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: "TEST reference_equivalence: PASS (all 4583 samples matched reference)" and "Mismatches: 0 in 4583 samples". The trajectory shows the agent derived the required behavior from the prompt, wrote /app/dut.sv, and explicitly reasoned about/locally tested first all-four grant to requester 0, 0-1-2-3 rotation, lock hold/drop, dead-air, idle, and reset behavior. No evidence of reading hidden tests or solution files appears in the trajectory.

Root causeThe agent implemented the specified four-requester round-robin arbiter with the required lock and dead-air semantics, and the behavioral verifier accepted it against the hidden reference. The tests check cycle-level functional equivalence and reasonable source hygiene constraints that are consistent with the instruction.

RecommendationN/A - task is fine

Trajectory

Tool-by-tool agent trajectory

16 tool calls · 4 tool types · 32 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.

I'll analyze this arbiter spec carefully, then write the module. Key decisions from the spec: - **Mealy (combinational) outputs**: grant reacts to `req`/`lock` within the same cycle ("the cycle they ask", "the cycle lock drops", dead-air on `req` low). - **State**: `last` (last served, search starts just past it), `held`/`held_who` (lock latch). Reset → `last=3` so first search starts at 0. Let me reason through the tricky transitions before writing: - Latch cycle (lock high on a grant) → enter hold. - Hold + req high + lock high → continue; lock low → holder owns this cycle, release next. - Hold + req low → dead air (grant 0), counts as served, hold over; if lock still high, next real winner re-latches by the start rule.

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
);

  // Fairness / hold state
  logic [1:0] last;      // last requester served; search starts just past it
  logic       held;      // a locked hold is active
  logic [1:0] held_who;  // which requester is held

  // Combinational round-robin arbitration starting just past `last`
  logic [1:0] astart;
  logic       arb_valid;
  logic [1:0] arb_idx;

  always_comb begin
    arb_valid = 1'b0;
    arb_idx   = 2'd0;
    astart    = last + 2'd1;
    for (int k = 0; k < 4; k++) begin
      if (!arb_valid && req[astart + k[1:0]]) begin
        arb_valid = 1'b1;
        arb_idx   = astart + k[1:0];
      end
    end
  end

  // Mealy outputs
  always_comb begin
    grant       = 4'b0000;
    grant_valid = 1'b0;
    if (!rst) begin
      if (held) begin
        // Hold lives only while the held requester keeps asking.
        if (req[held_who]) begin
          grant       = 4'b0001 << held_who;
          grant_valid = 1'b1;
        end
        // else: dead air (grant 0, grant_valid 0)
      end else begin
        if (arb_valid) begin
          grant       = 4'b0001 << arb_idx;
          grant_valid = 1'b1;
        end
      end
    end
  end

  // State update
  always_ff @(posedge clk) begin
    if (rst) begin
      last     <= 2'd3;   // first search starts at requester 0
      held     <= 1'b0;
      held_who <= 2'd0;
    end else if (held) begin
      if (req[held_who]) begin
        // Held winner owns this cycle; keep holding only while lock stays high.
        last <= held_who;
        held <= lock;
        // held_who unchanged
      end else begin
        // Dead air: hold over, but counts as served.
        last <= held_who;
        held <= 1'b0;
      end
    end else begin
      if (arb_valid) begin
        last <= arb_idx;
        // Lock high on a granted cycle latches this winner.
        held     <= lock;
        held_who <= arb_idx;
      end else begin
        // Idle keeps your place.
        held <= 1'b0;
      end
    end
  end

endmodule

Now let me verify it compiles with iverilog and simulates through the key scenarios.

Compile check with iverilog

shell

iverilog -g2012 -o /tmp/dut_test dut.sv -e 2>&1 | head; echo "---syntax check (no top TB yet)---"; iverilog -g2012 -o /tmp/x dut.sv 2>&1 | head; echo "exit=$?"

output

iverilog: invalid option -- 'e'
---syntax check (no top TB yet)---
dut.sv:20: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
exit=0

[stdout]
iverilog: invalid option -- 'e'
---syntax check (no top TB yet)---
dut.sv:20: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
exit=0

Compiles (exit 0), but there's a warning about the variable bit-select `req[astart + k]` , iverilog may not handle it correctly. Let me verify actual behavior with a testbench before trusting it.

Run testbench simulation

shell

cat > /tmp/tb.sv <<'EOF'
module tb;
  logic clk=0, rst, lock;
  logic [3:0] req;
  logic [3:0] grant;
  logic gv;
  TopModule d(.clk,.rst,.req,.lock,.grant(grant),.grant_valid(gv));
  always #5 clk=~clk;
  task step; @(posedge clk); #1; $display("t=%0t rst=%b req=%b lock=%b -> grant=%b gv=%b", $time, rst, req, lock, grant, gv); endtask
  initial begin
    rst=1; req=0; lock=0; @(posedge clk);#1; rst=0;
    $display("== all four forever ==");
    req=4'b1111; lock=0;
    step; step; step; step; step; step;
    $display("== lone requester (only 2) ==");
    req=4'b0100; step; step; step;
    $display("== lock latch then hold then drop ==");
    req=4'b1111; lock=1; step;      // grant someone, latch
    step;                            // held, lock high, continue
    lock=0; step;                    // lock drops, holder still owns
    step;                            // resume next
    $display("== dead air: hold but req low, lock high ==");
    req=4'b1111; lock=1; step;       // latch a winner
    // drop that winner's req: find who was granted
    $display("(now force granted one's req low)");
    req=4'b1110; step;               // if holder was 0, dead air
    lock=0; req=4'b1111; step; step;
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim dut.sv /tmp/tb.sv 2>&1 | grep -v "constant selects" ; vvp /tmp/sim 2>&1

output

== all four forever ==
t=16 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=26 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=36 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=46 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=56 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=66 rst=0 req=1111 lock=0 -> grant=0100 gv=1
== lone requester (only 2) ==
t=76 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=86 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=96 rst=0 req=0100 lock=0 -> grant=0100 gv=1
== lock latch then hold then drop ==
t=106 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=116 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=126 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=136 rst=0 req=1111 lock=0 -> grant=0010 gv=1
== dead air: hold but req low, lock high ==
t=146 rst=0 req=1111 lock=1 -> grant=0010 gv=1
(now force granted one's req low)
t=156 rst=0 req=1110 lock=1 -> grant=0010 gv=1
t=166 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=176 rst=0 req=1111 lock=0 -> grant=1000 gv=1
/tmp/tb.sv:27: $finish called at 176 (1s)

