Eliminate branches on constant conditions during codegen.

dart/lib/compiler/implementation/ssa/codegen.dart

Index: dart/lib/compiler/implementation/ssa/codegen.dart
diff --git a/dart/lib/compiler/implementation/ssa/codegen.dart b/dart/lib/compiler/implementation/ssa/codegen.dart
index 91f179dc4a9da9b9577922cbbe83d70de4e0b08f..aac27f7721d1aee0eb3062055c2afbf6b65a34cc 100644
--- a/dart/lib/compiler/implementation/ssa/codegen.dart
+++ b/dart/lib/compiler/implementation/ssa/codegen.dart
@@ -902,23 +902,37 @@ class SsaCodeGenerator implements HVisitor {
   }
 
   visitIf(HIf node) {
+    HInstruction condition = node.inputs[0];
+    int preVisitedBlocks = 1;

[Lasse Reichstein Nielsen, 2012/03/30 12:56:02]

I'd actually prefer to initialize it to zero, and only set it to 1 or 2 (or
increment it) after *actually* visiting the block.

[ahe, 2012/03/30 13:15:52]

Done.

     List<HBasicBlock> dominated = node.block.dominatedBlocks;
     HIfBlockInformation info = node.blockInformation;
-    startIf(node);
-    assert(!isGenerateAtUseSite(node));
-    startThen(node);
-    assert(node.thenBlock === dominated[0]);
-    visitSubGraph(info.thenGraph);
-    int preVisitedBlocks = 1;
-    endThen(node);
-    if (node.hasElse) {
-      startElse(node);
-      assert(node.elseBlock === dominated[1]);
-      visitSubGraph(info.elseGraph);
-      preVisitedBlocks = 2;
-      endElse(node);
-    }
-    endIf(node);
+    if (false && condition.isConstant()) {

[Lasse Reichstein Nielsen, 2012/03/30 12:56:02]

"false &&"?

[ahe, 2012/03/30 13:15:52]

Done.

+      HConstant constant = condition;
+      if (constant.constant.isTrue()) {
+        visitSubGraph(info.thenGraph);

[Lasse Reichstein Nielsen, 2012/03/30 12:56:02]

Increment preVisitedBlocks here, or set it to 1, or do that in an else branch of
the following 'if'.

+        if (node.hasElse) {
+          preVisitedBlocks = 2;
+        }
+      } else if (node.hasElse) {
+        preVisitedBlocks = 2;

[Lasse Reichstein Nielsen, 2012/03/30 12:56:02]

Here you set it to 1 (possibly with comment about skipping the then-branch).

+        visitSubGraph(info.elseGraph);

[Lasse Reichstein Nielsen, 2012/03/30 12:56:02]

And to 2 after visiting the else-part.

+      }
+    } else {
+      startIf(node);
+      assert(!isGenerateAtUseSite(node));
+      startThen(node);
+      assert(node.thenBlock === dominated[0]);
+      visitSubGraph(info.thenGraph);
+      endThen(node);
+      if (node.hasElse) {
+        startElse(node);
+        assert(node.elseBlock === dominated[1]);
+        visitSubGraph(info.elseGraph);
+        preVisitedBlocks = 2;
+        endElse(node);
+      }
+      endIf(node);
+    }
     if (info.joinBlock !== null && info.joinBlock.dominator !== node.block) {
       // The join block is dominated by a block in one of the branches.
       // The subgraph traversal never reached it, so we visit it here