Remove empty ifs

If we had an HIf where both branches were empty, we wouldn't emit
any code generation for the HIf. However, this still emits other
instructions e.g. outer ifs, instructions used in the if guard.

This CL detects when an HIf is empty and can be eliminated during
DCE.

Bug: 305225992
Fixes: 305225992
Test: art/test/testrunner/testrunner.py --host --64 -b --optimizing
Change-Id: I81ff5ee1326b6a27480b1af88d41955f1ca2b823

1 files changed, 11 insertions, 0 deletions

diff --git a/compiler/optimizing/dead_code_elimination.h b/compiler/optimizing/dead_code_elimination.h
index ddd01f7103..789962f93c 100644
--- a/compiler/optimizing/dead_code_elimination.h
+++ b/compiler/optimizing/dead_code_elimination.h
@@ -40,6 +40,17 @@ class HDeadCodeElimination : public HOptimization {
  private:
   void MaybeRecordDeadBlock(HBasicBlock* block);
   void MaybeRecordSimplifyIf();
+  // Detects and remove ifs that are empty e.g. it turns
+  //     1
+  //    / \
+  //   2   3
+  //   \  /
+  //    4
+  // where 2 and 3 are single goto blocks and 4 doesn't contain a Phi into:
+  //    1
+  //    |
+  //    4
+  bool RemoveEmptyIfs();
   // If `force_recomputation` is true, we will recompute the dominance information even when we
   // didn't delete any blocks. `force_loop_recomputation` is similar but it also forces the loop
   // information recomputation.