[masm] Shortcut some indirect external reference loads

External references located within the isolate can be loaded as a
fixed offset from the root register. This avoids one load from memory
over the default indirect method (which reads from the external
reference table).

Bug: v8:6666
Change-Id: I569308c03f1a8c3feb1be7ceb072178b5be17b25
Reviewed-on: https://chromium-review.googlesource.com/1092739
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53661}

src/arm/macro-assembler-arm.cc

@@ -173,6 +173,15 @@ void TurboAssembler::LoadBuiltin(Register destination, int builtin_index) {
 
   ldr(destination, MemOperand(kRootRegister, roots_to_builtins_offset));
 }
+
+void TurboAssembler::LoadRootRegisterOffset(Register destination,
+                                            intptr_t offset) {
+  if (offset == 0) {
+    Move(destination, kRootRegister);
+  } else {
+    add(destination, kRootRegister, Operand(offset));
+  }
+}
 #endif  // V8_EMBEDDED_BUILTINS
 
 void TurboAssembler::Jump(Register target, Condition cond) { bx(target, cond); }

src/arm/macro-assembler-arm.h

@@ -322,6 +322,7 @@ class TurboAssembler : public TurboAssemblerBase {
   void LoadExternalReference(Register destination,
                              int reference_index) override;
   void LoadBuiltin(Register destination, int builtin_index) override;
+  void LoadRootRegisterOffset(Register destination, intptr_t offset) override;
 #endif  // V8_EMBEDDED_BUILTINS
 
   // Returns the size of a call in instructions. Note, the value returned is

src/arm64/macro-assembler-arm64.cc

@@ -1891,6 +1891,15 @@ void TurboAssembler::LoadBuiltin(Register destination, int builtin_index) {
       Heap::roots_to_builtins_offset() + builtin_index * kPointerSize;
   Ldr(destination, MemOperand(kRootRegister, roots_to_builtins_offset));
 }
+
+void TurboAssembler::LoadRootRegisterOffset(Register destination,
+                                            intptr_t offset) {
+  if (offset == 0) {
+    Move(destination, kRootRegister);
+  } else {
+    Add(destination, kRootRegister, Operand(offset));
+  }
+}
 #endif  // V8_EMBEDDED_BUILTINS
 
 void TurboAssembler::Jump(Register target, Condition cond) {

src/arm64/macro-assembler-arm64.h

@@ -880,6 +880,7 @@ class TurboAssembler : public TurboAssemblerBase {
   void LoadExternalReference(Register destination,
                              int reference_index) override;
   void LoadBuiltin(Register destination, int builtin_index) override;
+  void LoadRootRegisterOffset(Register destination, intptr_t offset) override;
 #endif  // V8_EMBEDDED_BUILTINS
 
   void Jump(Register target, Condition cond = al);

src/external-reference.cc

@@ -834,6 +834,24 @@ ExternalReference ExternalReference::ForDeoptEntry(Address entry) {
   return ExternalReference(entry);
 }
 
+bool ExternalReference::IsAddressableThroughRootRegister(
+    Isolate* isolate) const {
+  Address start = reinterpret_cast<Address>(isolate);
+  Address end = isolate->heap()->root_register_addressable_end();
+  return start <= address_ && address_ < end;
+}
+
+intptr_t ExternalReference::OffsetFromRootRegister(Isolate* isolate) const {
+  DCHECK(IsAddressableThroughRootRegister(isolate));
+#ifdef V8_TARGET_ARCH_X64
+  return static_cast<intptr_t>(address_) - kRootRegisterBias -
+         reinterpret_cast<intptr_t>(isolate->heap()->roots_array_start());
+#else
+  return static_cast<intptr_t>(address_) -
+         reinterpret_cast<intptr_t>(isolate->heap()->roots_array_start());
+#endif
+}
+
 ExternalReference ExternalReference::cpu_features() {
   DCHECK(CpuFeatures::initialized_);
   return ExternalReference(&CpuFeatures::supported_);

src/external-reference.h

@@ -285,6 +285,11 @@ class ExternalReference BASE_EMBEDDED {
 
   Address address() const { return address_; }
 
+  // An address is addressable through kRootRegister if it is located within
+  // [isolate, roots_ + root_register_addressable_end_offset[.
+  bool IsAddressableThroughRootRegister(Isolate* isolate) const;
+  intptr_t OffsetFromRootRegister(Isolate* isolate) const;
+
  private:
   explicit ExternalReference(Address address) : address_(address) {}
 

src/heap/heap.h

@@ -1099,6 +1099,11 @@ class Heap {
     return kRootsBuiltinsOffset;
   }
 
+  Address root_register_addressable_end() {
+    return reinterpret_cast<Address>(roots_array_start()) +
+           kRootRegisterAddressableEndOffset;
+  }
+
   // Sets the stub_cache_ (only used when expanding the dictionary).
   void SetRootCodeStubs(SimpleNumberDictionary* value);
 
@@ -2216,6 +2221,12 @@ class Heap {
       ExternalReferenceTable::SizeInBytes();
   Object* builtins_[Builtins::builtin_count];
 
