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      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/Analysis/TargetLibraryInfo.h"
     20 #include "llvm/ProfileData/InstrProf.h"
     21 #include <memory>
     22 #include <vector>
     23 
     24 namespace llvm {
     25 
     26 class Function;
     27 class Instruction;
     28 class Value;
     29 
     30 /// Utility analysis that determines what values are worth profiling.
     31 /// The actual logic is inside the ValueProfileCollectorImpl, whose job is to
     32 /// populate the Candidates vector.
     33 ///
     34 /// Value profiling an expression means to track the values that this expression
     35 /// takes at runtime and the frequency of each value.
     36 /// It is important to distinguish between two sets of value profiles for a
     37 /// particular expression:
     38 ///  1) The set of values at the point of evaluation.
     39 ///  2) The set of values at the point of use.
     40 /// In some cases, the two sets are identical, but it's not unusual for the two
     41 /// to differ.
     42 ///
     43 /// To elaborate more, consider this C code, and focus on the expression `nn`:
     44 ///  void foo(int nn, bool b) {
     45 ///    if (b)  memcpy(x, y, nn);
     46 ///  }
     47 /// The point of evaluation can be as early as the start of the function, and
     48 /// let's say the value profile for `nn` is:
     49 ///     total=100; (value,freq) set = {(8,10), (32,50)}
     50 /// The point of use is right before we call memcpy, and since we execute the
     51 /// memcpy conditionally, the value profile of `nn` can be:
     52 ///     total=15; (value,freq) set = {(8,10), (4,5)}
     53 ///
     54 /// For this reason, a plugin is responsible for computing the insertion point
     55 /// for each value to be profiled. The `CandidateInfo` structure encapsulates
     56 /// all the information needed for each value profile site.
     57 class ValueProfileCollector {
     58 public:
     59   struct CandidateInfo {
     60     Value *V;                   // The value to profile.
     61     Instruction *InsertPt;      // Insert the VP lib call before this instr.
     62     Instruction *AnnotatedInst; // Where metadata is attached.
     63   };
     64 
     65   ValueProfileCollector(Function &Fn, TargetLibraryInfo &TLI);
     66   ValueProfileCollector(ValueProfileCollector &&) = delete;
     67   ValueProfileCollector &operator=(ValueProfileCollector &&) = delete;
     68 
     69   ValueProfileCollector(const ValueProfileCollector &) = delete;
     70   ValueProfileCollector &operator=(const ValueProfileCollector &) = delete;
     71   ~ValueProfileCollector();
     72 
     73   /// returns a list of value profiling candidates of the given kind
     74   std::vector<CandidateInfo> get(InstrProfValueKind Kind) const;
     75 
     76 private:
     77   class ValueProfileCollectorImpl;
     78   std::unique_ptr<ValueProfileCollectorImpl> PImpl;
     79 };
     80 
     81 } // namespace llvm
     82 
     83 #endif
     84