare
>>> fields has
>>> > signed integers (int<w>), usually this is more useful for some metadata
>>> > fields.
>>> > As mentioned in the spec, you cannot declare fields a "int" (without a
>>> > width). This is reserved for evaluation of compile-time constants. The
>>> "int"
>>> > type is used by the compiler but is hidden from the P4 programmer. For
>>> > example, let's look at the following code:
>>> > ethernet.etherType = (bit<16>) (11111111 - 1);
>>> > The compiler will use the internal "int" type to represent compile-time
>>> > constants 11111111 and 1. It will also compute the difference,
>>> 11111110,
>>> > which is also of type "int". This is what we call "arbitrary-precision
>>> > arithmetic" since the compiler can handle arbitrarily large compile
>>> time
>>> > constants. Finally the compiler should give you a warning because
>>> 11111110
>>> > cannot be represented using 16 bits and the most significant bits will
>>> be
>>> > lost.
>>> >
>>> > Best,
>>> >
>>> > Antonin
>>> >
>>> >
>>> > On Thu, Jun 16, 2016 at 4:13 PM, Javier <richardqa at gmail.com> wrote:
>>> >>
>>> >> Hi everyone,
>>> >>
>>> >> I am trying to use the operations on "arbitrary-precison constant
>>> >> integers", however I dont get to know how to declare these constant
>>> >> integers. In the P4 spec for v1.1 tell that "the only values that can
>>> have
>>> >> the type int are compile-time constants" but it doesn't show how to
>>> use it.
>>> >> For example, i want to use the operator modulo to get the modulo of
>>> two
>>> >> variables, where and how should I declare these variables?.
>>> >>
>>> >> Thank in advance,
>>> >> Javier
>>> >>
>>> >>
>>> >> --
>>> >> Saludos Cordiales
>>> >> Javier
>>> >> Fingerprint: 7AF8 4E03 66C2 2663 3A77  7067 6B5A D8F9 EE91 47BE
>>> >>
>>> >> _______________________________________________
>>> >> P4-dev mailing list
>>> >> P4-dev at lists.p4.org
>>> >> http://lists.p4.org/mailman/listinfo/p4-dev_lists.p4.org
>>> >
>>> >
>>> >
>>> >
>>> > --
>>> > Antonin
>>> >
>>> > _______________________________________________
>>> > P4-dev mailing list
>>> > P4-dev at lists.p4.org
>>> > http://lists.p4.org/mailman/listinfo/p4-dev_lists.p4.org
>>>
>>
>>
>>
>> --
>> Antonin
>>
>
>
>
> --
> Saludos Cordiales
> Javier
> PhD Student (UNICAMP)
> http://www.linkedin.com/in/richardqa
> Fingerprint: 7AF8 4E03 66C2 2663 3A77  7067 6B5A D8F9 EE91 47BE
>



-- 
Antonin
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