[stdout]
== all four forever ==
t=16 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=26 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=36 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=46 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=56 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=66 rst=0 req=1111 lock=0 -> grant=0100 gv=1
== lone requester (only 2) ==
t=76 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=86 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=96 rst=0 req=0100 lock=0 -> grant=0100 gv=1
== lock latch then hold then drop ==
t=106 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=116 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=126 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=136 rst=0 req=1111 lock=0 -> grant=0010 gv=1
== dead air: hold but req low, lock high ==
t=146 rst=0 req=1111 lock=1 -> grant=0010 gv=1
(now force granted one's req low)
t=156 rst=0 req=1110 lock=1 -> grant=0010 gv=1
t=166 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=176 rst=0 req=1111 lock=0 -> grant=1000 gv=1
/tmp/tb.sv:27: $finish called at 176 (1s)

Bug confirmed: first grant went to requester **1**, not 0 , the iverilog variable bit-select warning ("all bits included") broke arbitration. I'll rewrite using constant indices into `req` (rank-by-distance) to avoid that.

/app/dut.sv

− removed

  // Combinational round-robin arbitration starting just past `last`
  logic [1:0] astart;
  logic       arb_valid;
  logic [1:0] arb_idx;

  always_comb begin
    arb_valid = 1'b0;
    arb_idx   = 2'd0;
    astart    = last + 2'd1;
    for (int k = 0; k < 4; k++) begin
      if (!arb_valid && req[astart + k[1:0]]) begin
        arb_valid = 1'b1;
        arb_idx   = astart + k[1:0];
      end
    end
  end

+ added

  // Combinational round-robin arbitration starting just past `last`.
  // Each asking requester's rank = distance from the start position; the
  // smallest rank wins. Uses only constant indices into `req`.
  logic [1:0] astart;
  logic       arb_valid;
  logic [1:0] arb_idx;
  logic [2:0] best_rank;
  logic [1:0] rank;

  always_comb begin
    arb_valid = 1'b0;
    arb_idx   = 2'd0;
    best_rank = 3'd4;             // larger than any real rank (0..3)
    astart    = last + 2'd1;
    for (int i = 0; i < 4; i++) begin
      if (req[i]) begin
        rank = i[1:0] - astart;  // 2-bit modulo-4 distance
        if ({1'b0, rank} < best_rank) begin
          best_rank = {1'b0, rank};
          arb_valid = 1'b1;
          arb_idx   = i[1:0];
        end
      end
    end
  end

Recompile and rerun testbench

shell

iverilog -g2012 -o /tmp/sim dut.sv /tmp/tb.sv 2>&1; echo "compile-exit=$?"; vvp /tmp/sim 2>&1

output

dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
compile-exit=0
== all four forever ==
t=16 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=26 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=36 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=46 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=56 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=66 rst=0 req=1111 lock=0 -> grant=0100 gv=1
== lone requester (only 2) ==
t=76 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=86 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=96 rst=0 req=0100 lock=0 -> grant=0100 gv=1
== lock latch then hold then drop ==
t=106 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=116 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=126 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=136 rst=0 req=1111 lock=0 -> grant=0010 gv=1
== dead air: hold but req low, lock high ==
t=146 rst=0 req=1111 lock=1 -> grant=0010 gv=1
(now force granted one's req low)
t=156 rst=0 req=1110 lock=1 -> grant=0010 gv=1
t=166 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=176 rst=0 req=1111 lock=0 -> grant=1000 gv=1
/tmp/tb.sv:27: $finish called at 176 (1s)

[stdout]
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
compile-exit=0
== all four forever ==
t=16 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=26 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=36 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=46 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=56 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=66 rst=0 req=1111 lock=0 -> grant=0100 gv=1
== lone requester (only 2) ==
t=76 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=86 rst=0 req=0100 lock=0 -> grant=0100 gv=1
t=96 rst=0 req=0100 lock=0 -> grant=0100 gv=1
== lock latch then hold then drop ==
t=106 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=116 rst=0 req=1111 lock=1 -> grant=1000 gv=1
t=126 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=136 rst=0 req=1111 lock=0 -> grant=0010 gv=1
== dead air: hold but req low, lock high ==
t=146 rst=0 req=1111 lock=1 -> grant=0010 gv=1
(now force granted one's req low)
t=156 rst=0 req=1110 lock=1 -> grant=0010 gv=1
t=166 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=176 rst=0 req=1111 lock=0 -> grant=1000 gv=1
/tmp/tb.sv:27: $finish called at 176 (1s)

Still off-by-one (first grant is requester 1, not 0) and the warning persists at line 24. Let me probe the internal state to find the real cause.

Probe internal arbiter state

shell

cat > /tmp/tb2.sv <<'EOF'
module tb;
  logic clk=0, rst, lock;
  logic [3:0] req;
  logic [3:0] grant; logic gv;
  TopModule d(.clk,.rst,.req,.lock,.grant(grant),.grant_valid(gv));
  always #5 clk=~clk;
  initial begin
    rst=1; req=0; lock=0;
    @(posedge clk);#1;
    $display("after rst edge: last=%0d held=%b astart=%0d", d.last, d.held, d.astart);
    rst=0;
    req=4'b1111; lock=0;
    #1 $display("comb before edge: astart=%0d arb_valid=%b arb_idx=%0d", d.astart, d.arb_valid, d.arb_idx);
    @(posedge clk);#1;
    $display("cyc1: grant=%b last=%0d astart=%0d arb_idx=%0d", grant, d.last, d.astart, d.arb_idx);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim2 dut.sv /tmp/tb2.sv 2>&1 | grep -v constant; vvp /tmp/sim2 2>&1

output

after rst edge: last=3 held=0 astart=0
comb before edge: astart=0 arb_valid=1 arb_idx=0
cyc1: grant=0010 last=0 astart=1 arb_idx=1
/tmp/tb2.sv:16: $finish called at 16 (1s)

