LLVM  11.0.0git
ValueProfileCollector.h
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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 
20 #include "llvm/IR/Function.h"
21 #include "llvm/IR/PassManager.h"
22 #include "llvm/Pass.h"
24 
25 namespace llvm {
26 
27 /// Utility analysis that determines what values are worth profiling.
28 /// The actual logic is inside the ValueProfileCollectorImpl, whose job is to
29 /// populate the Candidates vector.
30 ///
31 /// Value profiling an expression means to track the values that this expression
32 /// takes at runtime and the frequency of each value.
33 /// It is important to distinguish between two sets of value profiles for a
34 /// particular expression:
35 /// 1) The set of values at the point of evaluation.
36 /// 2) The set of values at the point of use.
37 /// In some cases, the two sets are identical, but it's not unusual for the two
38 /// to differ.
39 ///
40 /// To elaborate more, consider this C code, and focus on the expression `nn`:
41 /// void foo(int nn, bool b) {
42 /// if (b) memcpy(x, y, nn);
43 /// }
44 /// The point of evaluation can be as early as the start of the function, and
45 /// let's say the value profile for `nn` is:
46 /// total=100; (value,freq) set = {(8,10), (32,50)}
47 /// The point of use is right before we call memcpy, and since we execute the
48 /// memcpy conditionally, the value profile of `nn` can be:
49 /// total=15; (value,freq) set = {(8,10), (4,5)}
50 ///
51 /// For this reason, a plugin is responsible for computing the insertion point
52 /// for each value to be profiled. The `CandidateInfo` structure encapsulates
53 /// all the information needed for each value profile site.
55 public:
56  struct CandidateInfo {
57  Value *V; // The value to profile.
58  Instruction *InsertPt; // Insert the VP lib call before this instr.
59  Instruction *AnnotatedInst; // Where metadata is attached.
60  };
61 
65 
69 
70  /// returns a list of value profiling candidates of the given kind
71  std::vector<CandidateInfo> get(InstrProfValueKind Kind) const;
72 
73 private:
75  std::unique_ptr<ValueProfileCollectorImpl> PImpl;
76 };
77 
78 } // namespace llvm
79 
80 #endif
This class represents lattice values for constants.
Definition: AllocatorList.h:23
ValueProfileCollector(Function &Fn, TargetLibraryInfo &TLI)
InstrProfValueKind
Definition: InstrProf.h:237
ValueProfileCollectorImpl inherits the API of PluginChainFinal.
Provides information about what library functions are available for the current target.
Utility analysis that determines what values are worth profiling.
ValueProfileCollector & operator=(ValueProfileCollector &&)=delete
LLVM Value Representation.
Definition: Value.h:74
This header defines various interfaces for pass management in LLVM.