[clang-tools-extra] [llvm-profdata] Do not create numerical strings for MD5 function names read from a Sample Profile. (PR #66164)

[William Junda Huang via cfe-commits]

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+//===--- ProfileFuncRef.h - Sample profile function name ---*- C++ -*-===//
+//
+// 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 defines the StringRefOrHashCode class. It is to represent function
+// names in a sample profile, which can be in one of two forms - either a
+// regular string, or a 64-bit hash code.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_PROFILEDATA_PROFILEFUNCREF_H
+#define LLVM_PROFILEDATA_PROFILEFUNCREF_H
+
+#include "llvm/ADT/DenseMapInfo.h"
+#include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/MD5.h"
+#include "llvm/Support/raw_ostream.h"
+#include <cstdint>
+
+namespace llvm {
+namespace sampleprof {
+
+/// This class represents a function name that is read from a sample profile. It
+/// comes with two forms: a string or a hash code. For efficient storage, a
+/// sample profile may store function names as 64-bit MD5 values, so when
+/// reading the profile, this class can represnet them without converting it to
+/// a string first.
+/// When representing a hash code, we utilize the Length field to store it, and
+/// Data is set to null. When representing a string, it is same as StringRef,
+/// and can be pointer-casted as one.
+/// We disallow implicit cast to StringRef because there are too many instances
+/// that it may cause break the code, such as using it in a StringMap.
+class ProfileFuncRef {
+
+  const char *Data = nullptr;
+
+  /// Use uint64_t instead of size_t so that it can also hold a MD5 value.
+  uint64_t LengthOrHashCode = 0;
+
+  /// Extension to memcmp to handle hash code representation. If both are hash
+  /// values, Lhs and Rhs are both null, function returns 0 (and needs an extra
+  /// comparison using getIntValue). If only one is hash code, it is considered
+  /// less than the StringRef one. Otherwise perform normal string comparison.
+  static int compareMemory(const char *Lhs, const char *Rhs, uint64_t Length) {
+    if (Lhs == Rhs)
+      return 0;
+    if (!Lhs)
+      return -1;
+    if (!Rhs)
+      return 1;
+    return ::memcmp(Lhs, Rhs, (size_t)Length);
+  }
+
+public:
+  ProfileFuncRef() = default;
+
+  /// Constructor from a StringRef. Check if Str is a number, which is generated
+  /// by converting a MD5 sample profile to a format that does not support MD5,
+  /// and if so, convert the numerical string to a hash code first. We assume
+  /// that no function name (from a profile) can be a pure number.
+  explicit ProfileFuncRef(StringRef Str)
+      : Data(Str.data()), LengthOrHashCode(Str.size()) {
+    // Only need to check for base 10 digits, fail faster if otherwise.
+    if (Str.size() > 0 && isdigit(Str[0]) &&
+        !Str.getAsInteger(10, LengthOrHashCode))
+      Data = nullptr;
+  }
+
+  /// Constructor from a hash code.
+  explicit ProfileFuncRef(uint64_t HashCode)
+      : Data(nullptr), LengthOrHashCode(HashCode) {
+    assert(HashCode != 0);
+  }
+
+  /// Check for equality. Similar to StringRef::equals, but will also cover for
+  /// the case where one or both are hash codes. Comparing their int values are
+  /// sufficient. A hash code ProfileFuncRef is considered not equal to a
+  /// StringRef ProfileFuncRef regardless of actual contents.
+  bool equals(const ProfileFuncRef &Other) const {
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huangjd wrote:

The code here is made consistent with StringRef, which is implemented this way. 

https://github.com/llvm/llvm-project/pull/66164