[stdout]
after rst edge: last=3 held=0 astart=0
comb before edge: astart=0 arb_valid=1 arb_idx=0
cyc1: grant=0010 last=0 astart=1 arb_idx=1
/tmp/tb2.sv:16: $finish called at 16 (1s)

The logic is actually correct , my testbench was mis-reading. The combinational probe showed `arb_idx=0` (requester 0 wins) *during* cycle 1, but I sampled `grant` *after* the edge when `last` had already advanced to 0, so I was seeing cycle 2's grant. Let me use a proper cycle-accurate testbench that reads outputs before each edge.

Cycle-accurate testbench sampling before edges

shell

cat > /tmp/tb3.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req;
  logic [3:0] grant; logic gv;
  TopModule d(.clk,.rst,.req,.lock,.grant(grant),.grant_valid(gv));
  always #5 clk=~clk;
  integer n=0;
  // drive inputs just after posedge; sample just before next posedge
  task automatic cyc(input [3:0] r, input logic l, input string tag);
    req=r; lock=l; #1;                      // settle in this cycle
    $display("cyc%0d %-10s rst=%b req=%b lock=%b -> grant=%b gv=%b", n, tag, rst, req, lock, grant, gv);
    @(posedge clk); n++;
  endtask
  initial begin
    // reset cycle
    rst=1; req=0; lock=0; #1; @(posedge clk); n++; rst=0;
    $display("== all four forever (expect 0,1,2,3,0,1) ==");
    cyc(4'b1111,0,"all"); cyc(4'b1111,0,"all"); cyc(4'b1111,0,"all");
    cyc(4'b1111,0,"all"); cyc(4'b1111,0,"all"); cyc(4'b1111,0,"all");
    $display("== lone req2 keeps winning ==");
    cyc(4'b0100,0,"lone"); cyc(4'b0100,0,"lone"); cyc(4'b0100,0,"lone");
    $display("== idle keeps place (last served=2) then req 0&1 -> expect 0 ==");
    cyc(4'b0000,0,"idle"); cyc(4'b0000,0,"idle"); cyc(4'b0011,0,"resume");
    $display("== lock latch/hold/drop ==");
    cyc(4'b1111,1,"latch");   // grant next after last; lock latches
    cyc(4'b1111,1,"hold");    // held, lock high -> same
    cyc(4'b1111,0,"lockdrop");// lock drops, holder still owns this cycle
    cyc(4'b1111,0,"resume");  // resume just past held
    $display("== dead air: latch winner, then its req low w/ lock high ==");
    cyc(4'b1111,1,"latch2");
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim3 dut.sv /tmp/tb3.sv 2>&1 | grep -v constant; vvp /tmp/sim3 2>&1

output

== all four forever (expect 0,1,2,3,0,1) ==
cyc1 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc2 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc3 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc4 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc5 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc6 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
== lone req2 keeps winning ==
cyc7 lone       rst=0 req=0100 lock=0 -> grant=0000 gv=0
cyc8 lone       rst=0 req=0100 lock=0 -> grant=0000 gv=0
cyc9 lone       rst=0 req=0100 lock=0 -> grant=0000 gv=0
== idle keeps place (last served=2) then req 0&1 -> expect 0 ==
cyc10 idle       rst=0 req=0000 lock=0 -> grant=0000 gv=0
cyc11 idle       rst=0 req=0000 lock=0 -> grant=0000 gv=0
cyc12 resume     rst=0 req=0011 lock=0 -> grant=0000 gv=0
== lock latch/hold/drop ==
cyc13 latch      rst=0 req=1111 lock=1 -> grant=0000 gv=0
cyc14 hold       rst=0 req=1111 lock=1 -> grant=0000 gv=0
cyc15 lockdrop   rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc16 resume     rst=0 req=1111 lock=0 -> grant=0000 gv=0
== dead air: latch winner, then its req low w/ lock high ==
cyc17 latch2     rst=0 req=1111 lock=1 -> grant=0000 gv=0
/tmp/tb3.sv:30: $finish called at 175 (1s)

[stdout]
== all four forever (expect 0,1,2,3,0,1) ==
cyc1 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc2 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc3 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc4 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc5 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc6 all        rst=0 req=1111 lock=0 -> grant=0000 gv=0
== lone req2 keeps winning ==
cyc7 lone       rst=0 req=0100 lock=0 -> grant=0000 gv=0
cyc8 lone       rst=0 req=0100 lock=0 -> grant=0000 gv=0
cyc9 lone       rst=0 req=0100 lock=0 -> grant=0000 gv=0
== idle keeps place (last served=2) then req 0&1 -> expect 0 ==
cyc10 idle       rst=0 req=0000 lock=0 -> grant=0000 gv=0
cyc11 idle       rst=0 req=0000 lock=0 -> grant=0000 gv=0
cyc12 resume     rst=0 req=0011 lock=0 -> grant=0000 gv=0
== lock latch/hold/drop ==
cyc13 latch      rst=0 req=1111 lock=1 -> grant=0000 gv=0
cyc14 hold       rst=0 req=1111 lock=1 -> grant=0000 gv=0
cyc15 lockdrop   rst=0 req=1111 lock=0 -> grant=0000 gv=0
cyc16 resume     rst=0 req=1111 lock=0 -> grant=0000 gv=0
== dead air: latch winner, then its req low w/ lock high ==
cyc17 latch2     rst=0 req=1111 lock=1 -> grant=0000 gv=0
/tmp/tb3.sv:30: $finish called at 175 (1s)

grant is 0 everywhere now , but the earlier probe showed `arb_valid=1`. Something about the output block isn't settling. Let me probe directly with this timing.

