Add some comments.

Author: rlavaee

llvm/include/llvm/CodeGen/BasicBlockSectionsProfileReader.h

@@ -42,9 +42,14 @@ struct BBClusterInfo {
   unsigned PositionInCluster;
 };
 
-struct BBPosition {
+// Assuming a block is split into subblocks across its callsites, this struct
+// 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).
+struct SubblockID {
   UniqueBBID BBID;
-  unsigned CallsiteIndex;
+  unsigned SubblockIndex;
 };
 
 // This represents the raw input profile for one function.
@@ -55,7 +60,9 @@ struct FunctionPathAndClusterInfo {
   // the edge a -> b (a is not cloned). The index of the path in this vector
   // determines the `UniqueBBID::CloneID` of the cloned blocks in that path.
   SmallVector<SmallVector<unsigned>> ClonePaths;
-  SmallVector<BBPosition> PrefetchTargets;
+  // Code prefetch targets, specified by the subblock ID of which beginning must
+  // be targetted for prefetching.
+  SmallVector<SubblockID> PrefetchTargets;
   // Node counts for each basic block.
   DenseMap<UniqueBBID, uint64_t> NodeCounts;
   // Edge counts for each edge.
@@ -92,7 +99,9 @@ class BasicBlockSectionsProfileReader {
   uint64_t getEdgeCount(StringRef FuncName, const UniqueBBID &SrcBBID,
                         const UniqueBBID &SinkBBID) const;
 
-  SmallVector<BBPosition>
+  // Returns the prefetch targets (identified by their containing subblocks) for
+  // function `FuncName`.
+  SmallVector<SubblockID>
   getPrefetchTargetsForFunction(StringRef FuncName) const;
 
 private:
@@ -204,7 +213,7 @@ class BasicBlockSectionsProfileReaderWrapperPass : public ImmutablePass {
   uint64_t getEdgeCount(StringRef FuncName, const UniqueBBID &SrcBBID,
                         const UniqueBBID &DestBBID) const;
 
-  SmallVector<BBPosition>
+  SmallVector<SubblockID>
   getPrefetchTargetsForFunction(StringRef FuncName) const;
 
   // Initializes the FunctionNameToDIFilename map for the current module and

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -119,7 +119,6 @@
 #include "llvm/Support/Format.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Path.h"
-#include "llvm/Support/SMLoc.h"
 #include "llvm/Support/VCSRevision.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
@@ -1982,7 +1981,6 @@ void AsmPrinter::emitFunctionBody() {
 
   FunctionCallGraphInfo FuncCGInfo;
   const auto &CallSitesInfoMap = MF->getCallSitesInfo();
-
   for (auto &MBB : *MF) {
     // Print a label for the basic block.
     emitBasicBlockStart(MBB);

llvm/lib/CodeGen/InsertCodePrefetch.cpp

@@ -1,4 +1,4 @@
-//===-- InsertCodePrefetch.cpp ---=========-----------------------------===//
+//===-- InsertCodePrefetch.cpp ---=========--------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -7,9 +7,14 @@
 //===----------------------------------------------------------------------===//
 //
 /// \file
-/// Prefetch insertion pass implementation.
+/// Code Prefetch Insertion Pass.
 //===----------------------------------------------------------------------===//
-/// Prefetch insertion pass.
+/// 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.
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SmallVector.h"
@@ -20,15 +25,11 @@
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
-#include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/Passes.h"
-#include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/InitializePasses.h"
-#include "llvm/MC/MCContext.h"
-#include "llvm/Target/TargetMachine.h"
 
 using namespace llvm;
-#define DEBUG_TYPE "prefetchinsertion"
+#define DEBUG_TYPE "insert-code-prefetch"
 
 namespace {
 class InsertCodePrefetch : public MachineFunctionPass {
@@ -40,13 +41,12 @@ class InsertCodePrefetch : public MachineFunctionPass {
   }
 
   StringRef getPassName() const override {
-    return "X86 Cide Prefetch Inserter Pass";
+    return "Code Prefetch Inserter Pass";
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override;
 
-  /// Identify basic blocks that need separate sections and prepare to emit them
-  /// accordingly.
+  // Sets prefetch targets based on the bb section profile.
   bool runOnMachineFunction(MachineFunction &MF) override;
 };
 
@@ -57,10 +57,10 @@ class InsertCodePrefetch : public MachineFunctionPass {
 //===----------------------------------------------------------------------===//
 
 char InsertCodePrefetch::ID = 0;
-INITIALIZE_PASS_BEGIN(InsertCodePrefetch, DEBUG_TYPE, "Reads prefetch", true,
+INITIALIZE_PASS_BEGIN(InsertCodePrefetch, DEBUG_TYPE, "Code prefetch insertion", true,
                       false)
 INITIALIZE_PASS_DEPENDENCY(BasicBlockSectionsProfileReaderWrapperPass)
-INITIALIZE_PASS_END(InsertCodePrefetch, DEBUG_TYPE, "Reads prefetch", true,
+INITIALIZE_PASS_END(InsertCodePrefetch, DEBUG_TYPE, "Code prefetch insertion", true,
                     false)
 
 bool InsertCodePrefetch::runOnMachineFunction(MachineFunction &MF) {