Support for typeof keyword

[mihaibudiu]

Personnel

• Owner: @mbudiu
• Supporters: @vgurevich

Design

• Document: this PR

Implementation

• p4-spec: this PR
• p4c: Experimental support for typeof keyword p4c#3017

Process

• LDWG discussed:
• LDWG approved:
• Merged into p4-spec:
• Merged into p4c:

[mihaibudiu]

This would be only really useful in conjunction with #1011. If #1011 is not accepted, a different design should be used for this feature. The reason is that typedef declarations being global confines the use of typeof only for constant expressions, and P4 currently does not have constant expressions of some types, such as header stacks.

[jnfoster]

We discussed this feature at the April '22 LDWG meeting.

• Folks should try this feature out, as well as Allow local typedef declarations #1011 and come with feedback for the May meeting.
• @mbudiu-vmw will also prototype a version that allows typeof anywhere a type can be indicated... if Bison allows it.

[jnfoster]

@vgurevich especially, please take a look!

[vgurevich]

@jnfoster -- noted. I'll try to refactor some programs and at least compile them with p4-test backend.