tests(load_queue): model L0 cache fast-path in constrained-random

  The load_queue constrained-random test could fail deterministically under
  Verilator (e.g. seed 1772249751) with a DUT/model count mismatch.

  Root cause:
  - DUT can complete a load via L0 cache fast-path without issuing a memory read.
  - The Python LQ model in constrained-random only modeled the mem-read issue
    path, so model state lagged DUT when cache-hit completion occurred.

  Fix:
  - Add `cache_hit_complete()` to `LQModel` to mirror DUT cache-hit behavior.
  - Update `test_constrained_random` to call model cache-hit completion when
    DUT `cache_hit_fast_path` is asserted, otherwise use normal mem-issue path.

  Result:
  - Reproduced failing seed now passes.
  - Full `load_queue` cocotb suite passes for the same command/seed.

Author: adambagley

verif/cocotb_tests/tomasulo/load_queue/lq_model.py

@@ -242,6 +242,26 @@ def apply_forward(self, sq_forward: SQForwardResult) -> None:
             e.forwarded = True
             e.data = sq_forward.data & MASK64
 
+    def cache_hit_complete(self) -> None:
+        """Model DUT cache-hit fast path for the current Phase B candidate.
+
+        On an L0 cache hit, the DUT marks the candidate's data as valid without
+        issuing a memory request.
+        """
+        _, mem_idx = self._issue_scan()
+        if mem_idx is None:
+            return
+
+        e = self.entries[mem_idx]
+
+        # Mirror load_queue.sv cache_hit_fast_path gating.
+        if e.is_mmio:
+            return
+        if e.is_fp and e.size == MEM_SIZE_DOUBLE:
+            return
+
+        e.data_valid = True
+
     def issue_to_memory(
         self, all_older_known: bool, sq_forward: SQForwardResult
     ) -> dict | None:

verif/cocotb_tests/tomasulo/load_queue/test_load_queue.py

@@ -685,7 +685,9 @@ async def test_constrained_random(dut: Any) -> None:
             await Timer(1, unit="ns")
 
             mem_req = dut_if.read_mem_request()
-            if mem_req["en"]:
+            if bool(dut.cache_hit_fast_path.value):
+                model.cache_hit_complete()
+            elif mem_req["en"]:
                 model.issue_to_memory(True, SQForwardResult())
             await dut_if.step()
             dut_if.drive_sq_all_older_known(False)