SyncValsverifier → artifact → classifier → verdict
SyncVals · Trajectory

round-robin-arbiter-grant

claude-code claude-opus-4-8 ✓ resolved GOOD_SUCCESS ↑ View task
Solved from the instruction alone, tests/ and solution/ were withheld from the agent's workspace and restored only for grading.
Reward = tests/test.sh exit code (0 → resolved); the classification below is post-hoc and cannot change it.
Classification , post-hoc; cannot change the reward
GOOD_SUCCESSLegitimate solve, implements the asked-for behavior; tests verify real functionality.
SubtypeLegitimate Solution
EvidenceVerifier output shows `TEST reference_equivalence: PASS (all 4583 samples matched reference)` and `Mismatches: 0 in 4583 samples`. Agent trajectory shows it created `/app/dut.sv` with a real round-robin state machine using `base`, `hold`, and `held`, including dead-air handling (`if (req[held]) ... else grant 0`) and lock-release behavior (`base <= held + 2'd1`).
Root causeThe agent correctly implemented the specified four-requester round-robin arbiter with lock hold, dead-air, idle, and synchronous reset semantics. The tests compare behavior against a reference over deterministic and random stimulus and the implementation matched every sampled cycle.
RecommendationN/A - task is fine
Trajectory
Tool-by-tool agent trajectory
3 tool calls · 2 tool types · 8 steps
write a verilog module TopModule sharing one resource among four requesters. ports: clk, rst, req[3:0], lock, outputs grant[3:0], grant_valid. grant is one-hot; grant_valid marks cycles it names someone. the very first time all four ask at once, requester 0 wins. fairness is a circular queue: the one just served goes to the back. walk up from just past the last winner, wrapping 3 to 0; grant the first asker. all four asking forever gives 0,1,2,3,0,1,... a lone requester keeps winning; idle keeps your place. lock high on a granted cycle latches that winner: grant stays nailed to it, grant_valid high, while lock stays high; everyone else waits. lock with no grant does nothing. when lock drops the held winner still owns that cycle; sharing resumes next cycle just past it. a hold lives only while the held requester keeps asking. a held cycle with its req low, even the cycle lock drops, is dead air: grant 0, grant_valid 0, whoever else asks; the hold is over. sharing resumes next cycle just past the dead winner, it counts as served; a lock still high needs no re-arm, latching the next real winner by the start rule. rst is synchronous active-high: clears any hold, restarts the order. write plain synthesizable systemverilog to /app/dut.sv; no preprocessor directives, system tasks, or force/release; ascii only. graded by iverilog -g2012 against a hidden reference every cycle.
Looking at this spec, I need to build a round-robin arbiter with lock-hold and dead-air semantics. Let me reason through the state machine and write it.
write · /app/dut.sv
module TopModule (
    input  logic       clk,
    input  logic       rst,
    input  logic [3:0] req,
    input  logic       lock,
    output logic [3:0] grant,
    output logic       grant_valid
);

    // State: base = where the next round-robin search starts.
    //        hold/held = currently latched onto held requester by lock.
    logic [1:0] base;
    logic       hold;
    logic [1:0] held;

    // Round-robin search: first asker walking up from base, wrapping 3->0.
    logic [1:0] winner;
    logic       found;
    logic [1:0] idx;

    always_comb begin
        found  = 1'b0;
        winner = 2'd0;
        idx    = 2'd0;
        for (int k = 0; k < 4; k++) begin
            idx = base + k;          // truncates to 2 bits => mod-4 wrap
            if (req[idx] && !found) begin
                found  = 1'b1;
                winner = idx;
            end
        end
    end

    // Outputs are combinational (Mealy): grant reacts to req/lock this cycle.
    always_comb begin
        grant       = 4'b0000;
        grant_valid = 1'b0;
        if (!rst) begin
            if (hold) begin
                // A hold lives only while the held requester keeps asking.
                if (req[held]) begin
                    grant       = (4'b0001 << held);
                    grant_valid = 1'b1;
                end
                // else: dead air -> grant 0, grant_valid 0
            end else begin
                if (found) begin
                    grant       = (4'b0001 << winner);
                    grant_valid = 1'b1;
                end
                // else: idle -> no grant
            end
        end
    end

    // Registered state, synchronous active-high reset.
    always_ff @(posedge clk) begin
        if (rst) begin
            base <= 2'd0;
            hold <= 1'b0;
            held <= 2'd0;
        end else if (hold) begin
            if (req[held] && lock) begin
                // Keep the hold nailed to the same winner.
                hold <= 1'b1;
                held <= held;
                base <= base;
            end else begin
                // Hold ends: lock drop cycle (req still high) or dead air
                // (req low). Either way it counts as served; resume past it.
                hold <= 1'b0;
                base <= held + 2'd1;
            end
        end else begin
            if (found) begin
                if (lock) begin
                    // Latch this winner; base set when the hold later ends.
                    hold <= 1'b1;
                    held <= winner;
                    base <= base;
                end else begin
                    hold <= 1'b0;
                    base <= winner + 2'd1;
                end
            end else begin
                // No grant: lock does nothing, idle keeps your place.
                hold <= 1'b0;
                base <= base;
            end
        end
    end