+  // kRootRegister may be used to address any location that starts at the
+  // Isolate and ends at this point. Fields past this point are not guaranteed
+  // to live at a static offset from kRootRegister.
+  static constexpr int kRootRegisterAddressableEndOffset =
+      kRootsBuiltinsOffset + Builtins::builtin_count * kPointerSize;
+
   size_t code_range_size_;
   size_t max_semi_space_size_;
   size_t initial_semispace_size_;

src/mips/macro-assembler-mips.cc

@@ -3623,6 +3623,15 @@ void TurboAssembler::LoadBuiltin(Register destination, int builtin_index) {
       Heap::roots_to_builtins_offset() + builtin_index * kPointerSize;
   lw(destination, MemOperand(kRootRegister, roots_to_builtins_offset));
 }
+
+void TurboAssembler::LoadRootRegisterOffset(Register destination,
+                                            intptr_t offset) {
+  if (offset == 0) {
+    Move(destination, kRootRegister);
+  } else {
+    Addu(destination, kRootRegister, offset);
+  }
+}
 #endif  // V8_EMBEDDED_BUILTINS
 
 void TurboAssembler::Jump(Register target, int16_t offset, Condition cond,

src/mips/macro-assembler-mips.h

@@ -251,6 +251,7 @@ class TurboAssembler : public TurboAssemblerBase {
   void LoadExternalReference(Register destination,
                              int reference_index) override;
   void LoadBuiltin(Register destination, int builtin_index) override;
+  void LoadRootRegisterOffset(Register destination, intptr_t offset) override;
 #endif  // V8_EMBEDDED_BUILTINS
 
 // Jump, Call, and Ret pseudo instructions implementing inter-working.

src/mips64/macro-assembler-mips64.cc

@@ -4132,6 +4132,15 @@ void TurboAssembler::LoadBuiltin(Register destination, int builtin_index) {
       Heap::roots_to_builtins_offset() + builtin_index * kPointerSize;
   Ld(destination, MemOperand(kRootRegister, roots_to_builtins_offset));
 }
+
+void TurboAssembler::LoadRootRegisterOffset(Register destination,
+                                            intptr_t offset) {
+  if (offset == 0) {
+    Move(destination, kRootRegister);
+  } else {
+    Daddu(destination, kRootRegister, Operand(offset));
+  }
+}
 #endif  // V8_EMBEDDED_BUILTINS
 
 void TurboAssembler::Jump(Register target, Condition cond, Register rs,

src/mips64/macro-assembler-mips64.h

@@ -271,6 +271,7 @@ class TurboAssembler : public TurboAssemblerBase {
   void LoadExternalReference(Register destination,
                              int reference_index) override;
   void LoadBuiltin(Register destination, int builtin_index) override;
+  void LoadRootRegisterOffset(Register destination, intptr_t offset) override;
 #endif  // V8_EMBEDDED_BUILTINS
 
 // Jump, Call, and Ret pseudo instructions implementing inter-working.

src/turbo-assembler.cc

@@ -67,16 +67,22 @@ void TurboAssemblerBase::IndirectLoadExternalReference(
   CHECK(isolate()->ShouldLoadConstantsFromRootList());
   CHECK(root_array_available_);
 
-  // Encode as an index into the external reference table stored on the isolate.
+  if (reference.IsAddressableThroughRootRegister(isolate())) {
+    // Some external references can be efficiently loaded as an offset from
+    // kRootRegister.
+    intptr_t offset = reference.OffsetFromRootRegister(isolate());
+    LoadRootRegisterOffset(destination, offset);
+  } else {
+    // Otherwise, do a memory load from the external reference table.
 
-  ExternalReferenceEncoder encoder(isolate());
-  ExternalReferenceEncoder::Value v = encoder.Encode(reference.address());
-  CHECK(!v.is_from_api());
+    // Encode as an index into the external reference table stored on the
+    // isolate.
+    ExternalReferenceEncoder encoder(isolate());
+    ExternalReferenceEncoder::Value v = encoder.Encode(reference.address());
+    CHECK(!v.is_from_api());
 
-  // TODO(jgruber, v8:6666): Addresses within the isolate can be loaded through
-  // an offset from the roots pointer, skipping one level of indirection.
-
-  LoadExternalReference(destination, v.index());
+    LoadExternalReference(destination, v.index());
+  }
 }
 #endif  // V8_EMBEDDED_BUILTINS
 

src/turbo-assembler.h

@@ -45,6 +45,8 @@ class TurboAssemblerBase : public Assembler {
   virtual void LoadExternalReference(Register destination,
                                      int reference_index) = 0;
   virtual void LoadBuiltin(Register destination, int builtin_index) = 0;
+  virtual void LoadRootRegisterOffset(Register destination,
+                                      intptr_t offset) = 0;
 #endif  // V8_EMBEDDED_BUILTINS
 
   virtual void LoadRoot(Register destination, Heap::RootListIndex index) = 0;

src/x64/macro-assembler-x64.cc

@@ -172,6 +172,16 @@ void TurboAssembler::LoadBuiltin(Register destination, int builtin_index) {
 
   movp(destination, Operand(kRootRegister, roots_to_builtins_offset));
 }
+
+void TurboAssembler::LoadRootRegisterOffset(Register destination,
+                                            intptr_t offset) {
+  DCHECK(is_int32(offset));
+  if (offset == 0) {
+    Move(destination, kRootRegister);
+  } else {
+    leap(destination, Operand(kRootRegister, static_cast<int32_t>(offset)));
+  }
+}
 #endif  // V8_EMBEDDED_BUILTINS
 
 void TurboAssembler::LoadAddress(Register destination,

src/x64/macro-assembler-x64.h

@@ -366,6 +366,7 @@ class TurboAssembler : public TurboAssemblerBase {
   void LoadBuiltin(Register destination, int builtin_index) override;
   void LoadExternalReference(Register destination,
                              int reference_index) override;
+  void LoadRootRegisterOffset(Register destination, intptr_t offset) override;
 #endif  // V8_EMBEDDED_BUILTINS
 
   // Operand pointing to an external reference.