cpu: Fix the usage of const DynInstPtr

Summary: Usage of const DynInstPtr& when possible and introduction of
move operators to RefCountingPtr.

In many places, scoped references to dynamic instructions do a copy of
the DynInstPtr when a reference would do. This is detrimental to
performance. On top of that, in case there is a need for reference
tracking for debugging, the redundant copies make the process much more
painful than it already is.

Also, from the theoretical point of view, a function/method that
defines a convenience name to access an instruction should not be
considered an owner of the data, i.e., doing a copy and not a reference
is not justified.

On a related topic, C++11 introduces move semantics, and those are
useful when, for example, there is a class modelling a HW structure that
contains a list, and has a getHeadOfList function, to prevent doing a
copy to an internal variable -> update pointer, remove from the list ->
update pointer, return value making a copy to the assined variable ->
update pointer, destroy the returned value -> update pointer.

Change-Id: I3bb46c20ef23b6873b469fd22befb251ac44d2f6
Signed-off-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/13105
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cpu/o3/decode.hh b/src/cpu/o3/decode.hh
index 006219a50..a1e29a6b5 100644
--- a/src/cpu/o3/decode.hh
+++ b/src/cpu/o3/decode.hh
@@ -194,7 +194,7 @@ class DefaultDecode
     /** Squashes if there is a PC-relative branch that was predicted
      * incorrectly. Sends squash information back to fetch.
      */
-    void squash(DynInstPtr &inst, ThreadID tid);
+    void squash(const DynInstPtr &inst, ThreadID tid);
 
   public:
     /** Squashes due to commit signalling a squash. Changes status to