[Feat] Add partial support for loops (ForStatement) inside action bodies

[Vineet1101]

This PR provides partial implementation for p4lang/project-ideas#33 by adding support for ForStatement execution inside BMv2 action bodies.

Proposed Changes

1. backends/bmv2/common/check_unsupported.h:
   • Removed the ForStatement rejection block.
   • Updated the ForInStatement error message to be more specific (since for-in loops still rely on midend lowering that is not yet fully supported).
2. backends/bmv2/common/action.cpp:
   • Added ForStatement preorder handling to convertActionBody().
   • Loops compile cleanly to JSON using a condition label check and a backward unconditional _jump.
3. testdata/p4_16_samples/forloop-bmv2.p4:
   • Added a rigorous test asserting the behavior of standard for-loops, nested for-loops, and conditionals wrapped inside loops.

Verification & Proof of Compilation

The changes successfully lower P4 loop statements into valid BMv2 JSON structures. Below is the generated primitive stack from our test program forloop-bmv2.p4:

1. Simple Loop (action sum_loop)

for (bit<8> i = 1; i <= h.h.count; i = i + 1) {
    h.h.sum = h.h.sum + (bit<32>)i;
}

Compiled BMv2 JSON Primitives:

  [0] assign(h.sum, 0)
  [1] assign(scalars.i_0, 1)          <--- Init
  [2] _jump_if_zero(expr, jump_to=6)  <--- Condition (if false, jump to end)
  [3] assign(h.sum, expr)             <--- Body
  [4] assign(scalars.i_0, expr)       <--- Update
  [5] _jump(jump_to=2)                <--- Jump back to Condition
  [6] assign(standard_metadata.egress_spec, 0)

2. Nested Loops (action nested_loop)

for (bit<8> i = 0; i < h.h.count; i = i + 1) {
    for (bit<8> j = 0; j < h.h.count; j = j + 1) {
        h.h.product = h.h.product + 1;
    }
}

Compiled BMv2 JSON Primitives:

  [0] assign(h.product, 0)
  [1] assign(scalars.i_1, 0)          <--- Outer Init
  [2] _jump_if_zero(expr, jump_to=10) <--- Outer Condition
  [3] assign(scalars.j_0, 0)          <--- Inner Init
  [4] _jump_if_zero(expr, jump_to=8)  <--- Inner Condition
  [5] assign(h.product, expr)         <--- Inner Body
  [6] assign(scalars.j_0, expr)       <--- Inner Update
  [7] _jump(jump_to=4)                <--- Inner Jump back
  [8] assign(scalars.i_1, expr)       <--- Outer Update
  [9] _jump(jump_to=2)                <--- Outer Jump back

All existing BMv2 tests and validations pass with P4TEST_REPLACE=1.

[Vineet1101]

@fruffy @jafingerhut can you please review it

[fruffy]

Could you supply some stf test cases with the program to demonstrate various kind of packet input/output scenarios? Your example calculates sums based on the table entries and input header values, we can use that

[jafingerhut]

Do these changes support break or continue statements?

[Vineet1101]

Do these changes support break or continue statements?

I was trying to implement forstatement first if this gets merged I will implement other two statements. If you want I can implement them as well in this PR

[Vineet1101]

Could you supply some stf test cases with the program to demonstrate various kind of packet input/output scenarios? Your example calculates sums based on the table entries and input header values, we can use that

ok let me try

[Vineet1101]

Do these changes support break or continue statements?

now it supports break and continue as well

[Vineet1101]

Could you supply some stf test cases with the program to demonstrate various kind of packet input/output scenarios? Your example calculates sums based on the table entries and input header values, we can use that

done can you review it

[jafingerhut]

Looks like several test failures in the latest version of the changes.

[jafingerhut]

Looks like several test failures in the latest version of the changes.

OK, the C++ lint / formatting checks should be something you can fix with clang-format, but the testgen failures might be due to p4testgen not supporting for loops?

@fruffy - I am not here asking you to support for loops in p4testgen any time soon, as that sounds like too big of a request. Would you recommend marking test programs using for loops with XFAIL for now?

[jafingerhut]

Looks like several test failures in the latest version of the changes.

OK, the C++ lint / formatting checks should be something you can fix with clang-format, but the testgen failures might be due to p4testgen not supporting for loops?

@fruffy - I am not here asking you to support for loops in p4testgen any time soon, as that sounds like too big of a request. Would you recommend marking test programs using for loops with XFAIL for now?

FYI, there are already P4 test programs that have for loops in them that are exercised by p4testgen, but I believe all of the existing ones are ones that a p4c pass written by Chris Dodd completely unroll at compile time, so there are no for lops in the IR by the time the back end sees it. Perhapt p4testgen can unroll or otherwise handle the same loops that p4c pass unrolls?

I suspect this PR has P4 test programs that are not unrolled by that pass, because the number of iterations is not possible (or not easy enough) to determine at compile time.

[fruffy]

Perhapt p4testgen can unroll or otherwise handle the same loops that p4c pass unrolls?

Yes, the easiest approach is to just add the pass that unrolls loops and things should work as expected. At some point, if we want to support differential testing, we can support stepping through loops in the interpreter.

@Vineet1101 You can either try to add the loops unrolling pass to P4Testgen's BMv2 mid end here: https://github.com/p4lang/p4c/blob/main/backends/p4tools/modules/testgen/targets/bmv2/target.cpp#L112

Or add the failing tests to XFails.