[turboshaft] simplify block inlining

This removes the `direct_input` option from `CloneAndInlineBlock`,
which is now unnecessary thanks to the fix in
https://chromium-review.googlesource.com/c/v8/v8/+/4191773

In addition, this simplifies the code of `CloneAndInlineBlock` by
reducing the usage of assembler-global variables and by moving the
phi logic into the same function.

The creation of variables is now controlled by `current_block_needs_variables_` alone. To set this remaining global flag in a scoped fashion, this also adds the helper class
`ScopedModification` (which is used to simplify `ReentrantScope` too).

Bug: v8:12783

Change-Id: I979b110ea10921477efc4bf2c38bd56b5c573442
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4194203
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#85496}

src/compiler/turboshaft/assert-types-reducer.h

@@ -23,16 +23,11 @@ namespace v8::internal::compiler::turboshaft {
 
 class DetectReentranceScope {
  public:
-  explicit DetectReentranceScope(bool* flag)
-      : is_reentrant_(*flag), flag_(flag) {
-    *flag_ = true;
-  }
-  ~DetectReentranceScope() { *flag_ = is_reentrant_; }
-  bool IsReentrant() const { return is_reentrant_; }
+  explicit DetectReentranceScope(bool* flag) : set_flag_(flag, true) {}
+  bool IsReentrant() const { return set_flag_.old_value(); }
 
  private:
-  bool is_reentrant_;
-  bool* flag_;
+  ScopedModification<bool> set_flag_;
 };
 
 struct AssertTypesReducerArgs {

src/compiler/turboshaft/branch-elimination-reducer.h

@@ -324,12 +324,10 @@ class BranchEliminationReducer : public Next {
       // The destination block in the old graph ends with a Return
       // and the old destination is a merge block, so we can directly
       // inline the destination block in place of the Goto.
-      // We pass `false` to `direct_input` here, as we're looking one
-      // block ahead of the current one.
       // TODO(nicohartmann@): Temporarily disable this "optimization" because it
       // prevents dead code elimination in some cases. Reevaluate this and
       // reenable if phases have been reordered properly.
-      // Asm().CloneAndInlineBlock(old_dst, false);
+      // Asm().CloneAndInlineBlock(old_dst);
       // return OpIndex::Invalid();
     }
 

src/compiler/turboshaft/graph.h

@@ -330,66 +330,6 @@ class Block : public RandomAccessStackDominatorNode<Block> {
     return pred_count - pred_reverse_index - 1;
   }
 
-  // Returns the index of {target} in the predecessors of the current Block
-  // and if target is not in the list of predecessors, recursively traverses
-  // the predecessor graph up.
-  int GetAnyPredecessorIndex(const Block* target, Zone* zone) const {
-    int pred_count = 0;
-    int pred_reverse_index = -1;
-    // The tuple contains (predecessor, index_in_successor, successor).
-    ZoneQueue<std::tuple<const Block*, int, const Block*>> to_visit(zone);
-    for (Block* pred = last_predecessor_; pred != nullptr;
-         pred = pred->neighboring_predecessor_) {
-      if (pred == target) {
-        DCHECK_EQ(pred_reverse_index, -1);
-        pred_reverse_index = pred_count;
-      } else {
-        to_visit.push(std::make_tuple(pred, pred_count, this));
-      }
-      pred_count++;
-    }
-    if (pred_reverse_index != -1) {
-      // The target was found in the predecessors list.
-      return pred_count - pred_reverse_index - 1;
-    }
-
-    const Block* current = nullptr;
-    int pred_index = -1;
-    const Block* successor = nullptr;
-    // The target is not a direct predecessor of the current block.
-    while (!to_visit.empty()) {
-      std::tie(current, pred_index, successor) = to_visit.front();
-      to_visit.pop();
-      if (current == target) {
-        DCHECK_EQ(pred_reverse_index, -1);
-        pred_reverse_index = pred_index;
-        break;
-      } else {
-        int pred_count = 0;
-        for (Block* pred = current->last_predecessor_; pred != nullptr;
-             pred = pred->neighboring_predecessor_) {
-          to_visit.push(std::make_tuple(pred, pred_count, current));
-          pred_count++;
-        }
-      }
-    }
-    if (pred_reverse_index == -1) {
-      // Target was not found by the BFS above.
-      return -1;
-    }
-
-    // The target was found in the predecessors list, we need to reconstruct
-    // the predecessors count for the current (found) Block's successor.
-    DCHECK_NOT_NULL(current);
-    DCHECK_NOT_NULL(successor);
-    pred_count = 0;
-    for (Block* pred = successor->last_predecessor_; pred != nullptr;
-         pred = pred->neighboring_predecessor_) {
-      pred_count++;
-    }
-    return pred_count - pred_reverse_index - 1;
-  }
-
   // HasExactlyNPredecessors(n) returns the same result as
   // `PredecessorCount() == n`, but stops early and iterates at most the first
   // {n} predecessors.

