lib/Transforms/Instrumentation/ValueProfileCollector.h

   1 //===- ValueProfileCollector.h - determine what to value profile ----------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file contains a utility class, ValueProfileCollector, that is used to
  10 // determine what kind of llvm::Value's are worth value-profiling, at which
  11 // point in the program, and which instruction holds the Value Profile metadata.
  12 // Currently, the only users of this utility is the PGOInstrumentation[Gen|Use]
  13 // passes.
  14 //===----------------------------------------------------------------------===//
  15
  16 #ifndef LLVM_ANALYSIS_PROFILE_GEN_ANALYSIS_H
  17 #define LLVM_ANALYSIS_PROFILE_GEN_ANALYSIS_H
  18
  19 #include "llvm/IR/Function.h"
  20 #include "llvm/IR/PassManager.h"
  21 #include "llvm/Pass.h"
  22 #include "llvm/ProfileData/InstrProf.h"
  23
  24 namespace llvm {
  25
  26 /// Utility analysis that determines what values are worth profiling.
  27 /// The actual logic is inside the ValueProfileCollectorImpl, whose job is to
  28 /// populate the Candidates vector.
  29 ///
  30 /// Value profiling an expression means to track the values that this expression
  31 /// takes at runtime and the frequency of each value.
  32 /// It is important to distinguish between two sets of value profiles for a
  33 /// particular expression:
  34 ///  1) The set of values at the point of evaluation.
  35 ///  2) The set of values at the point of use.
  36 /// In some cases, the two sets are identical, but it's not unusual for the two
  37 /// to differ.
  38 ///
  39 /// To elaborate more, consider this C code, and focus on the expression `nn`:
  40 ///  void foo(int nn, bool b) {
  41 ///    if (b)  memcpy(x, y, nn);
  42 ///  }
  43 /// The point of evaluation can be as early as the start of the function, and
  44 /// let's say the value profile for `nn` is:
  45 ///     total=100; (value,freq) set = {(8,10), (32,50)}
  46 /// The point of use is right before we call memcpy, and since we execute the
  47 /// memcpy conditionally, the value profile of `nn` can be:
  48 ///     total=15; (value,freq) set = {(8,10), (4,5)}
  49 ///
  50 /// For this reason, a plugin is responsible for computing the insertion point
  51 /// for each value to be profiled. The `CandidateInfo` structure encapsulates
  52 /// all the information needed for each value profile site.
  53 class ValueProfileCollector {
  54 public:
  55   struct CandidateInfo {
  56     Value *V;                   // The value to profile.
  57     Instruction *InsertPt;      // Insert the VP lib call before this instr.
  58     Instruction *AnnotatedInst; // Where metadata is attached.
  59   };
  60
  61   ValueProfileCollector(Function &Fn);
  62   ValueProfileCollector(ValueProfileCollector &&) = delete;
  63   ValueProfileCollector &operator=(ValueProfileCollector &&) = delete;
  64
  65   ValueProfileCollector(const ValueProfileCollector &) = delete;
  66   ValueProfileCollector &operator=(const ValueProfileCollector &) = delete;
  67   ~ValueProfileCollector();
  68
  69   /// returns a list of value profiling candidates of the given kind
  70   std::vector<CandidateInfo> get(InstrProfValueKind Kind) const;
  71
  72 private:
  73   class ValueProfileCollectorImpl;
  74   std::unique_ptr<ValueProfileCollectorImpl> PImpl;
  75 };
  76
  77 } // namespace llvm
  78
  79 #endif