[PGO] Refactor Value Profiling into a plugin based oracle and create a well defined API for the plugins.

Summary: This PR creates a utility class called ValueProfileCollector that tells PGOInstrumentationGen and PGOInstrumentationUse what to value-profile and where to attach the profile metadata. It then refactors logic scattered in PGOInstrumentation.cpp into two plugins that plug into the ValueProfileCollector.

Authored By: Wael Yehia <wyehia@ca.ibm.com>

Reviewer: davidxl, tejohnson, xur

Reviewed By: davidxl, tejohnson, xur

Subscribers: llvm-commits

Tag: #llvm

Differential Revision: https://reviews.llvm.org/D67920

Patch By Wael Yehia <wyehia@ca.ibm.com>

llvm-svn: 373601

1 files changed, 79 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/ValueProfileCollector.h b/llvm/lib/Transforms/Instrumentation/ValueProfileCollector.h
new file mode 100644
index 00000000000..ff883c8d0c7
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+++ b/llvm/lib/Transforms/Instrumentation/ValueProfileCollector.h
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+//===- ValueProfileCollector.h - determine what to value profile ----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains a utility class, ValueProfileCollector, that is used to
+// determine what kind of llvm::Value's are worth value-profiling, at which
+// point in the program, and which instruction holds the Value Profile metadata.
+// Currently, the only users of this utility is the PGOInstrumentation[Gen|Use]
+// passes.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_PROFILE_GEN_ANALYSIS_H
+#define LLVM_ANALYSIS_PROFILE_GEN_ANALYSIS_H
+
+#include "llvm/IR/Function.h"
+#include "llvm/IR/PassManager.h"
+#include "llvm/Pass.h"
+#include "llvm/ProfileData/InstrProf.h"
+
+namespace llvm {
+
+/// Utility analysis that determines what values are worth profiling.
+/// The actual logic is inside the ValueProfileCollectorImpl, whose job is to
+/// populate the Candidates vector.
+///
+/// Value profiling an expression means to track the values that this expression
+/// takes at runtime and the frequency of each value.
+/// It is important to distinguish between two sets of value profiles for a
+/// particular expression:
+///  1) The set of values at the point of evaluation.
+///  2) The set of values at the point of use.
+/// In some cases, the two sets are identical, but it's not unusual for the two
+/// to differ.
+///
+/// To elaborate more, consider this C code, and focus on the expression `nn`:
+///  void foo(int nn, bool b) {
+///    if (b)  memcpy(x, y, nn);
+///  }
+/// The point of evaluation can be as early as the start of the function, and
+/// let's say the value profile for `nn` is:
+///     total=100; (value,freq) set = {(8,10), (32,50)}
+/// The point of use is right before we call memcpy, and since we execute the
+/// memcpy conditionally, the value profile of `nn` can be:
+///     total=15; (value,freq) set = {(8,10), (4,5)}
+///
+/// For this reason, a plugin is responsible for computing the insertion point
+/// for each value to be profiled. The `CandidateInfo` structure encapsulates
+/// all the information needed for each value profile site.
+class ValueProfileCollector {
+public:
+  struct CandidateInfo {
+    Value *V;                   // The value to profile.
+    Instruction *InsertPt;      // Insert the VP lib call before this instr.
+    Instruction *AnnotatedInst; // Where metadata is attached.
+  };
+
+  ValueProfileCollector(Function &Fn);
+  ValueProfileCollector(ValueProfileCollector &&) = delete;
+  ValueProfileCollector &operator=(ValueProfileCollector &&) = delete;
+
+  ValueProfileCollector(const ValueProfileCollector &) = delete;
+  ValueProfileCollector &operator=(const ValueProfileCollector &) = delete;
+  ~ValueProfileCollector();
+
+  /// returns a list of value profiling candidates of the given kind
+  std::vector<CandidateInfo> get(InstrProfValueKind Kind) const;
+
+private:
+  class ValueProfileCollectorImpl;
+  std::unique_ptr<ValueProfileCollectorImpl> PImpl;
+};
+
+} // namespace llvm
+
+#endif