Move validation to validate.dart and add comments.

sdk/lib/_internal/compiler/implementation/ssa/validate.dart

 // Copyright (c) 2011, 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.
 
 part of ssa;
 
 class HValidator extends HInstructionVisitor {
   bool isValid = true;
   HGraph graph;
 
@@ skipping 18 matching lines @@
     if (block.first == null || block.last == null) {
       markInvalid("empty block");
     }
     if (block.last is !HControlFlow) {
       markInvalid("block ends with non-tail node.");
     }
     if (block.last is HIf && block.successors.length != 2) {
       markInvalid("If node without two successors");
     }
     if (block.last is HConditionalBranch && block.successors.length != 2) {
-      markInvalid("Conditional node without two successors");
+        markInvalid("Conditional node without two successors");
+      }
+    if (block.last is HLoopBranch) {

[ngeoffray, 2014/06/24 15:45:00]

Why just limiting to HLoopBranch? A generic critical edge detector might be
better.

[floitsch, 2014/06/24 18:29:43]

Done.

+      // Assert that the block we inserted to avoid critical edges satisfies
+      // our assumptions. That is, it must not contain any instructions
+      // (although it may contain phy-updates).

[ngeoffray, 2014/06/24 15:45:01]

phy -> phi

[floitsch, 2014/06/24 18:29:43]

Done.

+      HBasicBlock avoidCriticalEdgeBlock = block.successors.last;
+      if (avoidCriticalEdgeBlock.first is! HGoto) {
+        markInvalid("Critical edge block contains instructions");
+      }
     }
     if (block.last is HGoto && block.successors.length != 1) {
       markInvalid("Goto node with not exactly one successor");
     }
     if (block.last is HJump && block.successors.length != 1) {
       markInvalid("Break or continue node without one successor");
     }
     if ((block.last is HReturn || block.last is HThrow) &&
         (block.successors.length != 1 || !block.successors[0].isExitBlock())) {
       markInvalid("Return or throw node with > 1 successor "
@@ skipping 121 matching lines @@
       markInvalid("Instruction in wrong block");
     }
     if (!hasCorrectInputs()) {
       markInvalid("Incorrect inputs");
     }
     if (!hasCorrectUses()) {
       markInvalid("Incorrect uses");
     }
   }
 }