Add canonicalize optimization for branches.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 16846)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -26,6 +26,7 @@
     "Propagate IC data from unoptimized to optimized IC calls.");
 DECLARE_FLAG(bool, enable_type_checks);
 DECLARE_FLAG(int, max_polymorphic_checks);
+DECLARE_FLAG(bool, trace_optimization);
 
 Definition::Definition()
     : range_(NULL),
@@ -1686,6 +1687,34 @@
   return this;
 }
 
+
+Instruction* BranchInstr::Canonicalize(FlowGraphOptimizer* optimizer) {
+  if (comparison()->IsStrictCompare()) {
+    Definition* replacement = comparison()->Canonicalize(optimizer);

[Kevin Millikin (Google), 2013/01/10 10:35:00]

It might be simpler with an early return, and I like to avoid calling both IsXXX
and AsXXX on the same instruction.  Also, the code should be safe if
comparison()->Canonicalize() returns NULL (or at least assert that it doesn't). 
What do you think of:

Definition* replacement = comparison()->Canonicalize(optimizer);
if ((replacement == comparision()) || (replacement == NULL)) return this;
ComparisonInstr* comp = replacement->AsComparison();
if (comp == NULL) return this;

// Replace the comparison if the replacement is only used in this branch.
if ((comp->...) && ... && ...) ...

[Florian Schneider, 2013/01/10 13:18:41]

Done.

+    if ((replacement != comparison()) &&
+        replacement->IsComparison()) {
+      ComparisonInstr* comp = replacement->AsComparison();
+      if ((comp->input_use_list()->instruction() == this) &&
+          (comp->input_use_list()->next_use() == NULL) &&
+          (comp->env_use_list() == NULL)) {
+        // Replace the comparison if the replacement is used at this branch,
+        // and has exactly one use.
+        comp->RemoveFromGraph();
+        ReplaceWith(comp, NULL /* ignored */);

[Kevin Millikin (Google), 2013/01/10 10:35:00]

You should expand the comment about NULL so the reader doesn't have to look at
ReplaceWith.

// It's safe to pass a NULL instruction iterator because we're sure that the
replacement is not the canonicalization iterator's current instruction.

[Florian Schneider, 2013/01/10 13:18:41]

Done.

+        for (intptr_t i = 0; i < comp->InputCount(); ++i) {
+          Value* operand = comp->InputAt(i);
+          operand->set_instruction(this);
+        }
+        if (FLAG_trace_optimization) {
+          OS::Print("Merging comparison v%"Pd"\n", comp->ssa_temp_index());
+        }
+      }
+    }
+  }
+  return this;
+}
+
+
 Definition* StrictCompareInstr::Canonicalize(FlowGraphOptimizer* optimizer) {
   if (!right()->BindsToConstant()) return this;
   const Object& right_constant = right()->BoundConstant();