Fix the threshold added in r186434 (a re-apply of r185393) and updaated

to be a ManagedStatic in r218163 to not be a global variable written and
read to from within the innards of SpillPlacement.

This will fix a really scary race condition for anyone that has two
copies of LLVM running spill placement concurrently. Yikes!

This will also fix a really significant compile time hit that r218163
caused because the spill placement threshold read is actually in the
*very* hot path of this code. The memory fence on each read was showing
up as huge compile time regressions when spilling is responsible for
most of the compile time. For example, optimizing sanitized code showed
over 50% compile time regressions here. =/

llvm-svn: 218921

llvm/lib/CodeGen/SpillPlacement.cpp

@@ -61,27 +61,6 @@ void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
-namespace {
-static ManagedStatic<BlockFrequency> Threshold;
-}
-
-/// Decision threshold. A node gets the output value 0 if the weighted sum of
-/// its inputs falls in the open interval (-Threshold;Threshold).
-static BlockFrequency getThreshold() { return *Threshold; }
-
-/// \brief Set the threshold for a given entry frequency.
-///
-/// Set the threshold relative to \c Entry.  Since the threshold is used as a
-/// bound on the open interval (-Threshold;Threshold), 1 is the minimum
-/// threshold.
-static void setThreshold(const BlockFrequency &Entry) {
-  // Apparently 2 is a good threshold when Entry==2^14, but we need to scale
-  // it.  Divide by 2^13, rounding as appropriate.
-  uint64_t Freq = Entry.getFrequency();
-  uint64_t Scaled = (Freq >> 13) + bool(Freq & (1 << 12));
-  *Threshold = std::max(UINT64_C(1), Scaled);
-}
-
 /// Node - Each edge bundle corresponds to a Hopfield node.
 ///
 /// The node contains precomputed frequency data that only depends on the CFG,
@@ -127,9 +106,9 @@ struct SpillPlacement::Node {
 
   /// clear - Reset per-query data, but preserve frequencies that only depend on
   // the CFG.
-  void clear() {
+  void clear(const BlockFrequency &Threshold) {
     BiasN = BiasP = Value = 0;
-    SumLinkWeights = getThreshold();
+    SumLinkWeights = Threshold;
     Links.clear();
   }
 
@@ -167,7 +146,7 @@ struct SpillPlacement::Node {
 
   /// update - Recompute Value from Bias and Links. Return true when node
   /// preference changes.
-  bool update(const Node nodes[]) {
+  bool update(const Node nodes[], const BlockFrequency &Threshold) {
     // Compute the weighted sum of inputs.
     BlockFrequency SumN = BiasN;
     BlockFrequency SumP = BiasP;
@@ -187,9 +166,9 @@ struct SpillPlacement::Node {
     //  2. It helps tame rounding errors when the links nominally sum to 0.
     //
     bool Before = preferReg();
-    if (SumN >= SumP + getThreshold())
+    if (SumN >= SumP + Threshold)
       Value = -1;
-    else if (SumP >= SumN + getThreshold())
+    else if (SumP >= SumN + Threshold)
       Value = 1;
     else
       Value = 0;
@@ -228,7 +207,7 @@ void SpillPlacement::activate(unsigned n) {
   if (ActiveNodes->test(n))
     return;
   ActiveNodes->set(n);
-  nodes[n].clear();
+  nodes[n].clear(Threshold);
 
   // Very large bundles usually come from big switches, indirect branches,
   // landing pads, or loops with many 'continue' statements. It is difficult to
@@ -245,6 +224,18 @@ void SpillPlacement::activate(unsigned n) {
   }
 }
 
+/// \brief Set the threshold for a given entry frequency.
+///
+/// Set the threshold relative to \c Entry.  Since the threshold is used as a
+/// bound on the open interval (-Threshold;Threshold), 1 is the minimum
+/// threshold.
+void SpillPlacement::setThreshold(const BlockFrequency &Entry) {
+  // Apparently 2 is a good threshold when Entry==2^14, but we need to scale
+  // it.  Divide by 2^13, rounding as appropriate.
+  uint64_t Freq = Entry.getFrequency();
+  uint64_t Scaled = (Freq >> 13) + bool(Freq & (1 << 12));
+  Threshold = std::max(UINT64_C(1), Scaled);
+}
 
 /// addConstraints - Compute node biases and weights from a set of constraints.
 /// Set a bit in NodeMask for each active node.
@@ -311,7 +302,7 @@ bool SpillPlacement::scanActiveBundles() {
   Linked.clear();
   RecentPositive.clear();
   for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n)) {
-    nodes[n].update(nodes);
+    nodes[n].update(nodes, Threshold);
     // A node that must spill, or a node without any links is not going to
     // change its value ever again, so exclude it from iterations.
     if (nodes[n].mustSpill())
@@ -331,7 +322,7 @@ void SpillPlacement::iterate() {
   // First update the recently positive nodes. They have likely received new
   // negative bias that will turn them off.
   while (!RecentPositive.empty())
-    nodes[RecentPositive.pop_back_val()].update(nodes);
+    nodes[RecentPositive.pop_back_val()].update(nodes, Threshold);
 
   if (Linked.empty())
     return;
@@ -350,7 +341,7 @@ void SpillPlacement::iterate() {
            iteration == 0 ? Linked.rbegin() : std::next(Linked.rbegin()),
            E = Linked.rend(); I != E; ++I) {
       unsigned n = *I;
-      if (nodes[n].update(nodes)) {
+      if (nodes[n].update(nodes, Threshold)) {
         Changed = true;
         if (nodes[n].preferReg())
           RecentPositive.push_back(n);
@@ -364,7 +355,7 @@ void SpillPlacement::iterate() {
     for (SmallVectorImpl<unsigned>::const_iterator I =
            std::next(Linked.begin()), E = Linked.end(); I != E; ++I) {
       unsigned n = *I;
-      if (nodes[n].update(nodes)) {
+      if (nodes[n].update(nodes, Threshold)) {
         Changed = true;
         if (nodes[n].preferReg())
           RecentPositive.push_back(n);

llvm/lib/CodeGen/SpillPlacement.h

@@ -62,6 +62,10 @@ class SpillPlacement : public MachineFunctionPass {
   // Block frequencies are computed once. Indexed by block number.
   SmallVector<BlockFrequency, 8> BlockFrequencies;
 
+  /// Decision threshold. A node gets the output value 0 if the weighted sum of
+  /// its inputs falls in the open interval (-Threshold;Threshold).
+  BlockFrequency Threshold;
+
 public:
   static char ID; // Pass identification, replacement for typeid.
 
@@ -152,6 +156,7 @@ private:
   void releaseMemory() override;
 
   void activate(unsigned);
+  void setThreshold(const BlockFrequency &Entry);
 };
 
 } // end namespace llvm