[Statistics] Add a method to atomically update a statistic that contains a maximum

Summary:
There are several places in the codebase that try to calculate a maximum value in a Statistic object. We currently do this in one of two ways:

  MaxNumFoo = std::max(MaxNumFoo, NumFoo);

or

  MaxNumFoo = (MaxNumFoo > NumFoo) ? MaxNumFoo : NumFoo;

The first version reads from MaxNumFoo one time and uncontionally rwrites to it. The second version possibly reads it twice depending on the result of the first compare.  But we have no way of knowing if the value was changed by another thread between the reads and the writes.

This patch adds a method to the Statistic object that can ensure that we only store if our value is the max and the previous max didn't change after we read it. If it changed we'll recheck if our value should still be the max or not and try again.

This spawned from an audit I'm trying to do of all places we uses the implicit conversion to unsigned on the Statistics objects. See my previous thread on llvm-dev https://groups.google.com/forum/#!topic/llvm-dev/yfvxiorKrDQ

Reviewers: dberlin, chandlerc, hfinkel, dblaikie

Reviewed By: chandlerc

Subscribers: llvm-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D33301

llvm-svn: 303318

llvm/include/llvm/ADT/Statistic.h

@@ -101,6 +101,16 @@ public:
     return init();
   }
 
+  void updateMax(unsigned V) {
+    unsigned PrevMax = Value.load(std::memory_order_relaxed);
+    // Keep trying to update max until we succeed or another thread produces
+    // a bigger max than us.
+    while (V > PrevMax && !Value.compare_exchange_weak(
+                              PrevMax, V, std::memory_order_relaxed)) {
+    }
+    init();
+  }
+
 #else  // Statistics are disabled in release builds.
 
   const Statistic &operator=(unsigned Val) {
@@ -131,6 +141,8 @@ public:
     return *this;
   }
 
+  void updateMax(unsigned V) {}
+
 #endif  // !defined(NDEBUG) || defined(LLVM_ENABLE_STATS)
 
 protected:

llvm/lib/Analysis/CallGraphSCCPass.cpp

@@ -478,9 +478,7 @@ bool CGPassManager::runOnModule(Module &M) {
       DEBUG(dbgs() << "  CGSCCPASSMGR: Stopped iteration after " << Iteration
                    << " times, due to -max-cg-scc-iterations\n");
 
-    if (Iteration > MaxSCCIterations)
+    MaxSCCIterations.updateMax(Iteration);
-      MaxSCCIterations = Iteration;
-    
   }
   Changed |= doFinalization(CG);
   return Changed;

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

@@ -110,8 +110,8 @@ StatepointLoweringState::allocateStackSlot(EVT ValueType,
          Builder.FuncInfo.StatepointStackSlots.size() &&
          "Broken invariant");
 
-  StatepointMaxSlotsRequired = std::max<unsigned long>(
+  StatepointMaxSlotsRequired.updateMax(
-      StatepointMaxSlotsRequired, Builder.FuncInfo.StatepointStackSlots.size());
+      Builder.FuncInfo.StatepointStackSlots.size());
 
   return SpillSlot;
 }

llvm/lib/Transforms/Scalar/SROA.cpp

@@ -3898,8 +3898,7 @@ AllocaInst *SROA::rewritePartition(AllocaInst &AI, AllocaSlices &AS,
   }
 
   NumAllocaPartitionUses += NumUses;
-  MaxUsesPerAllocaPartition =
+  MaxUsesPerAllocaPartition.updateMax(NumUses);
-      std::max<unsigned>(NumUses, MaxUsesPerAllocaPartition);
 
   // Now that we've processed all the slices in the new partition, check if any
   // PHIs or Selects would block promotion.
@@ -4016,8 +4015,7 @@ bool SROA::splitAlloca(AllocaInst &AI, AllocaSlices &AS) {
   }
 
   NumAllocaPartitions += NumPartitions;
-  MaxPartitionsPerAlloca =
+  MaxPartitionsPerAlloca.updateMax(NumPartitions);
-      std::max<unsigned>(NumPartitions, MaxPartitionsPerAlloca);
 
   // Migrate debug information from the old alloca to the new alloca(s)
   // and the individual partitions.