[Liftoff] Only allocate Labels on the heap on arm64

Arm64 is the only platform where Labels cannot be moved, because the
assembler keeps track of pointers to Labels. On all other platforms,
there is no need to heap-allocate the Labels.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I4e98203890a8c426afa8a1db21e31f30bab892fa
Reviewed-on: https://chromium-review.googlesource.com/783210Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49572}

src/wasm/baseline/liftoff-compiler.cc

@@ -30,6 +30,28 @@ namespace {
     if (FLAG_trace_liftoff) PrintF("[liftoff] " __VA_ARGS__); \
   } while (false)
 
+#if V8_TARGET_ARCH_ARM64
+// On ARM64, the Assembler keeps track of pointers to Labels to resolve
+// branches to distant targets. Moving labels would confuse the Assembler,
+// thus store the label on the heap and keep a unique_ptr.
+class MovableLabel {
+ public:
+  Label* get() { return label_.get(); }
+
+ private:
+  std::unique_ptr<Label> label_ = base::make_unique<Label>();
+};
+#else
+// On all other platforms, just store the Label directly.
+class MovableLabel {
+ public:
+  Label* get() { return &label_; }
+
+ private:
+  Label label_;
+};
+#endif
+
 class LiftoffCompiler {
  public:
   MOVE_ONLY_NO_DEFAULT_CONSTRUCTOR(LiftoffCompiler);
@@ -44,9 +66,7 @@ class LiftoffCompiler {
     MOVE_ONLY_WITH_DEFAULT_CONSTRUCTORS(Control);
 
     LiftoffAssembler::CacheState label_state;
-    // TODO(clemensh): Labels cannot be moved on arm64, but everywhere else.
-    // Find a better solution.
-    std::unique_ptr<Label> label = base::make_unique<Label>();
+    MovableLabel label;
   };
 
   using Decoder = WasmFullDecoder<validate, LiftoffCompiler>;
@@ -206,7 +226,7 @@ class LiftoffCompiler {
     if (!c->is_loop() && c->end_merge.reached) {
       __ cache_state()->Steal(c->label_state);
     }
-    if (!c->label->is_bound()) {
+    if (!c->label.get()->is_bound()) {
       __ bind(c->label.get());
     }
   }