Probe arb and grant with same timing

shell

cat > /tmp/tb4.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req;
  logic [3:0] grant; logic gv;
  TopModule d(.clk,.rst,.req,.lock,.grant(grant),.grant_valid(gv));
  always #5 clk=~clk;
  initial begin
    rst=1; req=0; lock=0; #1; @(posedge clk); rst=0;
    req=4'b1111; lock=0; #1;
    $display("last=%0d held=%b rst=%b | arb_valid=%b arb_idx=%0d best=%0d | grant=%b gv=%b",
             d.last, d.held, rst, d.arb_valid, d.arb_idx, d.best_rank, grant, gv);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim4 dut.sv /tmp/tb4.sv 2>&1 | grep -v constant; vvp /tmp/sim4 2>&1

output

last=x held=0 rst=0 | arb_valid=0 arb_idx=0 best=4 | grant=0000 gv=0
/tmp/tb4.sv:11: $finish called at 6 (1s)

[stdout]
last=x held=0 rst=0 | arb_valid=0 arb_idx=0 best=4 | grant=0000 gv=0
/tmp/tb4.sv:11: $finish called at 6 (1s)

`last=x` after reset , a **testbench race**: I set `rst=0` in the same timestep as the reset `@(posedge clk)`, so `always_ff` sometimes sampled `rst=0`. The DUT is fine; the grader drives inputs off-edge. Let me fix the TB to change inputs `#1` after each edge.

Race-free cycle-accurate testbench

shell

cat > /tmp/tb5.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req;
  logic [3:0] grant; logic gv;
  TopModule d(.clk,.rst,.req,.lock,.grant(grant),.grant_valid(gv));
  always #5 clk=~clk;
  integer n=0;
  // change inputs #1 after posedge (off-edge), sample #4 (well before next edge)
  task automatic cyc(input [3:0] r, input logic l, input string tag);
    @(posedge clk); #1; req=r; lock=l; #3;
    $display("cyc%0d %-9s req=%b lock=%b -> grant=%b gv=%b", n, tag, req, lock, grant, gv);
    n++;
  endtask
  initial begin
    rst=1; req=0; lock=0;
    @(posedge clk); #1; rst=0;   // reset applied cleanly at this edge
    $display("(after reset last=%0d)", d.last);
    $display("== all four forever: expect 0,1,2,3,0,1 ==");
    cyc(4'b1111,0,"all");cyc(4'b1111,0,"all");cyc(4'b1111,0,"all");
    cyc(4'b1111,0,"all");cyc(4'b1111,0,"all");cyc(4'b1111,0,"all");
    $display("== lone req2: expect 2,2,2 ==");
    cyc(4'b0100,0,"lone");cyc(4'b0100,0,"lone");cyc(4'b0100,0,"lone");
    $display("== idle x2 (keep place, last=2) then req0&1: expect none,none,0 ==");
    cyc(4'b0000,0,"idle");cyc(4'b0000,0,"idle");cyc(4'b0011,0,"resume");
    $display("== after serving 0: req all -> expect 1 ==");
    cyc(4'b1111,0,"nxt");
    $display("== lock: latch(expect2), hold(2), lockdrop(2 owns), resume(3) ==");
    cyc(4'b1111,1,"latch");cyc(4'b1111,1,"hold");cyc(4'b1111,0,"drop");cyc(4'b1111,0,"resume");
    $display("== dead air: latch winner then its req low+lock high ==");
    cyc(4'b1111,1,"latchA");        // expect grant to (last+1)
    cyc(4'b1011,1,"deadOR");        // if held was bit2, this keeps it; craft below
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim5 dut.sv /tmp/tb5.sv 2>&1 | grep -v constant; vvp /tmp/sim5 2>&1

output

(after reset last=3)
== all four forever: expect 0,1,2,3,0,1 ==
cyc0 all       req=1111 lock=0 -> grant=0001 gv=1
cyc1 all       req=1111 lock=0 -> grant=0010 gv=1
cyc2 all       req=1111 lock=0 -> grant=0100 gv=1
cyc3 all       req=1111 lock=0 -> grant=1000 gv=1
cyc4 all       req=1111 lock=0 -> grant=0001 gv=1
cyc5 all       req=1111 lock=0 -> grant=0010 gv=1
== lone req2: expect 2,2,2 ==
cyc6 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc7 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc8 lone      req=0100 lock=0 -> grant=0100 gv=1
== idle x2 (keep place, last=2) then req0&1: expect none,none,0 ==
cyc9 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc10 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc11 resume    req=0011 lock=0 -> grant=0001 gv=1
== after serving 0: req all -> expect 1 ==
cyc12 nxt       req=1111 lock=0 -> grant=0010 gv=1
== lock: latch(expect2), hold(2), lockdrop(2 owns), resume(3) ==
cyc13 latch     req=1111 lock=1 -> grant=0100 gv=1
cyc14 hold      req=1111 lock=1 -> grant=0100 gv=1
cyc15 drop      req=1111 lock=0 -> grant=0100 gv=1
cyc16 resume    req=1111 lock=0 -> grant=1000 gv=1
== dead air: latch winner then its req low+lock high ==
cyc17 latchA    req=1111 lock=1 -> grant=0001 gv=1
cyc18 deadOR    req=1011 lock=1 -> grant=0001 gv=1
/tmp/tb5.sv:31: $finish called at 199 (1s)

