cpu: Fix o3 front-end pipeline interlock behavior

The o3 pipeline interlock/stall logic is incorrect.  o3 unnecessicarily stalled
fetch and decode due to later stages in the pipeline.  In general, a stage
should usually only consider if it is stalled by the adjacent, downstream stage.
Forcing stalls due to later stages creates and results in bubbles in the
pipeline.  Additionally, o3 stalled the entire frontend (fetch, decode, rename)
on a branch mispredict while the ROB is being serially walked to update the
RAT (robSquashing). Only should have stalled at rename.

1 files changed, 6 insertions, 58 deletions

diff --git a/src/cpu/o3/fetch_impl.hh b/src/cpu/o3/fetch_impl.hh
index 98c1b6998..637e39957 100644
--- a/src/cpu/o3/fetch_impl.hh
+++ b/src/cpu/o3/fetch_impl.hh
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010-2013 ARM Limited
+ * Copyright (c) 2010-2014 ARM Limited
  * All rights reserved.
  *
  * The license below extends only to copyright in the software and shall
@@ -354,9 +354,6 @@ DefaultFetch<Impl>::resetStage()
         memReq[tid] = NULL;
 
         stalls[tid].decode = false;
-        stalls[tid].rename = false;
-        stalls[tid].iew = false;
-        stalls[tid].commit = false;
         stalls[tid].drain = false;
 
         fetchBufferPC[tid] = 0;
@@ -435,10 +432,6 @@ DefaultFetch<Impl>::drainSanityCheck() const
 
     for (ThreadID i = 0; i < numThreads; ++i) {
         assert(!memReq[i]);
-        assert(!stalls[i].decode);
-        assert(!stalls[i].rename);
-        assert(!stalls[i].iew);
-        assert(!stalls[i].commit);
         assert(fetchStatus[i] == Idle || stalls[i].drain);
     }
 
@@ -680,7 +673,11 @@ DefaultFetch<Impl>::finishTranslation(Fault fault, RequestPtr mem_req)
             fetchStatus[tid] = IcacheWaitResponse;
         }
     } else {
-        if (!(numInst < fetchWidth)) {
+        // Don't send an instruction to decode if it can't handle it.
+        // Asynchronous nature of this function's calling means we have to
+        // check 2 signals to see if decode is stalled.
+        if (!(numInst < fetchWidth) || stalls[tid].decode ||
+            fromDecode->decodeBlock[tid]) {
             assert(!finishTranslationEvent.scheduled());
             finishTranslationEvent.setFault(fault);
             finishTranslationEvent.setReq(mem_req);
@@ -802,15 +799,6 @@ DefaultFetch<Impl>::checkStall(ThreadID tid) const
     } else if (stalls[tid].decode) {
         DPRINTF(Fetch,"[tid:%i]: Stall from Decode stage detected.\n",tid);
         ret_val = true;
-    } else if (stalls[tid].rename) {
-        DPRINTF(Fetch,"[tid:%i]: Stall from Rename stage detected.\n",tid);
-        ret_val = true;
-    } else if (stalls[tid].iew) {
-        DPRINTF(Fetch,"[tid:%i]: Stall from IEW stage detected.\n",tid);
-        ret_val = true;
-    } else if (stalls[tid].commit) {
-        DPRINTF(Fetch,"[tid:%i]: Stall from Commit stage detected.\n",tid);
-        ret_val = true;
     }
 
     return ret_val;
@@ -952,36 +940,6 @@ DefaultFetch<Impl>::checkSignalsAndUpdate(ThreadID tid)
         stalls[tid].decode = false;
     }
 
-    if (fromRename->renameBlock[tid]) {
-        stalls[tid].rename = true;
-    }
-
-    if (fromRename->renameUnblock[tid]) {
-        assert(stalls[tid].rename);
-        assert(!fromRename->renameBlock[tid]);
-        stalls[tid].rename = false;
-    }
-
-    if (fromIEW->iewBlock[tid]) {
-        stalls[tid].iew = true;
-    }
-
-    if (fromIEW->iewUnblock[tid]) {
-        assert(stalls[tid].iew);
-        assert(!fromIEW->iewBlock[tid]);
-        stalls[tid].iew = false;
-    }
-
-    if (fromCommit->commitBlock[tid]) {
-        stalls[tid].commit = true;
-    }
-
-    if (fromCommit->commitUnblock[tid]) {
-        assert(stalls[tid].commit);
-        assert(!fromCommit->commitBlock[tid]);
-        stalls[tid].commit = false;
-    }
-
     // Check squash signals from commit.
     if (fromCommit->commitInfo[tid].squash) {
 
@@ -1013,16 +971,6 @@ DefaultFetch<Impl>::checkSignalsAndUpdate(ThreadID tid)
         branchPred->update(fromCommit->commitInfo[tid].doneSeqNum, tid);
     }
 
-    // Check ROB squash signals from commit.
-    if (fromCommit->commitInfo[tid].robSquashing) {
-        DPRINTF(Fetch, "[tid:%u]: ROB is still squashing.\n", tid);
-
-        // Continue to squash.
-        fetchStatus[tid] = Squashing;
-
-        return true;
-    }
-
     // Check squash signals from decode.
     if (fromDecode->decodeInfo[tid].squash) {
         DPRINTF(Fetch, "[tid:%u]: Squashing instructions due to squash "