optimizing: constructor fence redundancy elimination - remove dmb after LSE

Part one of a few upcoming CLs to optimize constructor fences.

This improves load-store-elimination; all singleton objects that are not
returned will have their associated constructor fence removed.

If the allocation is removed, so is the fence. Even if allocation is not
removed, fences can sometimes be removed.

This change is enabled by tracking the "this" object associated with the
constructor fence as an input. Fence inputs are considered weak; they do not keep
the "this" object alive; if the instructions for "this" are all deleted,
the fence can also be deleted.

Bug: 36656456
Test: art/test.py --host && art/test.py --target
Change-Id: I05659ab07e20d6e2ecd4be051b722726776f4ab1

1 files changed, 6 insertions, 0 deletions

diff --git a/compiler/optimizing/instruction_builder.h b/compiler/optimizing/instruction_builder.h
index 7fdc1883ca..2fb5c7b94d 100644
--- a/compiler/optimizing/instruction_builder.h
+++ b/compiler/optimizing/instruction_builder.h
@@ -62,6 +62,7 @@ class HInstructionBuilder : public ValueObject {
         current_block_(nullptr),
         current_locals_(nullptr),
         latest_result_(nullptr),
+        current_this_parameter_(nullptr),
         compiler_driver_(driver),
         code_generator_(code_generator),
         dex_compilation_unit_(dex_compilation_unit),
@@ -325,6 +326,11 @@ class HInstructionBuilder : public ValueObject {
   HBasicBlock* current_block_;
   ArenaVector<HInstruction*>* current_locals_;
   HInstruction* latest_result_;
+  // Current "this" parameter.
+  // Valid only after InitializeParameters() finishes.
+  // * Null for static methods.
+  // * Non-null for instance methods.
+  HParameterValue* current_this_parameter_;
 
   CompilerDriver* const compiler_driver_;