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util: optimizes HexStr

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In my benchmark, this rewrite improves runtime 27% (g++) to 46% (clang++) for the benchmark `HexStrBench`:

g++ 11.2.0
|             ns/byte |              byte/s |    err% |        ins/byte |        cyc/byte |    IPC |       bra/byte |   miss% |     total | benchmark
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|                0.94 |    1,061,381,310.36 |    0.7% |           12.00 |            3.01 |  3.990 |           1.00 |    0.0% |      0.01 | `HexStrBench` master
|                0.68 |    1,465,366,544.25 |    1.7% |            6.00 |            2.16 |  2.778 |           1.00 |    0.0% |      0.01 | `HexStrBench` branch

clang++ 13.0.1
|             ns/byte |              byte/s |    err% |        ins/byte |        cyc/byte |    IPC |       bra/byte |   miss% |     total | benchmark
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|                0.80 |    1,244,713,415.92 |    0.9% |           10.00 |            2.56 |  3.913 |           0.50 |    0.0% |      0.01 | `HexStrBench` master
|                0.43 |    2,324,188,940.72 |    0.2% |            4.00 |            1.37 |  2.914 |           0.25 |    0.0% |      0.01 | `HexStrBench` branch

Note that the idea for this change comes from denis2342 in PR 23364. This is a rewrite so no unaligned accesses occur.

Also, the lookup table is now calculated at compile time, which hopefully makes the code a bit easier to review.
This commit is contained in:
Martin Leitner-Ankerl 2022-03-23 07:08:32 +01:00
parent 4e2b99f72a
commit 5e61532e72
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1 changed files with 27 additions and 6 deletions
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33
src/util/strencodings.cpp
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@ -9,6 +9,7 @@
#include <tinyformat.h>
#include <algorithm>
#include <array>
#include <cstdlib>
#include <cstring>
#include <limits>
@ -508,17 +509,37 @@ std::string Capitalize(std::string str)
return str;
}
namespace {
using ByteAsHex = std::array<char, 2>;
constexpr std::array<ByteAsHex, 256> CreateByteToHexMap()
{
constexpr char hexmap[16] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
std::array<ByteAsHex, 256> byte_to_hex{};
for (size_t i = 0; i < byte_to_hex.size(); ++i) {
byte_to_hex[i][0] = hexmap[i >> 4];
byte_to_hex[i][1] = hexmap[i & 15];
}
return byte_to_hex;
}
} // namespace
std::string HexStr(const Span<const uint8_t> s)
{
std::string rv(s.size() * 2, '\0');
static constexpr char hexmap[16] = { '0', '1', '2', '3', '4', '5', '6', '7',
'8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
auto it = rv.begin();
static constexpr auto byte_to_hex = CreateByteToHexMap();
static_assert(sizeof(byte_to_hex) == 512);
char* it = rv.data();
for (uint8_t v : s) {
*it++ = hexmap[v >> 4];
*it++ = hexmap[v & 15];
std::memcpy(it, byte_to_hex[v].data(), 2);
it += 2;
}
assert(it == rv.end());
assert(it == rv.data() + rv.size());
return rv;
}