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, 2 insertions, 4 deletions

diff --git a/src/cpu/o3/decode_impl.hh b/src/cpu/o3/decode_impl.hh
index 15ec76f5a..51c1b9de1 100644
--- a/src/cpu/o3/decode_impl.hh
+++ b/src/cpu/o3/decode_impl.hh
@@ -288,7 +288,7 @@ DefaultDecode<Impl>::unblock(ThreadID tid)
 
 template<class Impl>
 void
-DefaultDecode<Impl>::squash(DynInstPtr &inst, ThreadID tid)
+DefaultDecode<Impl>::squash(const DynInstPtr &inst, ThreadID tid)
 {
     DPRINTF(Decode, "[tid:%i]: [sn:%i] Squashing due to incorrect branch "
             "prediction detected at decode.\n", tid, inst->seqNum);
@@ -650,8 +650,6 @@ DefaultDecode<Impl>::decodeInsts(ThreadID tid)
         ++decodeRunCycles;
     }
 
-    DynInstPtr inst;
-
     std::queue<DynInstPtr>
         &insts_to_decode = decodeStatus[tid] == Unblocking ?
         skidBuffer[tid] : insts[tid];
@@ -661,7 +659,7 @@ DefaultDecode<Impl>::decodeInsts(ThreadID tid)
     while (insts_available > 0 && toRenameIndex < decodeWidth) {
         assert(!insts_to_decode.empty());
 
-        inst = insts_to_decode.front();
+        DynInstPtr inst = std::move(insts_to_decode.front());
 
         insts_to_decode.pop();