Merge "optimizing: constructor fence redundancy elimination - remove dmb after LSE"

1 files changed, 34 insertions, 0 deletions

diff --git a/compiler/driver/compiler_driver.h b/compiler/driver/compiler_driver.h
index 874e35716c..fbab9dfbaf 100644
--- a/compiler/driver/compiler_driver.h
+++ b/compiler/driver/compiler_driver.h
@@ -179,6 +179,40 @@ class CompilerDriver {
                                      uint16_t class_def_index,
                                      bool requires)
       REQUIRES(!requires_constructor_barrier_lock_);
+
+  // Do the <init> methods for this class require a constructor barrier (prior to the return)?
+  // The answer is "yes", if and only if this class has any instance final fields.
+  // (This must not be called for any non-<init> methods; the answer would be "no").
+  //
+  // ---
+  //
+  // JLS 17.5.1 "Semantics of final fields" mandates that all final fields are frozen at the end
+  // of the invoked constructor. The constructor barrier is a conservative implementation means of
+  // enforcing the freezes happen-before the object being constructed is observable by another
+  // thread.
+  //
+  // Note: This question only makes sense for instance constructors;
+  // static constructors (despite possibly having finals) never need
+  // a barrier.
+  //
+  // JLS 12.4.2 "Detailed Initialization Procedure" approximately describes
+  // class initialization as:
+  //
+  //   lock(class.lock)
+  //     class.state = initializing
+  //   unlock(class.lock)
+  //
+  //   invoke <clinit>
+  //
+  //   lock(class.lock)
+  //     class.state = initialized
+  //   unlock(class.lock)              <-- acts as a release
+  //
+  // The last operation in the above example acts as an atomic release
+  // for any stores in <clinit>, which ends up being stricter
+  // than what a constructor barrier needs.
+  //
+  // See also QuasiAtomic::ThreadFenceForConstructor().
   bool RequiresConstructorBarrier(Thread* self,
                                   const DexFile* dex_file,
                                   uint16_t class_def_index)