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

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.

Differential Revision: https://reviews.llvm.org/D84357

Author: mtrofin

llvm/include/llvm/CodeGen/MachineBlockFrequencyInfo.h

@@ -58,9 +58,17 @@ class MachineBlockFrequencyInfo : public MachineFunctionPass {
   /// 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;
 
@@ -70,6 +78,9 @@ class MachineBlockFrequencyInfo : public MachineFunctionPass {
 
   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
@@ -81,6 +92,8 @@ class MachineBlockFrequencyInfo : public MachineFunctionPass {
   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;
 };
 

llvm/lib/CodeGen/LiveIntervals.cpp

@@ -868,9 +868,7 @@ float LiveIntervals::getSpillWeight(bool isDef, bool isUse,
 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

llvm/lib/CodeGen/RegAllocPBQP.cpp

@@ -451,8 +451,7 @@ class Coalescing : public PBQPRAConstraint {
         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))