endmodule
Let me verify it compiles with iverilog and simulates correctly with a quick self-test.
shell
cat > tb.sv <<'EOF'
module tb;
  logic clk, rst, lock;
  logic [3:0] req, grant;
  logic gv;
  TopModule d(.clk(clk),.rst(rst),.req(req),.lock(lock),.grant(grant),.grant_valid(gv));
  initial clk=0; always #5 clk=~clk;
  task step; @(posedge clk); #1; $display("req=%b lock=%b -> grant=%b gv=%b (rst=%b)",req,lock,grant,gv,rst); endtask
  initial begin
    rst=1; req=0; lock=0; step;
    // round robin all four
    rst=0; req=4'b1111; lock=0;
    step; step; step; step; step; // expect 0,1,2,3,0
    // lone requester bit2
    req=4'b0100; step; step; // expect 2,2
    // idle keeps place
    req=0; step;             // no grant
    req=4'b1111; step; step; // resumes: last winner was 2 -> base3 -> 3, then 0
    // lock hold on winner
    req=4'b1111; lock=1; step; // grant winner (base was1 ->1) latch
    lock=1; step;              // held stays
    req=4'b1111; lock=0; step; // drop cycle: held still owns
    step;                      // resumes past held
    // dead air: hold then req low
    req=4'b1111; lock=1; step; // latch a winner
    req=4'b0000; lock=1; step; // dead air even lock high
    req=4'b1111; lock=0; step; // resumes past dead winner, lock high case next
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim dut.sv tb.sv && vvp sim
output
req=0000 lock=0 -> grant=0000 gv=0 (rst=1)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
req=1111 lock=0 -> grant=0100 gv=1 (rst=0)
req=1111 lock=0 -> grant=1000 gv=1 (rst=0)
req=1111 lock=0 -> grant=0001 gv=1 (rst=0)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
req=0100 lock=0 -> grant=0100 gv=1 (rst=0)
req=0100 lock=0 -> grant=0100 gv=1 (rst=0)
req=0000 lock=0 -> grant=0000 gv=0 (rst=0)
req=1111 lock=0 -> grant=0001 gv=1 (rst=0)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
req=1111 lock=1 -> grant=0010 gv=1 (rst=0)
req=1111 lock=1 -> grant=0010 gv=1 (rst=0)
req=1111 lock=0 -> grant=0100 gv=1 (rst=0)
req=1111 lock=0 -> grant=1000 gv=1 (rst=0)
req=1111 lock=1 -> grant=1000 gv=1 (rst=0)
req=0000 lock=1 -> grant=0000 gv=0 (rst=0)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
tb.sv:27: $finish called at 176 (1s)

[stdout]
req=0000 lock=0 -> grant=0000 gv=0 (rst=1)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
req=1111 lock=0 -> grant=0100 gv=1 (rst=0)
req=1111 lock=0 -> grant=1000 gv=1 (rst=0)
req=1111 lock=0 -> grant=0001 gv=1 (rst=0)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
req=0100 lock=0 -> grant=0100 gv=1 (rst=0)
req=0100 lock=0 -> grant=0100 gv=1 (rst=0)
req=0000 lock=0 -> grant=0000 gv=0 (rst=0)
req=1111 lock=0 -> grant=0001 gv=1 (rst=0)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
req=1111 lock=1 -> grant=0010 gv=1 (rst=0)
req=1111 lock=1 -> grant=0010 gv=1 (rst=0)
req=1111 lock=0 -> grant=0100 gv=1 (rst=0)
req=1111 lock=0 -> grant=1000 gv=1 (rst=0)
req=1111 lock=1 -> grant=1000 gv=1 (rst=0)
req=0000 lock=1 -> grant=0000 gv=0 (rst=0)
req=1111 lock=0 -> grant=0010 gv=1 (rst=0)
tb.sv:27: $finish called at 176 (1s)
The displayed values are shifted because my testbench samples *after* the clock edge (post-`#1`), while grant is Mealy , it reflects `base` for the *current* cycle before the edge. Let me re-sample correctly to confirm the true per-cycle sequence.
shell
cat > tb2.sv <<'EOF'
module tb;
  logic clk, rst, lock;
  logic [3:0] req, grant; logic gv;
  TopModule d(.clk(clk),.rst(rst),.req(req),.lock(lock),.grant(grant),.grant_valid(gv));
  initial clk=0; always #5 clk=~clk;
  // sample just BEFORE the rising edge (steady-state combinational value)
  task drive(input [3:0] r, input l); req=r; lock=l; #4; $display("cyc req=%b lock=%b => grant=%b gv=%b rst=%b",req,lock,grant,gv,rst); @(posedge clk); #1; endtask
  initial begin
    #1;
    drive(4'b1111,0); // rst still 1 here? set below
  end
  initial begin
    rst=1; req=0; lock=0; #2;
    @(posedge clk); #1; rst=0;
    drive(4'b1111,0); // expect 0
    drive(4'b1111,0); // 1
    drive(4'b1111,0); // 2
    drive(4'b1111,0); // 3
    drive(4'b1111,0); // 0
    drive(4'b0100,0); // 2 lone
    drive(4'b0100,0); // 2
    drive(4'b0000,0); // idle none
    drive(4'b1111,0); // last winner 2 -> 3
    drive(4'b1111,0); // 0
    drive(4'b1111,1); // grant 1, latch
    drive(4'b1111,1); // hold 1
    drive(4'b1111,0); // drop cycle owns 1
    drive(4'b1111,0); // resume -> 2
    drive(4'b1111,1); // grant 3? latch
    drive(4'b0000,1); // dead air, none
    drive(4'b1111,0); // resume past dead -> next
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim2 dut.sv tb2.sv && vvp sim2 2>/dev/null | grep cyc
output
cyc req=1111 lock=0 => grant=0000 gv=0 rst=1
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0
cyc req=1111 lock=0 => grant=0010 gv=1 rst=0
cyc req=1111 lock=0 => grant=0100 gv=1 rst=0
cyc req=1111 lock=0 => grant=1000 gv=1 rst=0
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0
cyc req=0100 lock=0 => grant=0100 gv=1 rst=0
cyc req=0100 lock=0 => grant=0100 gv=1 rst=0
cyc req=0000 lock=0 => grant=0000 gv=0 rst=0
cyc req=1111 lock=0 => grant=1000 gv=1 rst=0
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0
cyc req=1111 lock=1 => grant=0010 gv=1 rst=0
cyc req=1111 lock=1 => grant=0010 gv=1 rst=0
cyc req=1111 lock=0 => grant=0010 gv=1 rst=0
cyc req=1111 lock=0 => grant=0100 gv=1 rst=0
cyc req=1111 lock=1 => grant=1000 gv=1 rst=0
cyc req=0000 lock=1 => grant=0000 gv=0 rst=0
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0

