[llvm][NFC] refactor setBlockFrequency for clarity.

[mtrofin]

Summary

The refactoring encapsulates frequency calculation in MachineBlockFrequencyInfo,
and renames the API to clarify its motivation. It should clarify
frequencies may not be reset 'freely' by users of the analysis, as the
API serves as a partial update to avoid a full analysis recomputation.

[davidxl]

Perhaps also keep setBlockFreq as a lower level interface? I am surprised that edge splitting is the only one user of it.

[mtrofin]

In D84427#2177332, @davidxl wrote:

Perhaps also keep setBlockFreq as a lower level interface? I am surprised that edge splitting is the only one user of it.

We can always add it back. Its absence makes things easier to a reader, I think, because it becomes more clear when the analysis result undergoes mutations - i.e. whenever the pass manager requests it, and during edge splitting, as a performance optimization.

[davidxl]

thanks for the cleanup

llvm/include/llvm/CodeGen/MachineBlockFrequencyInfo.h

float getBlockFreqRelativeToEntryBlock(const MachineBasicBlock *MBB) const {
     return getBlockFreq(MBB).getFrequency() * (1.0f / getEntryFreq());
   }
 
   Optional<uint64_t> getBlockProfileCount(const MachineBasicBlock *MBB) const;
   Optional<uint64_t> getProfileCountFromFreq(uint64_t Freq) const;
 
   bool isIrrLoopHeader(const MachineBasicBlock *MBB) const;
 
-  void setBlockFreq(const MachineBasicBlock *MBB, uint64_t Freq);
+  /// incrementally calculate block frequencies when we split edges, to avoid
+  /// full CFG traversal.
+  void onEdgeSplit(const MachineBasicBlock &NewPredecessor,
+                   const MachineBasicBlock &NewSuccessor,
+                   const MachineBranchProbabilityInfo &MBPI);
 
   const MachineFunction *getFunction() const;
   const MachineBranchProbabilityInfo *getMBPI() const;
 
   /// Pop up a ghostview window with the current block frequency propagation
   /// rendered using dot.
   void view(const Twine &Name, bool isSimple = true) const;
 

llvm/lib/CodeGen/MachineBlockFrequencyInfo.cpp

 }
 
 bool MachineBlockFrequencyInfo::isIrrLoopHeader(
     const MachineBasicBlock *MBB) const {
   assert(MBFI && "Expected analysis to be available");
   return MBFI->isIrrLoopHeader(MBB);
 }
 
-void MachineBlockFrequencyInfo::setBlockFreq(const MachineBasicBlock *MBB,
-                                             uint64_t Freq) {
+void MachineBlockFrequencyInfo::onEdgeSplit(
+    const MachineBasicBlock &NewPredecessor,
+    const MachineBasicBlock &NewSuccessor,
+    const MachineBranchProbabilityInfo &MBPI) {
   assert(MBFI && "Expected analysis to be available");
-  MBFI->setBlockFreq(MBB, Freq);
+  auto NewSuccFreq = MBFI->getBlockFreq(&NewPredecessor) *
+                     MBPI.getEdgeProbability(&NewPredecessor, &NewSuccessor);
+
+  MBFI->setBlockFreq(&NewSuccessor, NewSuccFreq.getFrequency());
 }
 
 const MachineFunction *MachineBlockFrequencyInfo::getFunction() const {
   return MBFI ? MBFI->getFunction() : nullptr;
 }
 
 const MachineBranchProbabilityInfo *MachineBlockFrequencyInfo::getMBPI() const {
   return MBFI ? &MBFI->getBPI() : nullptr;

llvm/lib/CodeGen/MachineSink.cpp

while (true) {
     // If we have anything we marked as toSplit, split it now.
     for (auto &Pair : ToSplit) {
       auto NewSucc = Pair.first->SplitCriticalEdge(Pair.second, *this);
       if (NewSucc != nullptr) {
         LLVM_DEBUG(dbgs() << " *** Splitting critical edge: "
                           << printMBBReference(*Pair.first) << " -- "
                           << printMBBReference(*NewSucc) << " -- "
                           << printMBBReference(*Pair.second) << '\n');
-        if (MBFI) {
-          auto NewSuccFreq = MBFI->getBlockFreq(Pair.first) *
-                             MBPI->getEdgeProbability(Pair.first, NewSucc);
-          MBFI->setBlockFreq(NewSucc, NewSuccFreq.getFrequency());
-        }
+        if (MBFI)
+          MBFI->onEdgeSplit(*Pair.first, *NewSucc, *MBPI);
+
         MadeChange = true;
         ++NumSplit;
       } else
         LLVM_DEBUG(dbgs() << " *** Not legal to break critical edge\n");
     }
     // If this iteration over the code changed anything, keep iterating.
     if (!MadeChange) break;
     EverMadeChange = true;