Add forward iterator to iterate instructions inside a basic block.

vm/intermediate_language.cc

Index: vm/intermediate_language.cc
===================================================================
--- vm/intermediate_language.cc	(revision 9430)
+++ vm/intermediate_language.cc	(working copy)
@@ -527,7 +527,7 @@
 }
 
 
-Instruction* Instruction::RemoveFromGraph() {
+Instruction* Instruction::RemoveFromGraph(RemoveReturnValue ret) {

[Kevin Millikin (Google), 2012/07/06 10:44:30]

Do you think we could (should) get rid of this function an put it in the
iterator?  That way we would already know whether we wanted to return previous
or next, and we could avoid the linear search for the block entry when removing
the last instruction.

Disadvantage is that we need an iterator focused on the instruction to remove it
from the graph....

   ASSERT(!IsBlockEntry());
   ASSERT(!IsBranch());
   ASSERT(!IsThrow());
@@ -535,10 +535,11 @@
   ASSERT(!IsReThrow());
   ASSERT(previous() != NULL);
   Instruction* next = successor();
-  previous()->set_successor(next);
+  Instruction* prev = previous();
+  prev->set_successor(next);
   if (next != NULL) {
     if (!next->IsBlockEntry()) {
-      next->set_previous(previous());
+      next->set_previous(prev);
     } else {
       // Removing the last instruction of a block.
       // Update last_instruction of the current basic block.
@@ -547,14 +548,14 @@
         current = current->previous();
       }
       ASSERT(current->AsBlockEntry()->last_instruction() == this);
-      current->AsBlockEntry()->set_last_instruction(previous());
+      current->AsBlockEntry()->set_last_instruction(prev);
     }
   }
   // Reset successor and previous instruction to indicate
   // that the instruction is removed from the graph.
   set_successor(NULL);
   set_previous(NULL);
-  return next;
+  return ret == kReturnPrevious ? prev : next;
 }