Bug 1323027: IonMonkey - Remove empty blocks, r=jandem

js/src/jit/Ion.cpp

@@ -1521,6 +1521,17 @@ OptimizeMIR(MIRGenerator* mir)
             return false;
     }
 
+    {
+        AutoTraceLog log(logger, TraceLogger_FoldEmptyBlocks);
+        if (!FoldEmptyBlocks(graph))
+            return false;
+        gs.spewPass("Fold Empty Blocks");
+        AssertBasicGraphCoherency(graph);
+
+        if (mir->shouldCancel("Fold Empty Blocks"))
+            return false;
+    }
+
     {
         AutoTraceLog log(logger, TraceLogger_FoldTests);
         if (!FoldTests(graph))

js/src/jit/IonAnalysis.cpp

@@ -923,6 +923,42 @@ jit::FoldTests(MIRGraph& graph)
     return true;
 }
 
+bool
+jit::FoldEmptyBlocks(MIRGraph& graph)
+{
+    for (MBasicBlockIterator iter(graph.begin()); iter != graph.end(); ) {
+        MBasicBlock* block = *iter;
+        iter++;
+
+        if (block->numPredecessors() != 1 || block->numSuccessors() != 1)
+            continue;
+
+        if (!block->phisEmpty())
+            continue;
+
+        if (block->outerResumePoint())
+            continue;
+
+        if (*block->begin() != *block->rbegin())
+            continue;
+
+        MBasicBlock* succ = block->getSuccessor(0);
+        MBasicBlock* pred = block->getPredecessor(0);
+
+        if (succ->numPredecessors() != 1)
+            continue;
+
+        size_t pos = pred->getSuccessorIndex(block);
+        pred->lastIns()->replaceSuccessor(pos, succ);
+
+        graph.removeBlock(block);
+
+        succ->addPredecessorSameInputsAs(pred, block);
+        succ->removePredecessor(block);
+    }
+    return true;
+}
+
 static void
 EliminateTriviallyDeadResumePointOperands(MIRGraph& graph, MResumePoint* rp)
 {

js/src/jit/IonAnalysis.h

@@ -24,6 +24,9 @@ PruneUnusedBranches(MIRGenerator* mir, MIRGraph& graph);
 MOZ_MUST_USE bool
 FoldTests(MIRGraph& graph);
 
+MOZ_MUST_USE bool
+FoldEmptyBlocks(MIRGraph& graph);
+
 MOZ_MUST_USE bool
 SplitCriticalEdges(MIRGraph& graph);
 

js/src/vm/TraceLoggingTypes.h

@@ -42,6 +42,7 @@
     /* Specific passes during ion compilation */      \
     _(PruneUnusedBranches)                            \
     _(FoldTests)                                      \
+    _(FoldEmptyBlocks)                                \
     _(SplitCriticalEdges)                             \
     _(RenumberBlocks)                                 \
     _(ScalarReplacement)                              \