Move validation to validate.dart and add comments.

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

Index: sdk/lib/_internal/compiler/implementation/ssa/validate.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/validate.dart b/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
index e751b4dcce6c206f6edc371e919fe2656978e10c..2858204ebe1acdd57b667c09be7483ac98c7387c 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
@@ -36,7 +36,16 @@ class HValidator extends HInstructionVisitor {
       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) {
+      // 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 phi-updates).
+      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");
@@ -67,6 +76,17 @@ class HValidator extends HInstructionVisitor {
         markInvalid("successor with lower id, but not a loop-header");
       }
     }
+    // Make sure we don't have a critical edge.
+    if (isValid && block.successors.length > 1 &&
+        block.last is! HTry && block.last is! HExitTry &&
+        block.last is! HSwitch) {

[ngeoffray, 2014/06/25 07:55:22]

Wow, did not realize there was that many special cases :-(

[floitsch, 2014/06/25 08:27:18]

Didn't know either, until all kinds of tests bombed...

+      for (HBasicBlock successor in block.successors) {
+        if (!isValid) break;
+        if (successor.predecessors.length >= 2) {
+          markInvalid("SSA graph contains critical edge.");
+        }
+      }
+    }
 
     // Check that the entries in the dominated-list are sorted.
     int lastId = 0;