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Circuit frontend (emp-tool/circuits/frontend/)

A small, optional layer for writing a pure circuit once and running it on any context — plaintext, garbled 2PC, ZK, … . You write the circuit in ordinary typed circuit-value code (Bit_T<Ctx>, UInt_T<Ctx,N>, Int_T<Ctx,N>, Float_T<Ctx,W>); the frontend lets you call it live, or compile it once into a reusable, context-free Circuit and run it on whatever context you hold — with no global backend and no per-bit virtual dispatch.

Everything lives in namespace emp::frontend (so run / compile don't pollute emp). Header-only over emp-tool; C++20. Pull it in with #include "emp-tool/circuits/frontend/frontend.h" (or directly #include "emp-tool/circuits/frontend/circuit_fn.h").

To reuse a compiled circuit across many calls without materializing each copy (replay one unit by reference instead of inlining it), see composition.md — the run / run_compose inline-vs-reuse pattern.

This is the BooleanContext frontend: it compiles and replays over the typed context-bound values (Bit_T<Ctx> / UInt_T<Ctx,N> / …), with the context passed explicitly and no global backend.

For the typed values it builds on, read circuits.md; for the gate-context concept it replays over, read the header of ir/context/context.h; for the session that owns I/O around it, read ir/session/session.h.

What a circuit is — a pure function over a context

A circuit body takes typed circuit values as arguments and returns a typed value; it does no I/O of its own (no input/reveal/OT). This is enforced structurally: a body is recorded through a RecordCtx, which has no I/O.

  • No secret input inside — pass secret/party inputs as arguments.
  • No reveal inside — reveal the returned value outside, through the session.
  • Public constants inside are fine — a.constant(5) (implicit form) or UInt_T<Ctx,N>::constant(ctx, 5) (explicit form) fold to constant gates. A value that may differ across parties or runs must be an argument.

I/O stays the session's job, around the circuit:

ClearSession sess;                                    // session owns the I/O boundary
using Ctx = ClearSession::ctx_t;                      // a protocol session (e.g. SH2PCSession) exposes the same surface
using UInt32 = UInt_T<Ctx, 32>;
auto a = sess.input<UInt32>(ALICE, av);               // session feeds inputs
auto b = sess.input<UInt32>(BOB,   bv);
auto c = frontend::run(sess.ctx(), circuit, a, b);          // pure replay over the context
uint32_t r = sess.reveal<uint32_t>(c, PUBLIC).value(); // session reveals -> std::optional<uint32_t>

Body forms

A body comes in two forms; compile/run detect which is invocable. A body callable in both is a contract error (disambiguate).

  • implicit context (default) — the typed values carry their own context, so the body needs none; make a constant from an argument with a.constant(v):

    auto add = [](auto a, auto b) { return a + b; };

  • explicit context (general) — the body takes Ctx& first. Required for nullary circuits and for making a constant with no argument to anchor on:

    auto bias = [](auto& ctx) {
    using C = std::remove_reference_t<decltype(ctx)>;
    return UInt_T<C,16>::constant(ctx, 4242);
};

The calling contract

One diagnostic site (circuit_fn_traits / circuit_contract in circuit_fn.h): a body must be callable with prvalue circuit-value arguments and return a circuit value by value. Arguments are passed by value, so a body cannot mutate them (a non-const lvalue-reference parameter is rejected). Returning a reference, returning void, returning a non-circuit value, taking a non-circuit argument, or being callable in both context forms are all compile-time errors with a precise message — in compile<ArgVs...> and live run alike.

A circuit value is anything satisfying the WireBundle concept (ir/wire_value.h): it exposes Wire, context_type, a static width(), context(), pack_wires / from_wires, and a rebind<Ctx> that maps the same value family onto another context. Adding the clear codec (clear_t / encode / decode) makes it the stronger WireValue — the form session I/O needs; the frontend itself requires only WireBundle (a clear codec is not needed to compile/run). The five built-in families (Bit_T, BitVec_T, UInt_T, Int_T, Float_T in circuits/typed.h) model WireBundle at fixed width; Bit_T / BitVec_T / Float_T also model WireValue at every width, while UInt_T / Int_T model WireValue only for width <= 64 (their clear codecs ride 64-bit integers) — use BitVec_T for typed session I/O past 64 bits. The runtime-width forms UInt_T<Ctx,0> / Int_T<Ctx,0> (width chosen at construction) intentionally do not model WireBundle — they have no static width() — so they cannot be a compile / run argument; convert to a fixed UInt_T<Ctx,N> (to_fixed<N>()) first if a runtime-width result must enter the frontend. (They remain session-I/O eligible through the runtime-width input / reveal overloads — they model RuntimeWidthValue.)

