[llvm][NFC] Add comments and common-case API to MachineBlockFrequencyInfo

[mtrofin]

Summary

Clarify the relation between a block's BlockFrequency and the
getEntryFreq() API, and added an API for the relatively common case of
finding a block's frequency relative to the entrypoint.

Added / moved some comments to header.

[mtrofin]

rebase

[hjyamauchi]

llvm/include/llvm/CodeGen/MachineBlockFrequencyInfo.h

68

I'd go with "Entry" (or "EntryBlock") rather than "Entrypoint" here (and also in the new comments), as I don't think the latter is used around here.

Also, I'd use float instead of double and add parentheses around the division (so the "1.0 / ..." part
happens before the multiplication) to preserve the original spill cost formula below exactly. Slight errors might matter (might not but to be safe.)

[mtrofin]

feedback

llvm/include/llvm/CodeGen/MachineBlockFrequencyInfo.h

68

Done - I suppose if a user wants to calculate double precision, they may still do so.

llvm/include/llvm/CodeGen/MachineBlockFrequencyInfo.h

void calculate(const MachineFunction &F,
                  const MachineLoopInfo &MLI);
 
   void releaseMemory() override;
 
   /// getblockFreq - Return block frequency. Return 0 if we don't have the
   /// information. Please note that initial frequency is equal to 1024. It means
   /// that we should not rely on the value itself, but only on the comparison to
   /// the other block frequencies. We do this to avoid using of floating points.
-  ///
+  /// For example, to get the frequency of a block relative to the entry block,
+  /// divide the integral value returned by this function (the
+  /// BlockFrequency::getFrequency() value) by getEntryFreq().
   BlockFrequency getBlockFreq(const MachineBasicBlock *MBB) const;
 
+  /// Compute the frequency of the block, relative to the entry block.
+  /// This API assumes getEntryFreq() is non-zero.
+  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);
 
   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;
 
   // Print the block frequency Freq to OS using the current functions entry
   // frequency to convert freq into a relative decimal form.
   raw_ostream &printBlockFreq(raw_ostream &OS, const BlockFrequency Freq) const;
 
   // Convenience method that attempts to look up the frequency associated with
   // BB and print it to OS.
   raw_ostream &printBlockFreq(raw_ostream &OS,
                               const MachineBasicBlock *MBB) const;
 
+  /// Divide a block's BlockFrequency::getFrequency() value by this value to
+  /// obtain the entry block - relative frequency of said block.
   uint64_t getEntryFreq() const;
 };
 
 } // end namespace llvm
 
 #endif // LLVM_CODEGEN_MACHINEBLOCKFREQUENCYINFO_H

llvm/lib/CodeGen/LiveIntervals.cpp

float LiveIntervals::getSpillWeight(bool isDef, bool isUse,
                                     const MachineBlockFrequencyInfo *MBFI,
                                     const MachineInstr &MI) {
   return getSpillWeight(isDef, isUse, MBFI, MI.getParent());
 }
 
 float LiveIntervals::getSpillWeight(bool isDef, bool isUse,
                                     const MachineBlockFrequencyInfo *MBFI,
                                     const MachineBasicBlock *MBB) {
-  BlockFrequency Freq = MBFI->getBlockFreq(MBB);
-  const float Scale = 1.0f / MBFI->getEntryFreq();
-  return (isDef + isUse) * (Freq.getFrequency() * Scale);
+  return (isDef + isUse) * MBFI->getBlockFreqRelativeToEntryBlock(MBB);
 }
 
 LiveRange::Segment
 LiveIntervals::addSegmentToEndOfBlock(unsigned reg, MachineInstr &startInst) {
   LiveInterval& Interval = createEmptyInterval(reg);
   VNInfo *VN = Interval.getNextValue(
       SlotIndex(getInstructionIndex(startInst).getRegSlot()),
       getVNInfoAllocator());

llvm/lib/CodeGen/RegAllocPBQP.cpp

for (const auto &MBB : MF) {
       for (const auto &MI : MBB) {
         // Skip not-coalescable or already coalesced copies.
         if (!CP.setRegisters(&MI) || CP.getSrcReg() == CP.getDstReg())
           continue;
 
         unsigned DstReg = CP.getDstReg();
         unsigned SrcReg = CP.getSrcReg();
 
-        const float Scale = 1.0f / MBFI.getEntryFreq();
-        PBQP::PBQPNum CBenefit = MBFI.getBlockFreq(&MBB).getFrequency() * Scale;
+        PBQP::PBQPNum CBenefit = MBFI.getBlockFreqRelativeToEntryBlock(&MBB);
 
         if (CP.isPhys()) {
           if (!MF.getRegInfo().isAllocatable(DstReg))
             continue;
 
           PBQPRAGraph::NodeId NId = G.getMetadata().getNodeIdForVReg(SrcReg);
 
           const PBQPRAGraph::NodeMetadata::AllowedRegVector &Allowed =