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Mailbox / semaphore (Tier A #5)

Status: partial — compiler builtins for Accellera-style IPC (int messages).

Choice: compiler builtins (not library classes)

IEEE mailbox / semaphore are built-in classes. A pure SV library cannot implement the required API here because:

Functions cannot take output/ref ports (try_get(v) needs a side-effecting write).
Class event properties are not supported yet (needed for efficient wait/wakeup).

This fork therefore implements native builtins, similar in shape to string / queue method sfuncs: empty netclass_t types named mailbox / semaphore, methods lowered to $ivl_mailbox$* / $ivl_semaphore$*, and vvp opcodes (%mbx/*, %sem/*) with real thread suspend/resume.

The older IVL_UVM “poor-man’s” classes (ivl_uvm_mbx, ivl_uvm_semaphore) remain in uvm/ for that library’s own tests; new Accellera-shaped code should use the builtins.

Supported in this slice

mailbox #(int) mb = new();   // #(T) accepted; element type is int/vec for v1
mb.put(7);
mb.get(v);                   // blocking
ok = mb.try_put(8);
ok = mb.try_get(v);          // nonblocking; writes v on success
n = mb.num();

semaphore sem = new(1);
sem.get(1);                  // blocking
sem.put(1);
ok = sem.try_get(1);

Also: unbounded new() / new(0), bounded mailbox new(N), blocking put when full, peek / try_peek.

Encoding

Layer	Role
Parse / pform	Builtin typedefs `mailbox` / `semaphore`; `mailbox #(T) x` as var decl (T ignored for now)
Netlist	Empty `netclass_t` “mailbox” / “semaphore”
Elab	`$ivl_mailbox$new/put/get/...`, `$ivl_semaphore$new/get/put/try_get`
Codegen	`%mbx/`, `%sem/`, `%box/vec4` / `%unbox/vec4` for int payloads
Runtime	`vvp_mailbox` / `vvp_semaphore` (`vvp/vvp_mailbox.{h,cc}`)

Deferred (do not claim)

True parameterized element types (mailbox #(string), class handles) beyond boxing ints
Explicit specialization of user param-classes interacting with mailbox
peek stress / priority / typed message checking
Replacing IVL_UVM’s ivl_uvm_mbx usages with the builtin

Example

examples/mailbox_sem/mbx_sem_basic.sv — prints PASSED.

./install/bin/iverilog -g2012 -o /tmp/mbx_sem.vvp examples/mailbox_sem/mbx_sem_basic.sv
./install/bin/vvp /tmp/mbx_sem.vvp

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