Recording value types — context-free signatures

compile is parameterized by the circuit value types over the recording context (RecordCtx). The emp::rec:: aliases (circuits/frontend/rec.h) name those types without spelling RecordCtx:

value (per context) recording alias (emp::rec::)
Bit_T<Ctx> rec::Bit (= Bit_T<RecordCtx>)
UInt_T<Ctx,N> rec::UInt<N> (= UInt_T<RecordCtx,N>)
Int_T<Ctx,N> rec::Int<N> (= Int_T<RecordCtx,N>)
Float_T<Ctx,W> rec::Float<W> (= Float_T<RecordCtx,W>)
BitVec_T<Ctx,N> rec::BitVec<N> (= BitVec_T<RecordCtx,N>)

The metadata a compiled signature needs — bit width and the per-context family map — lives on the value type itself (width(), rebind<Ctx>). The clear codec (clear_t, encode/decode) is NOT part of the compiled signature (compile/run needs only WireBundle, as above); it is used at the session I/O boundary, and where present it is exposed uniformly through emp::value_traits<T> (circuits/value_traits.h): value_traits<T>::width(), value_traits<T>::encode(v), value_traits<T>::decode(bits), value_traits<T>::rebind<Ctx>. A value's rebind<Ctx> re-attaches a context (UInt_T<RecordCtx,32>::rebind<ClearCtx> == UInt_T<ClearCtx,32>); run uses it to reconstruct the live result type.

compile / run

#include "emp-tool/circuits/frontend/circuit_fn.h"
#include "emp-tool/circuits/frontend/rec.h"
#include "emp-tool/ir/session/clear_session.h"
namespace cf = emp::frontend;
using namespace emp;

auto add  = [](auto a, auto b) { return a + b; };
auto circ = cf::compile<rec::UInt<32>, rec::UInt<32>>(add);   // record ONCE

ClearSession sess;                                            // run on any session's context
using Ctx = ClearSession::ctx_t;
auto x = sess.input<UInt_T<Ctx,32>>(ALICE, 7);
auto y = sess.input<UInt_T<Ctx,32>>(BOB,   5);
auto z = cf::run(sess.ctx(), circ, x, y);                     // replay -> UInt_T<Ctx,32>
  • compiled — compile<ArgVs...>(body) records the body once through a RecordCtx and returns a Circuit<RetV, ArgVs...> (the ArgVs are value types over RecordCtx; use the rec:: aliases). run(ctx, circ, args...) replays it on the live ctx (args by const-ref, rebound to that ctx; the context is explicit — no global backend). The same Circuit runs identically on the plaintext session's context, SH2PCCtx, etc. — user circuits are as portable as the built-in .empbc files.
  • live — run(body, args...) invokes the body directly on already-live typed values (it recovers the context from the first argument). Same contract.

compile is host-side and deterministic: all parties compile the identical program, then replay it in lockstep. (A Float_T body inlines its fp<W>_*.empbc gates into the recording, so a recorded float circuit is semantically — not necessarily gate-for-gate — equal to the standalone builtin.)

The compiled circuit object

compile returns a Circuit<RetV, ArgVs...> wrapping a circuit::CircuitArtifact (the flat BooleanProgram + a CircuitSignature of argument widths and the return width). The argument/return template parameters are value types over RecordCtx. The artifact is private and immutable; the constructor validates the program structurally and that the signature matches the declared value widths, so a stale or mis-typed loaded artifact is rejected at construction rather than silently mis-running. Accessors: circ.program(), circ.signature().

No analyses are baked into the circuit — gate counts, liveness, and the AND-depth schedule are free functions over the program (ir/passes.h), computed when wanted.

What's inside (internals)

  • circuits/typed.h — the typed values Bit_T/BitVec_T/UInt_T/Int_T/Float_T<Ctx> (each carries width()/clear_t/encode/decode/rebind<Ctx> inline) plus the bare-wire arithmetic kernels in emp::kernel.
  • circuits/value_traits.h — value_traits<T>: the uniform metadata accessor (width, clear codec, rebind<Ctx>) over a circuit value's own static members.
  • circuits/frontend/rec.h — rec::Bit/rec::BitVec<N>/rec::UInt<N>/rec::Int<N>/rec::Float<W>, the value types over RecordCtx used as compile arguments.
  • ir/wire_value.h — the generic WireBundle concept (the structural value contract) and WireValue (WireBundle + the clear codec).
  • ir/context/context.h — a convenience umbrella re-exporting the BooleanContext concept and the contexts ClearCtx (plaintext) and RecordCtx (records a BooleanProgram), plus the CountCtx / DigestCtx analysis helpers; the value-return replay it re-exports — execute_program(ctx, prog, inputs, ws) and ProgramWorkspace — is defined in ir/execute.h.
  • ir/artifact.h — CircuitArtifact (program + signature) + validate_artifact.
  • circuits/frontend/circuit_fn.h — the RecordValue concept (refining WireBundle), circuit_fn_traits / circuit_contract, Circuit<RetV,ArgVs...>, compile, run.
  • ir/passes.h — analyses over the IR (count_pass, liveness_pass, schedule_pass, layout_pass) as free functions.

Adding a new backend

Define a type satisfying the BooleanContext concept (a std::semiregular Wire plus value-return public_bit/and_gate/xor_gate/not_gate). Every compiled circuit then replays on it via run(ctx, circ, …) with no frontend changes — the generic replay walks the gate list issuing the context's gate ops. A round-sensitive protocol (e.g. GMW) gets efficiency by consuming the program's AND-depth schedule (a BulkBooleanContext + scheduled_execute_program), not the scalar replay.

Tests

  • test/circuits/test_circuit_fn.cpp — compile-once / run-on-any-context on ClearCtx (incl. both body forms, a nullary circuit, .constant(), fp32), plus the size-optimal 31-AND adder and deterministic recording.
  • test/circuits/circuit_fn_contract_probes.cpp — the contract's positive case + the negative cases that must fail to compile with the expected message.
  • emp-sh2pc/test/test_circuit_fn_sh2pc.cpp — the same compiled Circuit run two-party over the garbled SH2PCCtx (uint32 + fp32).