Optimize btree_iterator increment/decrement to avoid aliasing issues by using local variables instead of repeatedly writing to `this`.

Also update the benchmark to use DoNotOptimize to make sure the iterators are actually materialized.

PiperOrigin-RevId: 736229349
Change-Id: I7422544398210a48a7a40d81d341fd4beaf71861

absl/container/btree_benchmark.cc

diff --git a/absl/container/btree_benchmark.cc b/absl/container/btree_benchmark.cc
index d0dac37..b1525bd 100644
--- a/absl/container/btree_benchmark.cc
+++ b/absl/container/btree_benchmark.cc
@@ -406,22 +406,18 @@
 template <typename T>
 void BM_FwdIter(benchmark::State& state) {
   using V = typename remove_pair_const<typename T::value_type>::type;
-  using R = typename T::value_type const*;
 
   std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
   T container(values.begin(), values.end());
 
   auto iter = container.end();
 
-  R r = nullptr;
-
   while (state.KeepRunning()) {
     if (iter == container.end()) iter = container.begin();
-    r = &(*iter);
-    ++iter;
+    auto *v = &(*iter);
+    benchmark::DoNotOptimize(v);
+    benchmark::DoNotOptimize(++iter);
   }
-
-  benchmark::DoNotOptimize(r);
 }
 
 // Benchmark random range-construction of a container.

absl/container/internal/btree.h

diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
index a742829..bed6be1 100644
--- a/absl/container/internal/btree.h
+++ b/absl/container/internal/btree.h
@@ -2151,50 +2151,54 @@
 
 template <typename N, typename R, typename P>
 void btree_iterator<N, R, P>::increment_slow() {
-  if (node_->is_leaf()) {
-    assert(position_ >= node_->finish());
-    btree_iterator save(*this);
-    while (position_ == node_->finish() && !node_->is_root()) {
-      assert(node_->parent()->child(node_->position()) == node_);
-      position_ = node_->position();
-      node_ = node_->parent();
+  N* node = node_;
+  int position = position_;
+  if (node->is_leaf()) {
+    assert(position >= node->finish());
+    while (position == node->finish() && !node->is_root()) {
+      assert(node->parent()->child(node->position()) == node);
+      position = node->position();
+      node = node->parent();
     }
     // TODO(ezb): assert we aren't incrementing end() instead of handling.
-    if (position_ == node_->finish()) {
-      *this = save;
+    if (position == node->finish()) {
+      return;
     }
   } else {
-    assert(position_ < node_->finish());
-    node_ = node_->child(static_cast<field_type>(position_ + 1));
-    while (node_->is_internal()) {
-      node_ = node_->start_child();
+    assert(position < node->finish());
+    node = node->child(static_cast<field_type>(position + 1));
+    while (node->is_internal()) {
+      node = node->start_child();
     }
-    position_ = node_->start();
+    position = node->start();
   }
+  *this = {node, position};
 }
 
 template <typename N, typename R, typename P>
 void btree_iterator<N, R, P>::decrement_slow() {
-  if (node_->is_leaf()) {
-    assert(position_ <= -1);
-    btree_iterator save(*this);
-    while (position_ < node_->start() && !node_->is_root()) {
-      assert(node_->parent()->child(node_->position()) == node_);
-      position_ = node_->position() - 1;
-      node_ = node_->parent();
+  N* node = node_;
+  int position = position_;
+  if (node->is_leaf()) {
+    assert(position <= -1);
+    while (position < node->start() && !node->is_root()) {
+      assert(node->parent()->child(node->position()) == node);
+      position = node->position() - 1;
+      node = node->parent();
     }
     // TODO(ezb): assert we aren't decrementing begin() instead of handling.
-    if (position_ < node_->start()) {
-      *this = save;
+    if (position < node->start()) {
+      return;
     }
   } else {
-    assert(position_ >= node_->start());
-    node_ = node_->child(static_cast<field_type>(position_));
-    while (node_->is_internal()) {
-      node_ = node_->child(node_->finish());
+    assert(position >= node->start());
+    node = node->child(static_cast<field_type>(position));
+    while (node->is_internal()) {
+      node = node->child(node->finish());
     }
-    position_ = node_->finish() - 1;
+    position = node->finish() - 1;
   }
+  *this = {node, position};
 }
 
 template <typename N, typename R, typename P>