[stdout]
(after reset last=3)
== all four forever: expect 0,1,2,3,0,1 ==
cyc0 all       req=1111 lock=0 -> grant=0001 gv=1
cyc1 all       req=1111 lock=0 -> grant=0010 gv=1
cyc2 all       req=1111 lock=0 -> grant=0100 gv=1
cyc3 all       req=1111 lock=0 -> grant=1000 gv=1
cyc4 all       req=1111 lock=0 -> grant=0001 gv=1
cyc5 all       req=1111 lock=0 -> grant=0010 gv=1
== lone req2: expect 2,2,2 ==
cyc6 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc7 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc8 lone      req=0100 lock=0 -> grant=0100 gv=1
== idle x2 (keep place, last=2) then req0&1: expect none,none,0 ==
cyc9 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc10 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc11 resume    req=0011 lock=0 -> grant=0001 gv=1
== after serving 0: req all -> expect 1 ==
cyc12 nxt       req=1111 lock=0 -> grant=0010 gv=1
== lock: latch(expect2), hold(2), lockdrop(2 owns), resume(3) ==
cyc13 latch     req=1111 lock=1 -> grant=0100 gv=1
cyc14 hold      req=1111 lock=1 -> grant=0100 gv=1
cyc15 drop      req=1111 lock=0 -> grant=0100 gv=1
cyc16 resume    req=1111 lock=0 -> grant=1000 gv=1
== dead air: latch winner then its req low+lock high ==
cyc17 latchA    req=1111 lock=1 -> grant=0001 gv=1
cyc18 deadOR    req=1011 lock=1 -> grant=0001 gv=1
/tmp/tb5.sv:31: $finish called at 199 (1s)

All spec scenarios pass: first winner is **0**; cycling 0,1,2,3; lone requester holds; idle keeps place (search resumes from 3→0); lock latches/holds/owns-on-drop/resumes past holder. My "deadOR" craft was wrong (req0 stayed high). Let me verify the true dead-air path and reset-clears-hold explicitly.

Verify dead-air, lock-no-grant, reset-clears-hold

shell

cat > /tmp/tb6.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req;
  logic [3:0] grant; logic gv;
  TopModule d(.clk,.rst,.req,.lock,.grant(grant),.grant_valid(gv));
  always #5 clk=~clk;
  integer n=0;
  task automatic cyc(input logic rs, input [3:0] r, input logic l, input string tag);
    @(posedge clk); #1; rst=rs; req=r; lock=l; #3;
    $display("cyc%0d %-9s rst=%b req=%b lock=%b -> grant=%b gv=%b (held=%b who=%0d)",
             n,tag,rst,req,lock,grant,gv,d.held,d.held_who); n++;
  endtask
  initial begin
    rst=1;req=0;lock=0; @(posedge clk);#1;rst=0;
    $display("== latch req0, then req0 low + lock HIGH => dead air, then re-latch next ==");
    cyc(0,4'b1111,1,"latch0");   // last=3 -> grant0, latch who=0
    cyc(0,4'b1110,1,"deadair");  // req0 low, lock high -> grant0000 gv0, hold over
    cyc(0,4'b1110,1,"relatch");  // resume past 0: grant1, lock high -> re-latch who=1
    cyc(0,4'b1110,1,"hold1");    // held 1
    $display("== dead air on the very cycle lock drops (req of holder low, lock low) ==");
    cyc(0,4'b1100,0,"deadDrop"); // req1 low + lock low -> dead air grant0 gv0
    cyc(0,4'b1100,0,"resume");   // resume past 1 -> grant2
    $display("== lock with no grant does nothing (idle+lock) ==");
    cyc(0,4'b0000,1,"idleLk");   // no req, lock high -> grant0 gv0, no latch
    cyc(0,4'b1111,0,"after");    // resume past last served(2) -> grant3
    $display("== reset clears hold mid-hold ==");
    cyc(0,4'b1111,1,"latchX");   // grant (past 3 ->0), latch
    cyc(1,4'b1111,0,"RESET");    // rst high: outputs 0, clears
    cyc(0,4'b1111,0,"postR");    // fresh: grant0
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim6 dut.sv /tmp/tb6.sv 2>&1 | grep -v constant; vvp /tmp/sim6 2>&1

output

== latch req0, then req0 low + lock HIGH => dead air, then re-latch next ==
cyc0 latch0    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=0)
cyc1 deadair   rst=0 req=1110 lock=1 -> grant=0000 gv=0 (held=1 who=0)
cyc2 relatch   rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=0 who=0)
cyc3 hold1     rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=1 who=1)
== dead air on the very cycle lock drops (req of holder low, lock low) ==
cyc4 deadDrop  rst=0 req=1100 lock=0 -> grant=0000 gv=0 (held=1 who=1)
cyc5 resume    rst=0 req=1100 lock=0 -> grant=0100 gv=1 (held=0 who=1)
== lock with no grant does nothing (idle+lock) ==
cyc6 idleLk    rst=0 req=0000 lock=1 -> grant=0000 gv=0 (held=0 who=2)
cyc7 after     rst=0 req=1111 lock=0 -> grant=1000 gv=1 (held=0 who=2)
== reset clears hold mid-hold ==
cyc8 latchX    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=3)
cyc9 RESET     rst=1 req=1111 lock=0 -> grant=0000 gv=0 (held=1 who=0)
cyc10 postR     rst=0 req=1111 lock=0 -> grant=0001 gv=1 (held=0 who=0)
/tmp/tb6.sv:29: $finish called at 119 (1s)

[stdout]
== latch req0, then req0 low + lock HIGH => dead air, then re-latch next ==
cyc0 latch0    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=0)
cyc1 deadair   rst=0 req=1110 lock=1 -> grant=0000 gv=0 (held=1 who=0)
cyc2 relatch   rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=0 who=0)
cyc3 hold1     rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=1 who=1)
== dead air on the very cycle lock drops (req of holder low, lock low) ==
cyc4 deadDrop  rst=0 req=1100 lock=0 -> grant=0000 gv=0 (held=1 who=1)
cyc5 resume    rst=0 req=1100 lock=0 -> grant=0100 gv=1 (held=0 who=1)
== lock with no grant does nothing (idle+lock) ==
cyc6 idleLk    rst=0 req=0000 lock=1 -> grant=0000 gv=0 (held=0 who=2)
cyc7 after     rst=0 req=1111 lock=0 -> grant=1000 gv=1 (held=0 who=2)
== reset clears hold mid-hold ==
cyc8 latchX    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=3)
cyc9 RESET     rst=1 req=1111 lock=0 -> grant=0000 gv=0 (held=1 who=0)
cyc10 postR     rst=0 req=1111 lock=0 -> grant=0001 gv=1 (held=0 who=0)
/tmp/tb6.sv:29: $finish called at 119 (1s)

Every spec rule verified correct: dead-air (req low with lock high → grant 0, hold over, counts as served), re-latch without re-arm, dead-air on the very cycle lock drops, lock-with-no-grant does nothing, idle keeps place, and reset clears the hold and restarts order. One loose end: the iverilog warning. Let me confirm which line triggers it and eliminate it so nothing is left to chance in hidden cases.

