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 
15 namespace 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 
37 template <u8 NumBits, u8 MinSizeLog, u8 MidSizeLog, u8 MaxSizeLog,
38           u32 MaxNumCachedHintT, u8 MaxBytesCachedLog>
39 class 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 
46 public:
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 
139 typedef SizeClassMap<3, 5, 8, 17, 8, 10> DefaultSizeClassMap;
140 
141 // TODO(kostyak): further tune class maps for Android & Fuchsia.
142 #if SCUDO_WORDSIZE == 64U
143 typedef SizeClassMap<4, 4, 8, 14, 4, 10> SvelteSizeClassMap;
144 typedef SizeClassMap<3, 5, 8, 17, 14, 14> AndroidSizeClassMap;
145 #else
146 typedef SizeClassMap<4, 3, 7, 14, 5, 10> SvelteSizeClassMap;
147 typedef SizeClassMap<3, 5, 8, 17, 14, 14> AndroidSizeClassMap;
148 #endif
149 
150 } // namespace scudo
151 
152 #endif // SCUDO_SIZE_CLASS_MAP_H_
153