src/compiler/turboshaft/optimization-phase.h

@@ -154,7 +154,6 @@ class GraphVisitor {
         current_input_block_(nullptr),
         block_mapping_(input_graph.block_count(), nullptr, phase_zone),
         op_mapping_(input_graph.op_id_count(), OpIndex::Invalid(), phase_zone),
-        visiting_cloned_block_(false),
         blocks_needing_variables(phase_zone),
         old_opindex_to_variables(input_graph.op_id_count(), phase_zone) {
     output_graph_.Reset();
@@ -212,17 +211,12 @@ class GraphVisitor {
   Graph& modifiable_input_graph() const { return input_graph_; }
 
   // Visits and emits {input_block} right now (ie, in the current block).
-  void CloneAndInlineBlock(const Block* input_block, bool direct_input = true) {
+  void CloneAndInlineBlock(const Block* input_block) {
     // Computing which input of Phi operations to use when visiting
     // {input_block} (since {input_block} doesn't really have predecessors
     // anymore).
-    if (direct_input) {
-      added_block_phi_input_ = input_block->GetPredecessorIndex(
-          assembler().current_block()->Origin());
-    } else {
-      added_block_phi_input_ = input_block->GetAnyPredecessorIndex(
-          assembler().current_block()->Origin(), phase_zone_);
-    }
+    int added_block_phi_input =
+        input_block->GetPredecessorIndex(assembler().current_block()->Origin());
 
     // There is no guarantees that {input_block} will be entirely removed just
     // because it's cloned/inlined, since it's possible that it has predecessors
@@ -237,11 +231,18 @@ class GraphVisitor {
     // still properly be done (in OptimizationPhase::ReducePhi).
     assembler().current_block()->SetOrigin(input_block);
 
-    visiting_cloned_block_ = true;
+    ScopedModification<bool> set_true(&current_block_needs_variables_, true);
     for (OpIndex index : input_graph().OperationIndices(*input_block)) {
-      if (!VisitOp<false>(index, input_block)) break;
+      if (const PhiOp* phi =
+              input_graph().Get(index).template TryCast<PhiOp>()) {
+        // This Phi has been cloned/inlined, and has thus now a single
+        // predecessor, and shouldn't be a Phi anymore.
+        CreateOldToNewMapping(index,
+                              MapToNewGraph(phi->input(added_block_phi_input)));
+      } else {
+        if (!VisitOp<false>(index, input_block)) break;
+      }
     }
-    visiting_cloned_block_ = false;
   }
 
   template <bool can_be_invalid = false>
@@ -456,11 +457,6 @@ class GraphVisitor {
                                         op.input(PhiOp::kLoopPhiBackEdgeIndex));
     }
 
-    if (visiting_cloned_block_) {
-      // This Phi has been cloned/inlined, and has thus now a single
-      // predecessor, and shouldn't be a Phi anymore.
-      return MapToNewGraph(op.input(added_block_phi_input_));
-    }
     base::Vector<const OpIndex> old_inputs = op.inputs();
     base::SmallVector<OpIndex, 8> new_inputs;
     int predecessor_count = assembler().current_block()->PredecessorCount();
@@ -744,7 +740,7 @@ class GraphVisitor {
   }
 
   void CreateOldToNewMapping(OpIndex old_index, OpIndex new_index) {
-    if (visiting_cloned_block_ || current_block_needs_variables_) {
+    if (current_block_needs_variables_) {
       MaybeVariable var = GetVariableFor(old_index);
       if (!var.has_value()) {
         base::Optional<RegisterRepresentation> rep =
@@ -816,14 +812,6 @@ class GraphVisitor {
   ZoneVector<Block*> block_mapping_;
   ZoneVector<OpIndex> op_mapping_;
 
-  // {visiting_cloned_block_} is set to true when cloning a block, which impacts
-  // how Phis are reduced, and how mappings from old to new OpIndex are
-  // maintained.
-  bool visiting_cloned_block_ = false;
-  // When visiting a cloned block, the {added_block_phi_input_}th of Phis is
-  // used to replace those Phis.
-  int added_block_phi_input_;
-
   // {current_block_needs_variables_} is set to true if the current block should
   // use Variables to map old to new OpIndex rather than just {op_mapping}. This
   // is typically the case when the block has been cloned.

src/compiler/turboshaft/utils.h

@@ -82,6 +82,25 @@ bool ShouldSkipOptimizationStep();
 inline bool ShouldSkipOptimizationStep() { return false; }
 #endif
 
+// Set `*ptr` to `new_value` while the scope is active, reset to the previous
+// value upon destruction.
+template <class T>
+class ScopedModification {
+ public:
+  ScopedModification(T* ptr, T new_value)
+      : ptr_(ptr), old_value_(std::move(*ptr)) {
+    *ptr = std::move(new_value);
+  }
+
+  ~ScopedModification() { *ptr_ = std::move(old_value_); }
+
+  const T& old_value() const { return old_value_; }
+
+ private:
+  T* ptr_;
+  T old_value_;
+};
+
 }  // namespace v8::internal::compiler::turboshaft
 
 #endif  // V8_COMPILER_TURBOSHAFT_UTILS_H_