Initial implementation of sparse conditional constant propagation.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index df20d4d2653426c4e3b09e81352abb3cd95133ff..d23b879a18783126b8e768e7262050ce41fd730c 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -35,6 +35,8 @@ DEFINE_FLAG(bool, disassemble, false, "Disassemble dart code.");
 DEFINE_FLAG(bool, disassemble_optimized, false, "Disassemble optimized code.");
 DEFINE_FLAG(bool, trace_bailout, false, "Print bailout from ssa compiler.");
 DEFINE_FLAG(bool, trace_compiler, false, "Trace compiler operations.");
+DEFINE_FLAG(bool, cp, true,
+    "Do conditional constant propagation/unreachable code elimination.");
 DEFINE_FLAG(bool, cse, true, "Do common subexpression elimination.");
 DEFINE_FLAG(bool, licm, true, "Do loop invariant code motion.");
 DEFINE_FLAG(int, deoptimization_counter_threshold, 5,
@@ -213,13 +215,10 @@ static bool CompileParsedFunctionHelper(const ParsedFunction& parsed_function,
         flow_graph->ComputeUseLists();
         optimizer.SelectRepresentations();
 
-        if (FLAG_cse) {
-          flow_graph->ComputeUseLists();
-          DominatorBasedCSE::Optimize(flow_graph);
-        }
-        if (FLAG_licm) {
-          LICM::Optimize(flow_graph);
-        }
+        if (FLAG_cp || FLAG_cse) flow_graph->ComputeUseLists();
+        if (FLAG_cp) ConstantPropagator::Optimize(flow_graph);
+        if (FLAG_cse) DominatorBasedCSE::Optimize(flow_graph);
+        if (FLAG_licm) LICM::Optimize(flow_graph);
 
         // Perform register allocation on the SSA graph.
         FlowGraphAllocator allocator(*flow_graph);