ColdBlockInfo: overhaul analysis pass

The old analysis pass doesn't take into account profile data, nor does
it consider post-dominance. It primarily dealt with _fastPath/_slowPath.

A block that is dominated by a cold block is itself cold. That's true
whether it's forwards or backwards dominance.

We can also consider a call to any `Never` returning function as a
cold-exit, though the block(s) leading up to that call may be executed
frequently because of concurrency. For now, I'm ignoring the concurrency
case and assuming it's cold. To make use of this "no return" prediction,
use the `-enable-noreturn-prediction` flag, which is currently off by
default.

Author: kavon

include/swift/AST/SILOptions.h

@@ -122,6 +122,9 @@ class SILOptions {
   /// within the optimizer. This influences static branch prediction.
   bool EnableThrowsPrediction = false;
 
+  /// Controls whether to say that blocks ending in an 'unreachable' are cold.
+  bool EnableNoReturnCold = false;
+
   /// Should we run any SIL performance optimizations
   ///
   /// Useful when you want to enable -O LLVM opts but not -O SIL opts.

include/swift/Basic/ProfileCounter.h

@@ -27,8 +27,8 @@ class ProfileCounter {
   uint64_t count;
 
 public:
-  explicit ProfileCounter() : count(UINT64_MAX) {}
-  ProfileCounter(uint64_t Count) : count(Count) {
+  explicit constexpr ProfileCounter() : count(UINT64_MAX) {}
+  constexpr ProfileCounter(uint64_t Count) : count(Count) {
     if (Count == UINT64_MAX) {
       count = UINT64_MAX - 1;
     }
@@ -40,6 +40,22 @@ class ProfileCounter {
     return count;
   }
   explicit operator bool() const { return hasValue(); }
+
+  /// Saturating addition of another counter to this one, meaning that overflow
+  /// is avoided. If overflow would have happened, this function returns true
+  /// and the maximum representable value will be set in this counter.
+  bool add_saturating(ProfileCounter other) {
+    assert(hasValue() && other.hasValue());
+
+    // Will we go over the max representable value by adding other?
+    if (count > ((UINT64_MAX-1) - other.count)) {
+      count = UINT64_MAX - 1;
+      return true;
+    }
+
+    count += other.count;
+    return false;
+  }
 };
 } // end namespace swift
 

include/swift/Option/FrontendOptions.td

@@ -545,6 +545,11 @@ def enable_throws_prediction : Flag<["-"], "enable-throws-prediction">,
 def disable_throws_prediction : Flag<["-"], "disable-throws-prediction">,
   HelpText<"Disables optimization assumption that functions rarely throw errors.">;
 
+def enable_noreturn_prediction : Flag<["-"], "enable-noreturn-prediction">,
+  HelpText<"Enables optimization assumption that calls to no-return functions are cold.">;
+def disable_noreturn_prediction : Flag<["-"], "disable-noreturn-prediction">,
+  HelpText<"Disables optimization assumption that calls to no-return functions are cold.">;
+
 def disable_access_control : Flag<["-"], "disable-access-control">,
   HelpText<"Don't respect access control restrictions">;
 def enable_access_control : Flag<["-"], "enable-access-control">,

include/swift/SILOptimizer/Analysis/ColdBlockInfo.h

@@ -15,52 +15,70 @@
 
 #include "llvm/ADT/DenseMap.h"
 #include "swift/SIL/SILValue.h"
+#include <bitset>
 
 namespace swift {
 class DominanceAnalysis;
+class PostDominanceAnalysis;
 class SILBasicBlock;
+class CondBranchInst;
 
 /// Cache a set of basic blocks that have been determined to be cold or hot.
 ///
 /// This does not inherit from SILAnalysis because it is not worth preserving
 /// across passes.
 class ColdBlockInfo {
+public:
+  // Represents the temperatures of edges flowing into a block.
+  //
+  //         T = "top" -- both warm and cold edges
+  //        /  \
+  //     Warm  Cold
+  //        \  /
+  //         B = "bottom" -- neither warm or cold edges
+  struct State {
+    // Each state kind, as an integer, is its position in any bit vectors.
+    enum Temperature {
+      Warm = 0,
+      Cold = 1
+    };
+
+    // Number of states, excluding Top or Bottom, in this flow problem.
+    static constexpr unsigned NumStates = 2;
+  };
+private:
   DominanceAnalysis *DA;
+  PostDominanceAnalysis *PDA;
+
+  using Energy = std::bitset<State::NumStates>;
+  static std::string toString(Energy e);
 