Identify warning source line

shell

iverilog -g2012 -o /tmp/x dut.sv 2>&1; echo "---"; sed -n '20,26p' dut.sv | cat -n

output

dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
---
     1	  logic [1:0] arb_idx;
     2	  logic [2:0] best_rank;
     3	  logic [1:0] rank;
     4	
     5	  always_comb begin
     6	    arb_valid = 1'b0;
     7	    arb_idx   = 2'd0;

[stdout]
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
---
     1	  logic [1:0] arb_idx;
     2	  logic [2:0] best_rank;
     3	  logic [1:0] rank;
     4	
     5	  always_comb begin
     6	    arb_valid = 1'b0;
     7	    arb_idx   = 2'd0;

Line 24 is the `always_comb` , the warning comes from iverilog unrolling the `for` loop into "constant selects" (`req[i]`). Behavior is empirically correct, but I'll hand-unroll with literal indices to eliminate the warning entirely and remove any doubt for hidden cases.

/app/dut.sv

− removed

  always_comb begin
    arb_valid = 1'b0;
    arb_idx   = 2'd0;
    best_rank = 3'd4;             // larger than any real rank (0..3)
    astart    = last + 2'd1;
    for (int i = 0; i < 4; i++) begin
      if (req[i]) begin
        rank = i[1:0] - astart;  // 2-bit modulo-4 distance
        if ({1'b0, rank} < best_rank) begin
          best_rank = {1'b0, rank};
          arb_valid = 1'b1;
          arb_idx   = i[1:0];
        end
      end
    end
  end

+ added

  always_comb begin
    arb_valid = 1'b0;
    arb_idx   = 2'd0;
    best_rank = 3'd4;                 // larger than any real rank (0..3)
    astart    = last + 2'd1;
    rank      = 2'd0;
    // Requester 0
    if (req[0]) begin
      rank = 2'd0 - astart;
      if ({1'b0, rank} < best_rank) begin
        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd0;
      end
    end
    // Requester 1
    if (req[1]) begin
      rank = 2'd1 - astart;
      if ({1'b0, rank} < best_rank) begin
        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd1;
      end
    end
    // Requester 2
    if (req[2]) begin
      rank = 2'd2 - astart;
      if ({1'b0, rank} < best_rank) begin
        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd2;
      end
    end
    // Requester 3
    if (req[3]) begin
      rank = 2'd3 - astart;
      if ({1'b0, rank} < best_rank) begin
        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd3;
      end
    end
  end

Confirm no warnings and re-run behavioral suites

shell

iverilog -g2012 -o /tmp/x dut.sv 2>&1; echo "warnings-exit=$?"; echo "=== rerun full behavioral suites ==="; iverilog -g2012 -o /tmp/s5 dut.sv /tmp/tb5.sv 2>&1; vvp /tmp/s5 2>&1 | grep -E "grant|reset|expect" ; echo "--- suite 6 ---"; iverilog -g2012 -o /tmp/s6 dut.sv /tmp/tb6.sv 2>&1; vvp /tmp/s6 2>&1 | grep -E "grant"

output

dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
warnings-exit=0
=== rerun full behavioral suites ===
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
(after reset last=3)
== all four forever: expect 0,1,2,3,0,1 ==
cyc0 all       req=1111 lock=0 -> grant=0001 gv=1
cyc1 all       req=1111 lock=0 -> grant=0010 gv=1
cyc2 all       req=1111 lock=0 -> grant=0100 gv=1
cyc3 all       req=1111 lock=0 -> grant=1000 gv=1
cyc4 all       req=1111 lock=0 -> grant=0001 gv=1
cyc5 all       req=1111 lock=0 -> grant=0010 gv=1
== lone req2: expect 2,2,2 ==
cyc6 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc7 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc8 lone      req=0100 lock=0 -> grant=0100 gv=1
== idle x2 (keep place, last=2) then req0&1: expect none,none,0 ==
cyc9 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc10 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc11 resume    req=0011 lock=0 -> grant=0001 gv=1
== after serving 0: req all -> expect 1 ==
cyc12 nxt       req=1111 lock=0 -> grant=0010 gv=1
== lock: latch(expect2), hold(2), lockdrop(2 owns), resume(3) ==
cyc13 latch     req=1111 lock=1 -> grant=0100 gv=1
cyc14 hold      req=1111 lock=1 -> grant=0100 gv=1
cyc15 drop      req=1111 lock=0 -> grant=0100 gv=1
cyc16 resume    req=1111 lock=0 -> grant=1000 gv=1
cyc17 latchA    req=1111 lock=1 -> grant=0001 gv=1
cyc18 deadOR    req=1011 lock=1 -> grant=0001 gv=1
--- suite 6 ---
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
cyc0 latch0    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=0)
cyc1 deadair   rst=0 req=1110 lock=1 -> grant=0000 gv=0 (held=1 who=0)
cyc2 relatch   rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=0 who=0)
cyc3 hold1     rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=1 who=1)
cyc4 deadDrop  rst=0 req=1100 lock=0 -> grant=0000 gv=0 (held=1 who=1)
cyc5 resume    rst=0 req=1100 lock=0 -> grant=0100 gv=1 (held=0 who=1)
== lock with no grant does nothing (idle+lock) ==
cyc6 idleLk    rst=0 req=0000 lock=1 -> grant=0000 gv=0 (held=0 who=2)
cyc7 after     rst=0 req=1111 lock=0 -> grant=1000 gv=1 (held=0 who=2)
cyc8 latchX    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=3)
cyc9 RESET     rst=1 req=1111 lock=0 -> grant=0000 gv=0 (held=1 who=0)
cyc10 postR     rst=0 req=1111 lock=0 -> grant=0001 gv=1 (held=0 who=0)

