[turbofan] Access merge environment map more efficiently

src/compiler/bytecode-graph-builder.cc

Index: src/compiler/bytecode-graph-builder.cc
diff --git a/src/compiler/bytecode-graph-builder.cc b/src/compiler/bytecode-graph-builder.cc
index b3178262d44f2ab3a9cbc8e9f8dd48202443aba6..dc41e55dd6b41b4950c110b092ebba9736e50e75 100644
--- a/src/compiler/bytecode-graph-builder.cc
+++ b/src/compiler/bytecode-graph-builder.cc
@@ -1896,11 +1896,12 @@ void BytecodeGraphBuilder::VisitIllegal() {
 void BytecodeGraphBuilder::VisitNop() {}
 
 void BytecodeGraphBuilder::SwitchToMergeEnvironment(int current_offset) {
-  if (merge_environments_[current_offset] != nullptr) {
+  auto it = merge_environments_.find(current_offset);

[Michael Starzinger, 2016/12/01 14:47:48]

question: Any reason to use an iterator here and a reference type below? I would
vote for sticking to one or the other in both functions for the sake of
readability.

[Leszek Swirski, 2016/12/01 14:52:36]

Indeed there is: the find+iterator does not create a new entry in the map if one
didn't exist already, while the array-style accessor does. I could use insert()
below to also use an iterator if you want?

[Michael Starzinger, 2016/12/01 15:00:51]

Yes, I would prefer that. Thanks!

[Michael Starzinger, 2016/12/01 17:00:43]

As discussed offline: Either is fine with me. Take your pick. :)

+  if (it != merge_environments_.end()) {
     if (environment() != nullptr) {
-      merge_environments_[current_offset]->Merge(environment());
+      it->second->Merge(environment());
     }
-    set_environment(merge_environments_[current_offset]);
+    set_environment(it->second);
   }
 }
 
@@ -1914,15 +1915,16 @@ void BytecodeGraphBuilder::BuildLoopHeaderEnvironment(int current_offset) {
 
 void BytecodeGraphBuilder::MergeIntoSuccessorEnvironment(int target_offset) {
   BuildLoopExitsForBranch(target_offset);
-  if (merge_environments_[target_offset] == nullptr) {
+  Environment*& merge_environment = merge_environments_[target_offset];
+  if (merge_environment == nullptr) {
     // Append merge nodes to the environment. We may merge here with another
     // environment. So add a place holder for merge nodes. We may add redundant
     // but will be eliminated in a later pass.
     // TODO(mstarzinger): Be smarter about this!
     NewMerge();
-    merge_environments_[target_offset] = environment();
+    merge_environment = environment();
   } else {
-    merge_environments_[target_offset]->Merge(environment());
+    merge_environment->Merge(environment());
   }
   set_environment(nullptr);
 }