[stdout]
cyc req=1111 lock=0 => grant=0000 gv=0 rst=1
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0
cyc req=1111 lock=0 => grant=0010 gv=1 rst=0
cyc req=1111 lock=0 => grant=0100 gv=1 rst=0
cyc req=1111 lock=0 => grant=1000 gv=1 rst=0
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0
cyc req=0100 lock=0 => grant=0100 gv=1 rst=0
cyc req=0100 lock=0 => grant=0100 gv=1 rst=0
cyc req=0000 lock=0 => grant=0000 gv=0 rst=0
cyc req=1111 lock=0 => grant=1000 gv=1 rst=0
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0
cyc req=1111 lock=1 => grant=0010 gv=1 rst=0
cyc req=1111 lock=1 => grant=0010 gv=1 rst=0
cyc req=1111 lock=0 => grant=0010 gv=1 rst=0
cyc req=1111 lock=0 => grant=0100 gv=1 rst=0
cyc req=1111 lock=1 => grant=1000 gv=1 rst=0
cyc req=0000 lock=1 => grant=0000 gv=0 rst=0
cyc req=1111 lock=0 => grant=0001 gv=1 rst=0
Now every rule checks out cycle-by-cycle: - **First all-four → 0 wins**, then round-robin `0,1,2,3,0`. - **Lone requester** (bit 2) keeps winning `2,2`. - **Idle** produces no grant and keeps its place (next grant resumes past the last real winner `2` → `3`). - **Lock hold**: winner `1` latched, grant stays `0010` while lock high; the lock-drop cycle still owns `1`; sharing resumes next cycle at `2`. - **Dead air**: after latching winner `3`, `req` goes low with lock still high → `grant 0000, gv 0`; the held winner counts as served, so sharing resumes past it → `0`. The design (`/app/dut.sv`) is Mealy , grant reacts to `req`/`lock` in the same cycle , with three synchronous, active-high-reset state registers (`base`, `hold`, `held`). It compiles clean under `iverilog -g2012`, uses only synthesizable SystemVerilog, no preprocessor/system-tasks/force-release, ASCII only. Reset drives outputs to 0 and restarts the order at requester 0.
Final diff , what the agent changed

No file changes, the agent left the workspace untouched.

Verifier output , the proof behind the reward
tests/test.sh
TEST dut_present: PASS (/app/dut.sv exists)
TEST dut_forbidden: PASS (no forbidden constructs)
TEST dut_elaborates: PASS (DUT elaborates standalone)
TEST dut_compiles: PASS (compiled with held-back reference/testbench)
TEST simulation_completed: PASS (Mismatches: 0 in 4583 samples)
TEST reference_equivalence: PASS (all 4583 samples matched reference)
{"score": 1.000000, "results_file": "/tmp/h2t-grader-home.jdpzXn/runtime/cb_verifier_318-8986-6871-11861_318/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_442110d4592f400f. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_442110d4592f400f · verifier authoritative; classifier explanatory.