[stdout]
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
warnings-exit=0
=== rerun full behavioral suites ===
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
(after reset last=3)
== all four forever: expect 0,1,2,3,0,1 ==
cyc0 all       req=1111 lock=0 -> grant=0001 gv=1
cyc1 all       req=1111 lock=0 -> grant=0010 gv=1
cyc2 all       req=1111 lock=0 -> grant=0100 gv=1
cyc3 all       req=1111 lock=0 -> grant=1000 gv=1
cyc4 all       req=1111 lock=0 -> grant=0001 gv=1
cyc5 all       req=1111 lock=0 -> grant=0010 gv=1
== lone req2: expect 2,2,2 ==
cyc6 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc7 lone      req=0100 lock=0 -> grant=0100 gv=1
cyc8 lone      req=0100 lock=0 -> grant=0100 gv=1
== idle x2 (keep place, last=2) then req0&1: expect none,none,0 ==
cyc9 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc10 idle      req=0000 lock=0 -> grant=0000 gv=0
cyc11 resume    req=0011 lock=0 -> grant=0001 gv=1
== after serving 0: req all -> expect 1 ==
cyc12 nxt       req=1111 lock=0 -> grant=0010 gv=1
== lock: latch(expect2), hold(2), lockdrop(2 owns), resume(3) ==
cyc13 latch     req=1111 lock=1 -> grant=0100 gv=1
cyc14 hold      req=1111 lock=1 -> grant=0100 gv=1
cyc15 drop      req=1111 lock=0 -> grant=0100 gv=1
cyc16 resume    req=1111 lock=0 -> grant=1000 gv=1
cyc17 latchA    req=1111 lock=1 -> grant=0001 gv=1
cyc18 deadOR    req=1011 lock=1 -> grant=0001 gv=1
--- suite 6 ---
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:24: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
cyc0 latch0    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=0)
cyc1 deadair   rst=0 req=1110 lock=1 -> grant=0000 gv=0 (held=1 who=0)
cyc2 relatch   rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=0 who=0)
cyc3 hold1     rst=0 req=1110 lock=1 -> grant=0010 gv=1 (held=1 who=1)
cyc4 deadDrop  rst=0 req=1100 lock=0 -> grant=0000 gv=0 (held=1 who=1)
cyc5 resume    rst=0 req=1100 lock=0 -> grant=0100 gv=1 (held=0 who=1)
== lock with no grant does nothing (idle+lock) ==
cyc6 idleLk    rst=0 req=0000 lock=1 -> grant=0000 gv=0 (held=0 who=2)
cyc7 after     rst=0 req=1111 lock=0 -> grant=1000 gv=1 (held=0 who=2)
cyc8 latchX    rst=0 req=1111 lock=1 -> grant=0001 gv=1 (held=0 who=3)
cyc9 RESET     rst=1 req=1111 lock=0 -> grant=0000 gv=0 (held=1 who=0)
cyc10 postR     rst=0 req=1111 lock=0 -> grant=0001 gv=1 (held=0 who=0)

The warning persists even with literal `req[0]` selects , so it's not the loop. Behavior is provably correct (e.g. lone req2 → 0100, which would break if selects really "included all bits"). But let me isolate exactly what triggers this iverilog message so I can be certain it's benign.

Isolate the iverilog warning trigger

shell

cd /tmp
cat > m1.sv <<'EOF'
module m1(input logic [3:0] req, output logic o);
  logic [1:0] astart; logic [1:0] rank;
  always_comb begin
    astart = 2'd1;
    rank = 2'd0 - astart;
    o = req[0];
  end
endmodule
EOF
echo "--- test1: req[0] only ---"; iverilog -g2012 -o /dev/null m1.sv 2>&1 || true
cat > m2.sv <<'EOF'
module m2(input logic [3:0] req, input logic [1:0] astart, output logic [2:0] r);
  always_comb begin
    r = {1'b0, (2'd0 - astart)};
  end
endmodule
EOF
echo "--- test2: 2'd0 - astart in concat ---"; iverilog -g2012 -o /dev/null m2.sv 2>&1 || true
cat > m3.sv <<'EOF'
module m3(input logic [3:0] req, input logic [1:0] astart, output logic [1:0] idx);
  logic [2:0] best; logic [1:0] rank;
  always_comb begin
    best=3'd4; idx=2'd0; rank=2'd0;
    if (req[0]) begin rank = 2'd0 - astart; if ({1'b0,rank}<best) begin best={1'b0,rank}; idx=2'd0; end end
  end
endmodule
EOF
echo "--- test3: full pattern ---"; iverilog -g2012 -o /dev/null m3.sv 2>&1 || true

output

--- test1: req[0] only ---
m1.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
--- test2: 2'd0 - astart in concat ---
--- test3: full pattern ---
m3.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
Shell cwd was reset to /app

[stdout]
--- test1: req[0] only ---
m1.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
--- test2: 2'd0 - astart in concat ---
--- test3: full pattern ---
m3.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
[stderr]

Shell cwd was reset to /app

`o = req[0]` alone triggers it , this iverilog build emits that "sorry" message for *any* constant bit-select inside an `always_*` block. It's non-fatal (exit 0). My full arbiter tests already prove the selects return the correct individual bit (req=0100 → grant 0100, not 0001). Let me close the loop with a direct micro-sim proving `req[0]` really is bit 0, not "all bits".

Prove req[0] returns bit 0 not all-bits

shell

cd /tmp
cat > m1tb.sv <<'EOF'
module tb; logic [3:0] req; logic o; m1 u(.req,.o);
initial begin
  req=4'b0100; #1; $display("req=%b req[0]=%b (expect 0)", req, o);
  req=4'b0001; #1; $display("req=%b req[0]=%b (expect 1)", req, o);
  req=4'b1110; #1; $display("req=%b req[0]=%b (expect 0)", req, o);
end endmodule
EOF
iverilog -g2012 -o m1sim m1.sv m1tb.sv 2>&1 | grep -v sorry; vvp m1sim 2>&1

output

req=0100 req[0]=0 (expect 0)
req=0001 req[0]=1 (expect 1)
req=1110 req[0]=0 (expect 0)
Shell cwd was reset to /app

[stdout]
req=0100 req[0]=0 (expect 0)
req=0001 req[0]=1 (expect 1)
req=1110 req[0]=0 (expect 0)
[stderr]

Shell cwd was reset to /app

Confirmed: `req[0]` returns exactly bit 0. The "all bits will be included" message is benign iverilog verbosity with no effect on simulation (which is what the grader runs). Let me do a final review of the complete file.

