Convert some compiler passes to preserve valid def-use chains.

runtime/vm/compiler.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/compiler.h"
 
 #include "vm/assembler.h"
 
 #include "vm/ast_printer.h"
 #include "vm/code_generator.h"
@@ skipping 161 matching lines @@
     if (optimized) {
       TimerScope timer(FLAG_compiler_stats,
                        &CompilerStats::graphoptimizer_timer,
                        isolate);
 
       flow_graph->ComputeUseLists();
 
       FlowGraphOptimizer optimizer(flow_graph);
       optimizer.ApplyICData();
 
-      // Compute the use lists.
-      flow_graph->ComputeUseLists();
-
       // Inlining (mutates the flow graph)
       if (FLAG_use_inlining) {
         TimerScope timer(FLAG_compiler_stats,
                          &CompilerStats::graphinliner_timer);
         FlowGraphInliner inliner(flow_graph);
         inliner.Inline();
         // Use lists are maintained and validated by the inliner.
       }
 
       if (FLAG_trace_type_propagation) {
@@ skipping 12 matching lines @@
         OS::Print("After type propagation:\n");
         FlowGraphPrinter printer(*flow_graph);
         printer.PrintBlocks();
       }
 
       flow_graph->ComputeUseLists();
 
       // Use propagated class-ids to optimize further.
       optimizer.ApplyClassIds();
 
-      // Recompute use lists after applying class ids.
-      flow_graph->ComputeUseLists();
-
       // Do optimizations that depend on the propagated type information.
       optimizer.Canonicalize();
 
-      flow_graph->ComputeUseLists();
-
       if (FLAG_constant_propagation) {
         ConstantPropagator::Optimize(flow_graph);
         // A canonicalization pass to remove e.g. smi checks on smi constants.
         optimizer.Canonicalize();
       }
 
       // Unbox doubles. Performed after constant propagation to minimize
       // interference from phis merging double values and tagged
       // values comming from dead paths.
-      flow_graph->ComputeUseLists();
       optimizer.SelectRepresentations();
-      flow_graph->ComputeUseLists();
 
       if (FLAG_common_subexpression_elimination) {
         if (DominatorBasedCSE::Optimize(flow_graph)) {
           // Do another round of CSE to take secondary effects into account:
           // e.g. when eliminating dependent loads (a.x[0] + a.x[0])
           // TODO(fschneider): Change to a one-pass optimization pass.
           DominatorBasedCSE::Optimize(flow_graph);
         }
       }
       if (FLAG_loop_invariant_code_motion &&
@@ skipping 408 matching lines @@
       Object::Handle(isolate->object_store()->sticky_error());
     isolate->object_store()->clear_sticky_error();
     isolate->set_long_jump_base(base);
     return result.raw();
   }
   UNREACHABLE();
   return Object::null();
 }
 
 }  // namespace dart