gpu-compute: Dropping fetchs when no entry is reserved in the buffer

This changeset drops fetches if there is no entry reserved in the
fetch buffer for that instruction. This can happen due to a fetch
attempted to be issued in the same cycle where a branch instruction
flushed the fetch buffer, while an ITLB or I-cache request is still
pending.

Change-Id: I3b80dbd71af27ccf790b543bd5c034bb9b02624a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29932
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Onur Kayıran <onur.kayiran@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

src/gpu-compute/fetch_unit.cc

@@ -234,6 +234,16 @@ FetchUnit::fetch(PacketPtr pkt, Wavefront *wavefront)
     pkt = new Packet(oldPkt->req, oldPkt->cmd);
     delete oldPkt;
 
+    /**
+     * if we have not reserved an entry in the fetch buffer,
+     * stop fetching. this can happen due to a branch instruction
+     * flushing the fetch buffer while an ITLB or I-cache request is still
+     * pending, in the same cycle another instruction is trying to fetch.
+     */
+    if (!fetchBuf.at(wavefront->wfSlotId).isReserved(pkt->req->getVaddr())) {
+        return;
+    }
+
     /**
      * we should have reserved an entry in the fetch buffer
      * for this cache line. here we get the pointer to the

src/gpu-compute/fetch_unit.hh

@@ -120,6 +120,18 @@ class FetchUnit
             return reserved_pc->second;
         }
 
+        /**
+         * returns true if there is an entry reserved for this address,
+         * and false otherwise
+         */
+        bool
+        isReserved(Addr vaddr) const
+        {
+            auto reserved_pc = reservedPCs.find(vaddr);
+            bool is_reserved = (reserved_pc != reservedPCs.end());
+            return is_reserved;
+        }
+
         void fetchDone(Addr vaddr);
 
         /**