/app/dut.sv

contents

1	module TopModule(
2	  input  logic       clk,
3	  input  logic       rst,
4	  input  logic [3:0] req,
5	  input  logic       lock,
6	  output logic [3:0] grant,
7	  output logic       grant_valid
8	);
9	
10	  // Fairness / hold state
11	  logic [1:0] last;      // last requester served; search starts just past it
12	  logic       held;      // a locked hold is active
13	  logic [1:0] held_who;  // which requester is held
14	
15	  // Combinational round-robin arbitration starting just past `last`.
16	  // Each asking requester's rank = distance from the start position; the
17	  // smallest rank wins. Uses only constant indices into `req`.
18	  logic [1:0] astart;
19	  logic       arb_valid;
20	  logic [1:0] arb_idx;
21	  logic [2:0] best_rank;
22	  logic [1:0] rank;
23	
24	  always_comb begin
25	    arb_valid = 1'b0;
26	    arb_idx   = 2'd0;
27	    best_rank = 3'd4;                 // larger than any real rank (0..3)
28	    astart    = last + 2'd1;
29	    rank      = 2'd0;
30	    // Requester 0
31	    if (req[0]) begin
32	      rank = 2'd0 - astart;
33	      if ({1'b0, rank} < best_rank) begin
34	        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd0;
35	      end
36	    end
37	    // Requester 1
38	    if (req[1]) begin
39	      rank = 2'd1 - astart;
40	      if ({1'b0, rank} < best_rank) begin
41	        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd1;
42	      end
43	    end
44	    // Requester 2
45	    if (req[2]) begin
46	      rank = 2'd2 - astart;
47	      if ({1'b0, rank} < best_rank) begin
48	        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd2;
49	      end
50	    end
51	    // Requester 3
52	    if (req[3]) begin
53	      rank = 2'd3 - astart;
54	      if ({1'b0, rank} < best_rank) begin
55	        best_rank = {1'b0, rank}; arb_valid = 1'b1; arb_idx = 2'd3;
56	      end
57	    end
58	  end
59	
60	  // Mealy outputs
61	  always_comb begin
62	    grant       = 4'b0000;
63	    grant_valid = 1'b0;
64	    if (!rst) begin
65	      if (held) begin
66	        // Hold lives only while the held requester keeps asking.
67	        if (req[held_who]) begin
68	          grant       = 4'b0001 << held_who;
69	          grant_valid = 1'b1;
70	        end
71	        // else: dead air (grant 0, grant_valid 0)
72	      end else begin
73	        if (arb_valid) begin
74	          grant       = 4'b0001 << arb_idx;
75	          grant_valid = 1'b1;
76	        end
77	      end
78	    end
79	  end
80	
81	  // State update
82	  always_ff @(posedge clk) begin
83	    if (rst) begin
84	      last     <= 2'd3;   // first search starts at requester 0
85	      held     <= 1'b0;
86	      held_who <= 2'd0;
87	    end else if (held) begin
88	      if (req[held_who]) begin
89	        // Held winner owns this cycle; keep holding only while lock stays high.
90	        last <= held_who;
91	        held <= lock;
92	        // held_who unchanged
93	      end else begin
94	        // Dead air: hold over, but counts as served.
95	        last <= held_who;
96	        held <= 1'b0;
97	      end
98	    end else begin
99	      if (arb_valid) begin
100	        last <= arb_idx;
101	        // Lock high on a granted cycle latches this winner.
102	        held     <= lock;
103	        held_who <= arb_idx;
104	      end else begin
105	        // Idle keeps your place.
106	        held <= 1'b0;
107	      end
108	    end
109	  end
110	
111	endmodule
112

The module is complete and verified. Final summary: ## `/app/dut.sv` , shared-resource arbiter **Design:** Mealy machine (grant is combinational in `req`/`lock`, as the spec requires , reactions happen "the cycle" they occur). Two `always_comb` blocks compute arbitration and outputs; one `always_ff` updates state. **State:** - `last` , last requester served; the round-robin search starts at `last+1`. Reset to `3` so the very first search starts at requester 0. - `held` / `held_who` , the lock latch. **Arbitration:** each asking requester's rank = circular distance from `last+1`; smallest rank wins. Written with literal `req[0..3]` selects. **Every spec rule verified against `iverilog -g2012`:** - First all-four → **0**; sustained → 0,1,2,3,0,1… ✓ - Lone requester keeps winning; idle keeps place (served 2, idle, then req0&1 → 0) ✓ - Lock on a granted cycle latches; grant nailed while lock high; **holder still owns the cycle lock drops**; sharing resumes just past it ✓ - Dead air: held requester's `req` low (including the very cycle lock drops) → grant 0 / valid 0, hold over, counts as served ✓ - Re-latch with no re-arm: lock still high after dead air latches the next real winner ✓ - Lock with no grant does nothing (idle+lock, no latch) ✓ - Synchronous `rst` clears the hold, forces outputs low, restarts the order ✓ **On the compiler message:** `iverilog` prints `sorry: constant selects in always_* processes… (all bits will be included)` for *any* constant bit-select inside an `always_*` block in this build. It's non-fatal (exit 0) and I proved directly that `req[0]` returns bit 0 (not an OR of all bits), so simulation , which is what the grader runs , is correct. No preprocessor directives, system tasks, or force/release; ASCII only.

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.WuRBcr/runtime/cb_verifier_427-8262-1834-4569_427/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_b68c083c59054d3b. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_b68c083c59054d3b · verifier authoritative; classifier explanatory.