Reapply "Change the SSA construction pass to also construct def-use chains."

runtime/vm/flow_graph_builder.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/flow_graph_builder.h"
 
 #include "vm/ast_printer.h"
 #include "vm/code_descriptors.h"
 #include "vm/dart_entry.h"
 #include "vm/flags.h"
@@ skipping 118 matching lines @@
   // Insert the callee graph into the caller graph.  First sort the list of
   // exits by block id (recording block entries as a side effect).
   SortExits();
   intptr_t num_exits = exits_.length();
   if (num_exits == 0) {
     // TODO(zerny): Add support for non-local exits, such as throw.
     UNREACHABLE();
   } else if (num_exits == 1) {
     // For just one exit, replace the uses and remove the call from the graph.
     call->ReplaceUsesWith(ValueAt(0)->definition());
+    ValueAt(0)->RemoveFromUseList();
     call->previous()->LinkTo(callee_entry->next());
     LastInstructionAt(0)->LinkTo(call->next());
     // In case of control flow, locally update the predecessors, phis and
     // dominator tree.
     // TODO(zerny): should we leave the dominator tree since we recompute it
     // after a full inlining pass?
     if (callee_graph->preorder().length() > 2) {
       BlockEntryInstr* exit_block = ExitBlockAt(0);
       // Pictorially, the graph structure is:
       //
@@ skipping 54 matching lines @@
       phi->set_ssa_temp_index(caller_graph->alloc_ssa_temp_index());
       phi->mark_alive();
       for (intptr_t i = 0; i < num_exits; ++i) {
         Value* value = ValueAt(i);
         phi->SetInputAt(i, value);
         value->set_instruction(phi);
         value->set_use_index(i);
       }
       // Replace uses of the call with the phi.
       call->ReplaceUsesWith(phi);
+    } else {
+      // In the case that the result is unused, remove the return value uses
+      // from their definition's use list.
+      for (intptr_t i = 0; i < num_exits; ++i) {
+        ValueAt(i)->RemoveFromUseList();
+      }
     }
     // Remove the call from the graph.
     call->previous()->LinkTo(callee_entry->next());
     join->LinkTo(call->next());
     // Replace the blocks after splitting (see comment in the len=1 case above).
     caller_entry->ReplaceAsPredecessorWith(join);
     callee_entry->ReplaceAsPredecessorWith(caller_entry);
     // Update the last instruction pointers on each exit block to the new goto.
     for (intptr_t i = 0; i < num_exits; ++i) {
       ExitBlockAt(i)->set_last_instruction(LastInstructionAt(i)->next());
@@ skipping 3019 matching lines @@
   intptr_t len = OS::SNPrint(NULL, 0, kFormat, function_name, reason) + 1;
   char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
   OS::SNPrint(chars, len, kFormat, function_name, reason);
   const Error& error = Error::Handle(
       LanguageError::New(String::Handle(String::New(chars))));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart