V4 IR: do edge splitting after SSA transformation

This reduces the work for the dominator tree/frontier calculations,
because there are less blocks to consider. All blocks inserted by
splitting the critical edges, have (by definition) no effect on the
dominator calculations. However, the immediate dominators for all new
blocks needs to be added, because this information is used by the block
scheduling.

This change reduces memory/time usage during optimization passes,
especially when processing excessively big switch statements.

Change-Id: Ia69882e9dabdddffa1c98b1079012d8d988e1e8f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

1 files changed, 58 insertions, 8 deletions

diff --git a/src/qml/compiler/qv4ssa.cpp b/src/qml/compiler/qv4ssa.cpp
index f14205519d..7113dc7c26 100644
--- a/src/qml/compiler/qv4ssa.cpp
+++ b/src/qml/compiler/qv4ssa.cpp
@@ -409,7 +409,7 @@ class DominatorTree {
     typedef int BasicBlockIndex;
     enum { InvalidBasicBlockIndex = -1 };
 
-    const QVector<BasicBlock *> nodes;
+    QVector<BasicBlock *> nodes;
     int N;
     std::vector<int> dfnum; // BasicBlock index -> dfnum
     std::vector<int> vertex;
@@ -705,19 +705,57 @@ public:
         return nodes[idom[bb->index]];
     }
 
+    void dumpImmediateDominators() const
+    {
+        qDebug() << "Immediate dominators for" << idom.size() << "nodes:";
+        for (size_t i = 0, ei = idom.size(); i != ei; ++i)
+            if (idom[i] == InvalidBasicBlockIndex)
+                qDebug("\tnone -> L%d", int(i));
+            else
+                qDebug("\tL%d -> L%d", idom[i], int(i));
+    }
+
     void updateImmediateDominator(BasicBlock *bb, BasicBlock *newDominator)
     {
-        idom[bb->index] = newDominator->index;
+        Q_ASSERT(bb->index >= 0);
+
+        int blockIndex;
+        if (static_cast<std::vector<BasicBlockIndex>::size_type>(bb->index) >= idom.size()) {
+            // This is a new block, probably introduced by edge splitting. So, we'll have to grow
+            // the array before inserting the immediate dominator.
+            nodes.append(bb);
+            idom.resize(nodes.size(), InvalidBasicBlockIndex);
+            blockIndex = nodes.size() - 1;
+        } else {
+            blockIndex = getBlockIndex(bb);
+        }
+
+        idom[blockIndex] = getBlockIndex(newDominator);
     }
 
     bool dominates(BasicBlock *dominator, BasicBlock *dominated) const {
         // The index of the basic blocks might have changed, or the nodes array might have changed,
-        // or the block got deleted, so get the index from our copy of the array.
-        return dominates(nodes.indexOf(dominator), nodes.indexOf(dominated));
+        // so get the index from our copy of the array.
+        return dominates(getBlockIndex(dominator), getBlockIndex(dominated));
     }
 
 private:
+    int getBlockIndex(BasicBlock *bb) const {
+        if (!bb)
+            return InvalidBasicBlockIndex;
+
+        if (bb->index >= 0 && bb->index < nodes.size()) {
+            if (nodes.at(bb->index) == bb)
+                return bb->index;
+        }
+
+        return nodes.indexOf(bb);
+    }
+
     bool dominates(BasicBlockIndex dominator, BasicBlockIndex dominated) const {
+        // dominator can be Invalid when the dominated block has no dominator (i.e. the start node)
+        Q_ASSERT(dominated != InvalidBasicBlockIndex);
+
         for (BasicBlockIndex it = dominated; it != InvalidBasicBlockIndex; it = idom[it]) {
             if (it == dominator)
                 return true;
@@ -2471,7 +2509,7 @@ protected:
     }
 };
 
-void splitCriticalEdges(Function *f)
+void splitCriticalEdges(Function *f, DominatorTree &df)
 {
     for (int i = 0, ei = f->basicBlocks.size(); i != ei; ++i) {
         BasicBlock *bb = f->basicBlocks[i];
@@ -2518,6 +2556,9 @@ void splitCriticalEdges(Function *f)
                     } else {
                         Q_ASSERT(!"Unknown terminator!");
                     }
+
+                    // Set the immediate dominator of the new block to inBB
+                    df.updateImmediateDominator(newBB, inBB);
                 }
             }
         }
@@ -2667,6 +2708,12 @@ public:
         showMeTheCode(function);
         schedule(function->basicBlocks.first());
 
+#if defined(SHOW_SSA)
+        qDebug() << "Block sequence:";
+        foreach (BasicBlock *bb, sequence)
+            qDebug("\tL%d", bb->index);
+#endif // SHOW_SSA
+
         Q_ASSERT(function->basicBlocks.size() == sequence.size());
         function->basicBlocks = sequence;
         return loopsStartEnd;
@@ -3660,9 +3707,6 @@ void Optimizer::run(QQmlEnginePrivate *qmlEngine)
 
     if (!function->hasTry && !function->hasWith && !function->module->debugMode && doSSA) {
 //        qout << "SSA for " << (function->name ? qPrintable(*function->name) : "<anonymous>") << endl;
-//        qout << "Starting edge splitting..." << endl;
-        splitCriticalEdges(function);
-//        showMeTheCode(function);
 
         // Calculate the dominator tree:
         DominatorTree df(function->basicBlocks);
@@ -3690,6 +3734,11 @@ void Optimizer::run(QQmlEnginePrivate *qmlEngine)
         TypePropagation(defUses).run(function);
 //        showMeTheCode(function);
 
+        // Transform the CFG into edge-split SSA.
+//        qout << "Starting edge splitting..." << endl;
+        splitCriticalEdges(function, df);
+//        showMeTheCode(function);
+
         static bool doOpt = qgetenv("QV4_NO_OPT").isEmpty();
         if (doOpt) {
 //            qout << "Running SSA optimization..." << endl;
@@ -3709,6 +3758,7 @@ void Optimizer::run(QQmlEnginePrivate *qmlEngine)
 //        showMeTheCode(function);
 
 //        qout << "Doing block scheduling..." << endl;
+//        df.dumpImmediateDominators();
         startEndLoops = BlockScheduler(function, df).go();
 //        showMeTheCode(function);