Add comments and rename functions.

Author: rlavaee

llvm/include/llvm/CodeGen/BasicBlockSectionsProfileReader.h

@@ -46,7 +46,7 @@ struct BBClusterInfo {
 // uniquely identifies the subblock in block `BBID` which starts from right
 // after call number `SubblockIndex` (or the beginning of the block if
 // `SubblockIndex` is zero) to the call number `SubblockIndex+1` (or the end of
-// the block if there are are `SubblockIndex` calls in the basic block).
+// the block if `SubblockIndex` is the last call in the block).
 struct SubblockID {
   UniqueBBID BBID;
   unsigned SubblockIndex;

llvm/include/llvm/CodeGen/MachineBasicBlock.h

@@ -229,7 +229,12 @@ class MachineBasicBlock
   /// is only computed once and is cached.
   mutable MCSymbol *CachedMCSymbol = nullptr;
 
-  SmallVector<unsigned> PrefetchTargetIndexes;
+  /// Contains the subblock indices in this block that are targets of code prefetching.
+  /// The subblock indexed `i` specifies that region after the `i`th call (or the
+  /// beginning of the block if `i==0`) to before the`i+1`th callsite (or the
+  /// end of the block). The prefetch target is always the beginning of the
+  /// subblock.
+  SmallVector<unsigned> PrefetchTargetSubblockIndexes;
 
   /// Cached MCSymbol for this block (used if IsEHContTarget).
   mutable MCSymbol *CachedEHContMCSymbol = nullptr;
@@ -712,12 +717,12 @@ class MachineBasicBlock
 
   std::optional<UniqueBBID> getBBID() const { return BBID; }
 
-  const SmallVector<unsigned> &getPrefetchTargetIndexes() const {
-    return PrefetchTargetIndexes;
+  const SmallVector<unsigned> &getPrefetchTargetSubblockIndexes() const {
+    return PrefetchTargetSubblockIndexes;
   }
 
-  void setPrefetchTargetIndexes(const SmallVector<unsigned> &V) {
-    PrefetchTargetIndexes = V;
+  void setPrefetchTargetSubblockIndexes(const SmallVector<unsigned> &V) {
+    PrefetchTargetSubblockIndexes = V;
   }
 
   /// Returns the section ID of this basic block.

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -1986,17 +1986,20 @@ void AsmPrinter::emitFunctionBody() {
     emitBasicBlockStart(MBB);
     DenseMap<StringRef, unsigned> MnemonicCounts;
 
-    SmallVector<unsigned> PrefetchTargets = MBB.getPrefetchTargetIndexes();
+    SmallVector<unsigned> PrefetchTargets = MBB.getPrefetchTargetSubblockIndexes();
     auto PrefetchTargetIt = PrefetchTargets.begin();
     unsigned NumCalls = 0;
+    // Helper to emit a symbol for the prefetch target and proceed to the next
+    // one.
     auto EmitPrefetchTargetSymbolIfNeeded = [&]() {
-      if (PrefetchTargetIt == PrefetchTargets.end() ||
-          NumCalls < *PrefetchTargetIt)
-        return;
+      if (PrefetchTargetIt == PrefetchTargets.end()) return;
+      if (NumCalls < *PrefetchTargetIt) return;
       MCSymbol *PrefetchTargetSymbol = OutContext.getOrCreateSymbol(
           Twine("__llvm_prefetch_target_") + MF->getName() + Twine("_") +
           utostr(MBB.getBBID()->BaseID) + Twine("_") +
           utostr(*PrefetchTargetIt));
+      // If the function is weak-linkage it may be replaced by a strong version,
+      // in which case the prefetch targets should also be replaced.
       OutStreamer->emitSymbolAttribute(
           PrefetchTargetSymbol,
           MF->getFunction().isWeakForLinker() ? MCSA_Weak : MCSA_Global);
@@ -2159,6 +2162,8 @@ void AsmPrinter::emitFunctionBody() {
       for (auto &Handler : Handlers)
         Handler->endInstruction();
     }
+    // If the block ends with a call, we may need to emit a prefetch target
+    // at the end.
     EmitPrefetchTargetSymbolIfNeeded();
 
     // We must emit temporary symbol for the end of this basic block, if either

llvm/lib/CodeGen/InsertCodePrefetch.cpp

@@ -12,9 +12,9 @@
 /// This pass inserts code prefetch instructions according to the prefetch
 /// directives in the basic block section profile. The target of a prefetch can
 /// be the beginning of any dynamic basic block, that is the beginning of a
-/// machine basic block, or immediately after a callsite. A global symbol will
-/// be emitted at the position of the target so it can be addressed from the
-/// prefetch instruction.
+/// machine basic block, or immediately after a callsite. A global symbol is
+/// emitted at the position of the target so it can be addressed from the
+/// prefetch instruction from any module.
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SmallVector.h"
@@ -68,6 +68,8 @@ bool InsertCodePrefetch::runOnMachineFunction(MachineFunction &MF) {
          "BB Sections list not enabled!");
   if (hasInstrProfHashMismatch(MF))
     return false;
+  // Set each block's prefetch targets so AsmPrinter can emit a special symbol
+  // there.
   SmallVector<BBPosition> PrefetchTargets =
       getAnalysis<BasicBlockSectionsProfileReaderWrapperPass>()
           .getPrefetchTargetsForFunction(MF.getName());
@@ -78,9 +80,8 @@ bool InsertCodePrefetch::runOnMachineFunction(MachineFunction &MF) {
     auto R = PrefetchTargetsByBBID.find(*MBB.getBBID());
     if (R == PrefetchTargetsByBBID.end())
       continue;
-    MBB.setPrefetchTargetIndexes(R->second);
+    MBB.setPrefetchTargetSubblockIndexes(R->second);
   }
-
   return false;
 }