Eliminate branches on constant conditions during codegen.

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

 // 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.
 
 class SsaCodeGeneratorTask extends CompilerTask {
   SsaCodeGeneratorTask(Compiler compiler) : super(compiler);
   String get name() => 'SSA code generator';
 
 
   String generateMethod(WorkItem work, HGraph graph) {
@@ skipping 884 matching lines @@
       visitBasicBlock(node.finallyBlock);
       indent--;
     }
     addIndentation();
     buffer.add('}\n');
 
     visitBasicBlock(node.joinBlock);
   }
 
   visitIf(HIf node) {
+    HInstruction condition = node.inputs[0];
+    int preVisitedBlocks = 0;
     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);
+    if (condition.isConstant()) {
+      HConstant constant = condition;
+      if (constant.constant.isTrue()) {
+        visitSubGraph(info.thenGraph);
+      } else if (node.hasElse) {
+        visitSubGraph(info.elseGraph);
+      }
+      // We ignore the other branch, even if it isn't visited.
+      preVisitedBlocks = node.hasElse ? 2 : 1;
+    } else {
+      startIf(node);
+      assert(!isGenerateAtUseSite(node));
+      startThen(node);
+      assert(node.thenBlock === dominated[0]);
+      visitSubGraph(info.thenGraph);
+      preVisitedBlocks++;
+      endThen(node);
+      if (node.hasElse) {
+        startElse(node);
+        assert(node.elseBlock === dominated[1]);
+        visitSubGraph(info.elseGraph);
+        preVisitedBlocks++;
+        endElse(node);
+      }
+      endIf(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
       // instead.
       visitBasicBlock(info.joinBlock);
     }
 
     // Visit all the dominated blocks that are not part of the then or else
     // branches, and is not the join block.
     // Depending on how the then/else branches terminate
@@ skipping 922 matching lines @@
       startBailoutSwitch();
     }
   }
 
   void endElse(HIf node) {
     if (node.elseBlock.hasGuards()) {
       endBailoutSwitch();
     }
   }
 }