1//===-- size_class_map.h ----------------------------------------*- C++ -*-===//
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#ifndef SCUDO_SIZE_CLASS_MAP_H_
10#define SCUDO_SIZE_CLASS_MAP_H_
11
12#include "common.h"
13#include "string_utils.h"
14
15namespace scudo {
16
17// SizeClassMap maps allocation sizes into size classes and back, in an
18// efficient table-free manner.
19//
20// Class 0 is a special class that doesn't abide by the same rules as other
21// classes. The allocator uses it to hold batches.
22//
23// The other sizes are controlled by the template parameters:
24// - MinSizeLog: defines the first class as 2^MinSizeLog bytes.
25// - MaxSizeLog: defines the last class as 2^MaxSizeLog bytes.
26// - MidSizeLog: classes increase with step 2^MinSizeLog from 2^MinSizeLog to
27//               2^MidSizeLog bytes.
28// - NumBits: the number of non-zero bits in sizes after 2^MidSizeLog.
29//            eg. with NumBits==3 all size classes after 2^MidSizeLog look like
30//            0b1xx0..0 (where x is either 0 or 1).
31//
32// This class also gives a hint to a thread-caching allocator about the amount
33// of chunks that can be cached per-thread:
34// - MaxNumCachedHint is a hint for the max number of chunks cached per class.
35// - 2^MaxBytesCachedLog is the max number of bytes cached per class.
36
37template <u8 NumBits, u8 MinSizeLog, u8 MidSizeLog, u8 MaxSizeLog,
38          u32 MaxNumCachedHintT, u8 MaxBytesCachedLog>
39class SizeClassMap {
40  static const uptr MinSize = 1UL << MinSizeLog;
41  static const uptr MidSize = 1UL << MidSizeLog;
42  static const uptr MidClass = MidSize / MinSize;
43  static const u8 S = NumBits - 1;
44  static const uptr M = (1UL << S) - 1;
45
46public:
47  static const u32 MaxNumCachedHint = MaxNumCachedHintT;
48
49  static const uptr MaxSize = 1UL << MaxSizeLog;
50  static const uptr NumClasses =
51      MidClass + ((MaxSizeLog - MidSizeLog) << S) + 1;
52  static_assert(NumClasses <= 256, "");
53  static const uptr LargestClassId = NumClasses - 1;
54  static const uptr BatchClassId = 0;
55
56  static uptr getSizeByClassId(uptr ClassId) {
57    DCHECK_NE(ClassId, BatchClassId);
58    if (ClassId <= MidClass)
59      return ClassId << MinSizeLog;
60    ClassId -= MidClass;
61    const uptr T = MidSize << (ClassId >> S);
62    return T + (T >> S) * (ClassId & M);
63  }
64
65  static uptr getClassIdBySize(uptr Size) {
66    DCHECK_LE(Size, MaxSize);
67    if (Size <= MidSize)
68      return (Size + MinSize - 1) >> MinSizeLog;
69    const uptr L = getMostSignificantSetBitIndex(Size);
70    const uptr HBits = (Size >> (L - S)) & M;
71    const uptr LBits = Size & ((1UL << (L - S)) - 1);
72    const uptr L1 = L - MidSizeLog;
73    return MidClass + (L1 << S) + HBits + (LBits > 0);
74  }
75
76  static u32 getMaxCachedHint(uptr Size) {
77    DCHECK_LE(Size, MaxSize);
78    DCHECK_NE(Size, 0);
79    u32 N;
80    // Force a 32-bit division if the template parameters allow for it.
81    if (MaxBytesCachedLog > 31 || MaxSizeLog > 31)
82      N = static_cast<u32>((1UL << MaxBytesCachedLog) / Size);
83    else
84      N = (1U << MaxBytesCachedLog) / static_cast<u32>(Size);
85    return Max(1U, Min(MaxNumCachedHint, N));
86  }
87
88  static void print() {
89    ScopedString Buffer(1024);
90    uptr PrevS = 0;
91    uptr TotalCached = 0;
92    for (uptr I = 0; I < NumClasses; I++) {
93      if (I == BatchClassId)
94        continue;
95      const uptr S = getSizeByClassId(I);
96      if (S >= MidSize / 2 && (S & (S - 1)) == 0)
97        Buffer.append("\n");
98      const uptr D = S - PrevS;
99      const uptr P = PrevS ? (D * 100 / PrevS) : 0;
100      const uptr L = S ? getMostSignificantSetBitIndex(S) : 0;
101      const uptr Cached = getMaxCachedHint(S) * S;
102      Buffer.append(
103          "C%02zu => S: %zu diff: +%zu %02zu%% L %zu Cached: %zu %zu; id %zu\n",
104          I, getSizeByClassId(I), D, P, L, getMaxCachedHint(S), Cached,
105          getClassIdBySize(S));
106      TotalCached += Cached;
107      PrevS = S;
108    }
109    Buffer.append("Total Cached: %zu\n", TotalCached);
110    Buffer.output();
111  }
112
113  static void validate() {
114    for (uptr C = 0; C < NumClasses; C++) {
115      if (C == BatchClassId)
116        continue;
117      const uptr S = getSizeByClassId(C);
118      CHECK_NE(S, 0U);
119      CHECK_EQ(getClassIdBySize(S), C);
120      if (C < LargestClassId)
121        CHECK_EQ(getClassIdBySize(S + 1), C + 1);
122      CHECK_EQ(getClassIdBySize(S - 1), C);
123      if (C - 1 != BatchClassId)
124        CHECK_GT(getSizeByClassId(C), getSizeByClassId(C - 1));
125    }
126    // Do not perform the loop if the maximum size is too large.
127    if (MaxSizeLog > 19)
128      return;
129    for (uptr S = 1; S <= MaxSize; S++) {
130      const uptr C = getClassIdBySize(S);
131      CHECK_LT(C, NumClasses);
132      CHECK_GE(getSizeByClassId(C), S);
133      if (C - 1 != BatchClassId)
134        CHECK_LT(getSizeByClassId(C - 1), S);
135    }
136  }
137};
138
139typedef SizeClassMap<3, 5, 8, 17, 8, 10> DefaultSizeClassMap;
140
141// TODO(kostyak): further tune class maps for Android & Fuchsia.
142#if SCUDO_WORDSIZE == 64U
143typedef SizeClassMap<4, 4, 8, 14, 4, 10> SvelteSizeClassMap;
144typedef SizeClassMap<3, 5, 8, 17, 14, 14> AndroidSizeClassMap;
145#else
146typedef SizeClassMap<4, 3, 7, 14, 5, 10> SvelteSizeClassMap;
147typedef SizeClassMap<3, 5, 8, 17, 14, 14> AndroidSizeClassMap;
148#endif
149
150} // namespace scudo
151
152#endif // SCUDO_SIZE_CLASS_MAP_H_
153