cpu: Rewrite O3 draining to avoid stopping in microcode

Previously, the O3 CPU could stop in the middle of a microcode
sequence. This patch makes sure that the pipeline stops when it has
committed a normal instruction or exited from a microcode
sequence. Additionally, it makes sure that the pipeline has no
instructions in flight when it is drained, which should make draining
more robust.

Draining is controlled in the commit stage, which checks if the next
PC after a committed instruction is in microcode. If this isn't the
case, it requests a squash of all instructions after that the
instruction that just committed and immediately signals a drain stall
to the fetch stage. The CPU then continues to execute until the
pipeline and all associated buffers are empty.

1 files changed, 20 insertions, 11 deletions

diff --git a/src/cpu/o3/decode.hh b/src/cpu/o3/decode.hh
index 817c5263f..3424b1d07 100644
--- a/src/cpu/o3/decode.hh
+++ b/src/cpu/o3/decode.hh
@@ -1,4 +1,16 @@
 /*
+ * Copyright (c) 2012 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
  * Copyright (c) 2004-2006 The Regents of The University of Michigan
  * All rights reserved.
  *
@@ -89,6 +101,9 @@ class DefaultDecode
     /** DefaultDecode constructor. */
     DefaultDecode(O3CPU *_cpu, DerivO3CPUParams *params);
 
+    void startupStage();
+    void resetStage();
+
     /** Returns the name of decode. */
     std::string name() const;
 
@@ -107,17 +122,14 @@ class DefaultDecode
     /** Sets pointer to list of active threads. */
     void setActiveThreads(std::list<ThreadID> *at_ptr);
 
-    /** Drains the decode stage. */
-    bool drain();
+    /** Perform sanity checks after a drain. */
+    void drainSanityCheck() const;
 
-    /** Resumes execution after a drain. */
-    void resume() { }
-
-    /** Switches out the decode stage. */
-    void switchOut() { }
+    /** Has the stage drained? */
+    bool isDrained() const { return true; }
 
     /** Takes over from another CPU's thread. */
-    void takeOverFrom();
+    void takeOverFrom() { resetStage(); }
 
     /** Ticks decode, processing all input signals and decoding as many
      * instructions as possible.
@@ -268,9 +280,6 @@ class DefaultDecode
     /** List of active thread ids */
     std::list<ThreadID> *activeThreads;
 
-    /** Number of branches in flight. */
-    unsigned branchCount[Impl::MaxThreads];
-
     /** Maximum size of the skid buffer. */
     unsigned skidBufferMax;