-  /// Each block in this map has been determined to be either cold or hot.
-  llvm::DenseMap<const SILBasicBlock*, bool> ColdBlockMap;
+  /// Each block in this map has been determined to be cold and/or warm.
+  llvm::DenseMap<const SILBasicBlock*, Energy> EnergyMap;
+  bool changedMap = false;
 
   // This is a cache and shouldn't be copied around.
   ColdBlockInfo(const ColdBlockInfo &) = delete;
   ColdBlockInfo &operator=(const ColdBlockInfo &) = delete;
+  LLVM_DUMP_METHOD void dump() const;
 
-  /// Tri-value return code for checking branch hints.
-  enum BranchHint : unsigned {
-    None,
-    LikelyTrue,
-    LikelyFalse
-  };
-
-  enum {
-    RecursionDepthLimit = 3
-  };
+  void resetToCold(const SILBasicBlock *BB);
+  void set(const SILBasicBlock *BB, State::Temperature temp);
 
-  BranchHint getBranchHint(SILValue Cond, int recursionDepth);
+  using ExpectedValue = std::optional<bool>;
 
-  bool isSlowPath(const SILBasicBlock *FromBB, const SILBasicBlock *ToBB,
-                  int recursionDepth);
+  void setExpectedCondition(CondBranchInst *CondBranch, ExpectedValue value);
 
-  bool isCold(const SILBasicBlock *BB,
-              int recursionDepth);
+  ExpectedValue searchForExpectedValue(SILValue Cond,
+                                       unsigned recursionDepth = 0);
+  void searchForExpectedValue(CondBranchInst *CondBranch);
 
 public:
-  ColdBlockInfo(DominanceAnalysis *DA): DA(DA) {}
+  ColdBlockInfo(DominanceAnalysis *DA, PostDominanceAnalysis *PDA);
 
-  bool isSlowPath(const SILBasicBlock *FromBB, const SILBasicBlock *ToBB) {
-    return isSlowPath(FromBB, ToBB, 0);
-  }
+  void analyze(SILFunction *F);
 
-  bool isCold(const SILBasicBlock *BB) { return isCold(BB, 0); }
+  bool isCold(const SILBasicBlock *BB) const;
 };
 } // end namespace swift
 

include/swift/SILOptimizer/Utils/PerformanceInlinerUtils.h

@@ -429,6 +429,7 @@ class ShortestPathAnalysis {
       return;
 
     BlockInfoStorage.resize(numBlocks);
+    CBI.analyze(F);
 
     // First step: compute the length of the blocks.
     unsigned BlockIdx = 0;

lib/DriverTool/sil_opt_main.cpp

@@ -293,6 +293,10 @@ struct SILOptOptions {
   EnableThrowsPrediction = llvm::cl::opt<bool>("enable-throws-prediction",
                      llvm::cl::desc("Enables optimization assumption that functions rarely throw errors."));
 
+  llvm::cl::opt<bool>
+  EnableNoReturnCold = llvm::cl::opt<bool>("enable-noreturn-prediction",
+                     llvm::cl::desc("Enables optimization assumption that calls to no-return functions are cold."));
+
   llvm::cl::opt<bool>
   EnableMoveInoutStackProtection = llvm::cl::opt<bool>("enable-move-inout-stack-protector",
                     llvm::cl::desc("Enable the stack protector by moving values to temporaries."));
@@ -852,6 +856,7 @@ int sil_opt_main(ArrayRef<const char *> argv, void *MainAddr) {
   SILOpts.EnableSpeculativeDevirtualization = options.EnableSpeculativeDevirtualization;
   SILOpts.EnableAsyncDemotion = options.EnableAsyncDemotion;
   SILOpts.EnableThrowsPrediction = options.EnableThrowsPrediction;
+  SILOpts.EnableNoReturnCold = options.EnableNoReturnCold;
   SILOpts.IgnoreAlwaysInline = options.IgnoreAlwaysInline;
   SILOpts.EnableOSSAModules = options.EnableOSSAModules;
   SILOpts.EnableSILOpaqueValues = options.EnableSILOpaqueValues;

lib/Frontend/CompilerInvocation.cpp

@@ -2661,6 +2661,9 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   Opts.EnableThrowsPrediction = Args.hasFlag(
       OPT_enable_throws_prediction, OPT_disable_throws_prediction,
       Opts.EnableThrowsPrediction);
+  Opts.EnableNoReturnCold = Args.hasFlag(
+      OPT_enable_noreturn_prediction, OPT_disable_noreturn_prediction,
+      Opts.EnableNoReturnCold);
   Opts.EnableActorDataRaceChecks |= Args.hasFlag(
       OPT_enable_actor_data_race_checks,
       OPT_disable_actor_data_race_checks, /*default=*/false);