1 //===-- Implementation of memset and bzero --------------------------------===//
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 LLVM_LIBC_SRC_STRING_MEMORY_UTILS_MEMSET_IMPLEMENTATIONS_H
10 #define LLVM_LIBC_SRC_STRING_MEMORY_UTILS_MEMSET_IMPLEMENTATIONS_H
11 
12 #include "src/__support/architectures.h"
13 #include "src/string/memory_utils/elements.h"
14 #include "src/string/memory_utils/utils.h"
15 
16 #include <stddef.h> // size_t
17 
18 namespace __llvm_libc {
19 
20 // A general purpose implementation assuming cheap unaligned writes for sizes:
21 // 1, 2, 4, 8, 16, 32 and 64 Bytes. Note that some architecture can't store 32
22 // or 64 Bytes at a time, the compiler will expand them as needed.
23 //
24 // This implementation is subject to change as we benchmark more processors. We
25 // may also want to customize it for processors with specialized instructions
26 // that performs better (e.g. `rep stosb`).
27 //
28 // A note on the apparent discrepancy in the use of 32 vs 64 Bytes writes.
29 // We want to balance two things here:
30 //  - The number of redundant writes (when using `SetBlockOverlap`),
31 //  - The number of conditionals for sizes <=128 (~90% of memset calls are for
32 //    such sizes).
33 //
34 // For the range 64-128:
35 //  - SetBlockOverlap<64> uses no conditionals but always writes 128 Bytes this
36 //  is wasteful near 65 but efficient toward 128.
37 //  - SetAlignedBlocks<32> would consume between 3 and 4 conditionals and write
38 //  96 or 128 Bytes.
39 //  - Another approach could be to use an hybrid approach Copy<64>+Overlap<32>
40 //  for 65-96 and Copy<96>+Overlap<32> for 97-128
41 //
42 // Benchmarks showed that redundant writes were cheap (for Intel X86) but
43 // conditional were expensive, even on processor that do not support writing 64B
44 // at a time (pre-AVX512F). We also want to favor short functions that allow
45 // more hot code to fit in the iL1 cache.
46 //
47 // Above 128 we have to use conditionals since we don't know the upper bound in
48 // advance. SetAlignedBlocks<64> may waste up to 63 Bytes, SetAlignedBlocks<32>
49 // may waste up to 31 Bytes. Benchmarks showed that SetAlignedBlocks<64> was not
50 // superior for sizes that mattered.
51 inline static void inline_memset(char *dst, unsigned char value, size_t count) {
52 #if defined(LLVM_LIBC_ARCH_X86)
53   /////////////////////////////////////////////////////////////////////////////
54   // LLVM_LIBC_ARCH_X86
55   /////////////////////////////////////////////////////////////////////////////
56   using namespace __llvm_libc::x86;
57   if (count == 0)
58     return;
59   if (count == 1)
60     return SplatSet<_1>(dst, value);
61   if (count == 2)
62     return SplatSet<_2>(dst, value);
63   if (count == 3)
64     return SplatSet<_3>(dst, value);
65   if (count <= 8)
66     return SplatSet<HeadTail<_4>>(dst, value, count);
67   if (count <= 16)
68     return SplatSet<HeadTail<_8>>(dst, value, count);
69   if (count <= 32)
70     return SplatSet<HeadTail<_16>>(dst, value, count);
71   if (count <= 64)
72     return SplatSet<HeadTail<_32>>(dst, value, count);
73   if (count <= 128)
74     return SplatSet<HeadTail<_64>>(dst, value, count);
75   return SplatSet<Align<_32, Arg::Dst>::Then<Loop<_32>>>(dst, value, count);
76 #elif defined(LLVM_LIBC_ARCH_AARCH64)
77   /////////////////////////////////////////////////////////////////////////////
78   // LLVM_LIBC_ARCH_AARCH64
79   /////////////////////////////////////////////////////////////////////////////
80   using namespace __llvm_libc::aarch64_memset;
81   if (count == 0)
82     return;
83   if (count <= 3) {
84     SplatSet<_1>(dst, value);
85     if (count > 1)
86       SplatSet<Tail<_2>>(dst, value, count);
87     return;
88   }
89   if (count <= 8)
90     return SplatSet<HeadTail<_4>>(dst, value, count);
91   if (count <= 16)
92     return SplatSet<HeadTail<_8>>(dst, value, count);
93   if (count <= 32)
94     return SplatSet<HeadTail<_16>>(dst, value, count);
95   if (count <= 96) {
96     SplatSet<_32>(dst, value);
97     if (count <= 64)
98       return SplatSet<Tail<_32>>(dst, value, count);
99     SplatSet<Skip<32>::Then<_32>>(dst, value);
100     SplatSet<Tail<_32>>(dst, value, count);
101     return;
102   }
103   if (count < 448 || value != 0 || !AArch64ZVA(dst, count))
104     return SplatSet<Align<_16, Arg::_1>::Then<Loop<_64>>>(dst, value, count);
105 #else
106   /////////////////////////////////////////////////////////////////////////////
107   // Default
108   /////////////////////////////////////////////////////////////////////////////
109   using namespace ::__llvm_libc::scalar;
110 
111   if (count == 0)
112     return;
113   if (count == 1)
114     return SplatSet<_1>(dst, value);
115   if (count == 2)
116     return SplatSet<_2>(dst, value);
117   if (count == 3)
118     return SplatSet<_3>(dst, value);
119   if (count <= 8)
120     return SplatSet<HeadTail<_4>>(dst, value, count);
121   if (count <= 16)
122     return SplatSet<HeadTail<_8>>(dst, value, count);
123   if (count <= 32)
124     return SplatSet<HeadTail<_16>>(dst, value, count);
125   if (count <= 64)
126     return SplatSet<HeadTail<_32>>(dst, value, count);
127   if (count <= 128)
128     return SplatSet<HeadTail<_64>>(dst, value, count);
129   return SplatSet<Align<_32, Arg::Dst>::Then<Loop<_32>>>(dst, value, count);
130 #endif
131 }
132 
133 } // namespace __llvm_libc
134 
135 #endif // LLVM_LIBC_SRC_STRING_MEMORY_UTILS_MEMSET_IMPLEMENTATIONS_H
136