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, 5 insertions, 6 deletions

diff --git a/src/cpu/o3/rob_impl.hh b/src/cpu/o3/rob_impl.hh
index 5a9dc90f9..223f94caa 100644
--- a/src/cpu/o3/rob_impl.hh
+++ b/src/cpu/o3/rob_impl.hh
@@ -209,7 +209,7 @@ ROB<Impl>::countInsts(ThreadID tid)
 
 template <class Impl>
 void
-ROB<Impl>::insertInst(DynInstPtr &inst)
+ROB<Impl>::insertInst(const DynInstPtr &inst)
 {
     assert(inst);
 
@@ -252,10 +252,11 @@ ROB<Impl>::retireHead(ThreadID tid)
 
     assert(numInstsInROB > 0);
 
-    // Get the head ROB instruction.
+    // Get the head ROB instruction by copying it and remove it from the list
     InstIt head_it = instList[tid].begin();
 
-    DynInstPtr head_inst = (*head_it);
+    DynInstPtr head_inst = std::move(*head_it);
+    instList[tid].erase(head_it);
 
     assert(head_inst->readyToCommit());
 
@@ -269,8 +270,6 @@ ROB<Impl>::retireHead(ThreadID tid)
     head_inst->clearInROB();
     head_inst->setCommitted();
 
-    instList[tid].erase(head_it);
-
     //Update "Global" Head of ROB
     updateHead();
 
@@ -513,7 +512,7 @@ ROB<Impl>::squash(InstSeqNum squash_num, ThreadID tid)
 }
 
 template <class Impl>
-typename Impl::DynInstPtr
+const typename Impl::DynInstPtr&
 ROB<Impl>::readHeadInst(ThreadID tid)
 {
     if (threadEntries[tid] != 0) {