Improve and simplify removal of unreachable code

src/ia32/lithium-ia32.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 891 matching lines @@
   block->set_argument_count(argument_count_);
   next_block_ = NULL;
   current_block_ = NULL;
 }
 
 
 void LChunkBuilder::VisitInstruction(HInstruction* current) {
   HInstruction* old_current = current_instruction_;
   current_instruction_ = current;
   if (current->has_position()) position_ = current->position();
-  LInstruction* instr = current->CompileToLithium(this);
+
+  LInstruction* instr = NULL;
+  if (current->CanReplaceWithDummyUses()) {
+    HValue* first_operand = current->OperandCount() == 0
+        ? graph()->GetConstant1() :

[Jakob Kummerow, 2013/10/01 11:59:20]

nit: please put the ':' in the next line, under the '?'. Or move the next line
up here, right behind the ':'.

[danno, 2013/10/23 11:46:51]

Done.

+        current->OperandAt(0);
+    instr = DefineAsRegister(new(zone()) LDummyUse(UseAny(first_operand)));
+    for (int i = 1; i < current->OperandCount(); ++i) {
+      LInstruction* dummy =
+          new(zone()) LDummyUse(UseAny(current->OperandAt(1)));

[Jakob Kummerow, 2013/10/01 11:59:20]

bug: OperandAt(i)!

[danno, 2013/10/23 11:46:51]

Done.

+      dummy->set_hydrogen_value(current);
+      chunk_->AddInstruction(dummy, current_block_);
+    }
+  } else {
+    instr = current->CompileToLithium(this);
+  }
 
   if (instr != NULL) {
 #if DEBUG
     // Make sure that the lithium instruction has either no fixed register
     // constraints in temps or the result OR no uses that are only used at
     // start. If this invariant doesn't hold, the register allocator can decide
     // to insert a split of a range immediately before the instruction due to an
     // already allocated register needing to be used for the instruction's fixed
     // register constraint. In this case, The register allocator won't see an
     // interference between the split child and the use-at-start (it would if
@@ skipping 125 matching lines @@
   return result;
 }
 
 
 LInstruction* LChunkBuilder::DoGoto(HGoto* instr) {
   return new(zone()) LGoto(instr->FirstSuccessor());
 }
 
 
 LInstruction* LChunkBuilder::DoBranch(HBranch* instr) {
-  HValue* value = instr->value();
-  if (value->EmitAtUses()) {
-    ASSERT(value->IsConstant());
-    ASSERT(!value->representation().IsDouble());
-    HBasicBlock* successor = HConstant::cast(value)->BooleanValue()
-        ? instr->FirstSuccessor()
-        : instr->SecondSuccessor();
-    return new(zone()) LGoto(successor);
-  }
+  LInstruction* goto_instr = CheckElideControlInstruction(instr);
+  if (goto_instr != NULL) return goto_instr;
 
   ToBooleanStub::Types expected = instr->expected_input_types();
 
   // Tagged values that are not known smis or booleans require a
   // deoptimization environment. If the instruction is generic no
   // environment is needed since all cases are handled.
+  HValue* value = instr->value();
   Representation rep = value->representation();
   HType type = value->type();
   if (!rep.IsTagged() || type.IsSmi() || type.IsBoolean()) {
     return new(zone()) LBranch(UseRegister(value), NULL);
   }
 
   bool needs_temp = expected.NeedsMap() || expected.IsEmpty();
   LOperand* temp = needs_temp ? TempRegister() : NULL;
 
   // The Generic stub does not have a deopt, so we need no environment.
@@ skipping 651 matching lines @@
       left = UseRegisterAtStart(instr->left());
       right = UseRegisterAtStart(instr->right());
     }
     return new(zone()) LCompareNumericAndBranch(left, right);
   }
 }
 
 
 LInstruction* LChunkBuilder::DoCompareObjectEqAndBranch(
     HCompareObjectEqAndBranch* instr) {
+  LInstruction* goto_instr = CheckElideControlInstruction(instr);
+  if (goto_instr != NULL) return goto_instr;
   LOperand* left = UseRegisterAtStart(instr->left());
   LOperand* right = UseOrConstantAtStart(instr->right());
   return new(zone()) LCmpObjectEqAndBranch(left, right);
 }
 
 
 LInstruction* LChunkBuilder::DoCompareHoleAndBranch(
     HCompareHoleAndBranch* instr) {
   LOperand* value = UseRegisterAtStart(instr->value());
   return new(zone()) LCmpHoleAndBranch(value);
@@ skipping 973 matching lines @@
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseTempRegister(instr->index());
   return DefineSameAsFirst(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32