[Interpreter] Allocate context registers as temporary registers.

Rather than trying to pre-calculate the number of contexts required during
scope analysis, instead just allocate context registers in the register
allocator. This reduces frame size a bit due to reusing of registers when
the context isn't pushed.

 BUG=v8:6322, chromium:716265

Change-Id: I145e38fcb3797a3b86c91e90ea9326a6e55b9b89
Reviewed-on: https://chromium-review.googlesource.com/514087Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45522}

src/ast/scopes.cc

@@ -1409,19 +1409,6 @@ int Scope::ContextChainLengthUntilOutermostSloppyEval() const {
   return result;
 }
 
-int Scope::MaxNestedContextChainLength() {
-  int max_context_chain_length = 0;
-  for (Scope* scope = inner_scope_; scope != nullptr; scope = scope->sibling_) {
-    if (scope->is_function_scope()) continue;
-    max_context_chain_length = std::max(scope->MaxNestedContextChainLength(),
-                                        max_context_chain_length);
-  }
-  if (NeedsContext()) {
-    max_context_chain_length += 1;
-  }
-  return max_context_chain_length;
-}
-
 DeclarationScope* Scope::GetDeclarationScope() {
   Scope* scope = this;
   while (!scope->is_declaration_scope()) {

src/ast/scopes.h

@@ -407,10 +407,6 @@ class V8_EXPORT_PRIVATE Scope : public NON_EXPORTED_BASE(ZoneObject) {
   // sloppy eval call. One if this->calls_sloppy_eval().
   int ContextChainLengthUntilOutermostSloppyEval() const;
 
-  // The maximum number of nested contexts required for this scope and any inner
-  // scopes.
-  int MaxNestedContextChainLength();
-
   // Find the first function, script, eval or (declaration) block scope. This is
   // the scope where var declarations will be hoisted to in the implementation.
   DeclarationScope* GetDeclarationScope();

src/interpreter/bytecode-array-builder.cc

@@ -38,8 +38,8 @@ class RegisterTransferWriter final
 };
 
 BytecodeArrayBuilder::BytecodeArrayBuilder(
-    Isolate* isolate, Zone* zone, int parameter_count, int context_count,
-    int locals_count, FunctionLiteral* literal,
+    Isolate* isolate, Zone* zone, int parameter_count, int locals_count,
+    FunctionLiteral* literal,
     SourcePositionTableBuilder::RecordingMode source_position_mode)
     : zone_(zone),
       literal_(literal),
@@ -49,13 +49,11 @@ BytecodeArrayBuilder::BytecodeArrayBuilder(
       return_seen_in_block_(false),
       parameter_count_(parameter_count),
       local_register_count_(locals_count),
-      context_register_count_(context_count),
       register_allocator_(fixed_register_count()),
       bytecode_array_writer_(zone, &constant_array_builder_,
                              source_position_mode),
       register_optimizer_(nullptr) {
   DCHECK_GE(parameter_count_, 0);
-  DCHECK_GE(context_register_count_, 0);
   DCHECK_GE(local_register_count_, 0);
 
   if (FLAG_ignition_reo) {
@@ -67,16 +65,6 @@ BytecodeArrayBuilder::BytecodeArrayBuilder(
   return_position_ = literal ? literal->return_position() : kNoSourcePosition;
 }
 
-Register BytecodeArrayBuilder::first_context_register() const {
-  DCHECK_GT(context_register_count_, 0);
-  return Register(local_register_count_);
-}
-
-Register BytecodeArrayBuilder::last_context_register() const {
-  DCHECK_GT(context_register_count_, 0);
-  return Register(local_register_count_ + context_register_count_ - 1);
-}
-
 Register BytecodeArrayBuilder::Parameter(int parameter_index) const {
   DCHECK_GE(parameter_index, 0);
   // The parameter indices are shifted by 1 (receiver is the

src/interpreter/bytecode-array-builder.h

@@ -35,8 +35,8 @@ class V8_EXPORT_PRIVATE BytecodeArrayBuilder final
     : public NON_EXPORTED_BASE(ZoneObject) {
  public:
   BytecodeArrayBuilder(
-      Isolate* isolate, Zone* zone, int parameter_count, int context_count,
-      int locals_count, FunctionLiteral* literal = nullptr,
+      Isolate* isolate, Zone* zone, int parameter_count, int locals_count,
+      FunctionLiteral* literal = nullptr,
       SourcePositionTableBuilder::RecordingMode source_position_mode =
           SourcePositionTableBuilder::RECORD_SOURCE_POSITIONS);
 
@@ -54,17 +54,8 @@ class V8_EXPORT_PRIVATE BytecodeArrayBuilder final
     return local_register_count_;
   }
 
-  // Get number of contexts required for bytecode array.
-  int context_count() const {
-    DCHECK_GE(context_register_count_, 0);
-    return context_register_count_;
-  }
-
-  Register first_context_register() const;
-  Register last_context_register() const;
-
   // Returns the number of fixed (non-temporary) registers.
-  int fixed_register_count() const { return context_count() + locals_count(); }
+  int fixed_register_count() const { return locals_count(); }
 
   // Returns the number of fixed and temporary registers.
   int total_register_count() const {
@@ -558,7 +549,6 @@ class V8_EXPORT_PRIVATE BytecodeArrayBuilder final
   bool return_seen_in_block_;
   int parameter_count_;
   int local_register_count_;
-  int context_register_count_;
   int return_position_;
   BytecodeRegisterAllocator register_allocator_;
   BytecodeArrayWriter bytecode_array_writer_;

src/interpreter/bytecode-generator.cc

@@ -38,15 +38,9 @@ class BytecodeGenerator::ContextScope BASE_EMBEDDED {
     if (outer_) {
       depth_ = outer_->depth_ + 1;
 
-      int outer_reg_index =
-          builder()->first_context_register().index() + outer_->depth_;
-
-      // TODO(ignition): ensure overwriting of non-context registers with
-      // a Context can never occurs, and re-enable DCHECK.
-      // DCHECK_LE(outer_reg_index, builder()->last_context_register().index());
-
       // Push the outer context into a new context register.
-      Register outer_context_reg(outer_reg_index);
+      Register outer_context_reg =
+          generator_->register_allocator()->NewRegister();
       outer_->set_register(outer_context_reg);
       generator_->builder()->PushContext(outer_context_reg);
     }
@@ -766,7 +760,6 @@ BytecodeGenerator::BytecodeGenerator(CompilationInfo* info)
     : zone_(info->zone()),
       builder_(new (zone()) BytecodeArrayBuilder(
           info->isolate(), info->zone(), info->num_parameters_including_this(),
-          info->scope()->MaxNestedContextChainLength(),
           info->scope()->num_stack_slots(), info->literal(),
           info->SourcePositionRecordingMode())),
       info_(info),

test/cctest/interpreter/bytecode_expectations/TryCatch.golden

@@ -9,23 +9,23 @@ wrap: yes
 snippet: "
   try { return 1; } catch(e) { return 2; }
 "
-frame size: 3
+frame size: 2
 parameter count: 1
 bytecode array length: 30
 bytecodes: [
   /*   30 E> */ B(StackCheck),
-                B(Mov), R(context), R(1),
+                B(Mov), R(context), R(0),
   /*   40 S> */ B(LdaSmi), I8(1),
   /*   75 S> */ B(Return),
                 B(Jump), U8(21),
-                B(Star), R(2),
-                B(Ldar), R(closure),
-                B(CreateCatchContext), R(2), U8(0), U8(1),
                 B(Star), R(1),
+                B(Ldar), R(closure),
+                B(CreateCatchContext), R(1), U8(0), U8(1),
+                B(Star), R(0),
                 B(LdaTheHole),
                 B(SetPendingMessage),
-                B(Ldar), R(1),
-                B(PushContext), R(0),
+                B(Ldar), R(0),
+                B(PushContext), R(1),
   /*   63 S> */ B(LdaSmi), I8(2),
   /*   75 S> */ B(Return),
                 B(LdaUndefined),
@@ -45,39 +45,39 @@ snippet: "
   try { a = 1 } catch(e1) {};
   try { a = 2 } catch(e2) { a = 3 }
 "
-frame size: 4
+frame size: 3
 parameter count: 1
 bytecode array length: 61
 bytecodes: [
   /*   30 E> */ B(StackCheck),
-                B(Mov), R(context), R(2),
+                B(Mov), R(context), R(1),
   /*   47 S> */ B(LdaSmi), I8(1),
                 B(Star), R(0),
                 B(Jump), U8(20),
-                B(Star), R(3),
-                B(Ldar), R(closure),
-  /*   49 E> */ B(CreateCatchContext), R(3), U8(0), U8(1),
                 B(Star), R(2),
+                B(Ldar), R(closure),
+  /*   49 E> */ B(CreateCatchContext), R(2), U8(0), U8(1),
+                B(Star), R(1),
                 B(LdaTheHole),
                 B(SetPendingMessage),
-                B(Ldar), R(2),
-                B(PushContext), R(1),
-                B(PopContext), R(1),
-                B(Mov), R(context), R(2),
+                B(Ldar), R(1),
+                B(PushContext), R(2),
+                B(PopContext), R(2),
+                B(Mov), R(context), R(1),
   /*   75 S> */ B(LdaSmi), I8(2),
                 B(Star), R(0),
                 B(Jump), U8(24),
-                B(Star), R(3),
-                B(Ldar), R(closure),
-  /*   77 E> */ B(CreateCatchContext), R(3), U8(2), U8(3),
                 B(Star), R(2),
+                B(Ldar), R(closure),
+  /*   77 E> */ B(CreateCatchContext), R(2), U8(2), U8(3),
+                B(Star), R(1),
                 B(LdaTheHole),
                 B(SetPendingMessage),
-                B(Ldar), R(2),
-                B(PushContext), R(1),
+                B(Ldar), R(1),
+                B(PushContext), R(2),
   /*   95 S> */ B(LdaSmi), I8(3),
                 B(Star), R(0),
-                B(PopContext), R(1),
+                B(PopContext), R(2),
                 B(LdaUndefined),
   /*  103 S> */ B(Return),
 ]

test/cctest/interpreter/bytecode_expectations/WithStatement.golden

@@ -9,16 +9,16 @@ wrap: yes
 snippet: "
   with ({x:42}) { return x; }
 "
-frame size: 2
+frame size: 1
 parameter count: 1
 bytecode array length: 20
 bytecodes: [
   /*   30 E> */ B(StackCheck),
-  /*   34 S> */ B(CreateObjectLiteral), U8(0), U8(3), U8(1), R(1),
-                B(Ldar), R(1),
-                B(ToObject), R(1),
+  /*   34 S> */ B(CreateObjectLiteral), U8(0), U8(3), U8(1), R(0),
+                B(Ldar), R(0),
+                B(ToObject), R(0),
                 B(Ldar), R(closure),
-                B(CreateWithContext), R(1), U8(1),
+                B(CreateWithContext), R(0), U8(1),
                 B(PushContext), R(0),
   /*   50 S> */ B(LdaLookupSlot), U8(2),
   /*   62 S> */ B(Return),