Remove dead phis alongside dead instructions in DCE

A dead phi doesn't generate any code, but it may keep
other instructions alive.

Locally speed compiling, it improves generated code 0.01-0.1%.

Test: art/test/testrunner/testrunner.py --host --64 -b --optimizing
Change-Id: Ida024200baa7afeee8c5a729c95b573ed0c6e4e7

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler/optimizing/dead_code_elimination.cc b/compiler/optimizing/dead_code_elimination.cc
index 5784b93670..5b420db5be 100644
--- a/compiler/optimizing/dead_code_elimination.cc
+++ b/compiler/optimizing/dead_code_elimination.cc
@@ -919,6 +919,16 @@ void HDeadCodeElimination::RemoveDeadInstructions() {
         MaybeRecordStat(stats_, MethodCompilationStat::kRemovedDeadInstruction);
       }
     }
+
+    // Same for Phis.
+    for (HBackwardInstructionIterator phi_it(block->GetPhis()); !phi_it.Done(); phi_it.Advance()) {
+      DCHECK(phi_it.Current()->IsPhi());
+      HPhi* phi = phi_it.Current()->AsPhi();
+      if (phi->IsDeadAndRemovable()) {
+        block->RemovePhi(phi);
+        MaybeRecordStat(stats_, MethodCompilationStat::kRemovedDeadPhi);
+      }
+    }
   }
 }