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codegen.cpp
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codegen.cpp
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#include <algorithm>
#include <cstdio>
#include <cmath>
constexpr int NumValuesToCompute = 256;
int GCD(int a, int b)
{
if(b == 0)
return a;
return GCD(b, a % b);
}
int LCM(int a, int b)
{
return a * (b / GCD(a, b));
}
struct UltrapackGrouping
{
int elements_per_group;
int bits_per_group;
};
UltrapackGrouping groupings[NumValuesToCompute + 1];
void ComputeMinBitsPerValue(std::FILE *file, int num_values)
{
double bits_per_value = 10000;
int bits_per_group = 0;
int elts_per_group = 0;
for(int num_bits_per_group = 1; num_bits_per_group <= 56; num_bits_per_group++)
{
for(int num_elts_per_group = 1; num_elts_per_group < 100; num_elts_per_group++)
{
if(log(2) * num_bits_per_group < log(num_values) * num_elts_per_group)
break;
double bpv = static_cast<double>(num_bits_per_group) / static_cast<double>(num_elts_per_group);
if(bpv < bits_per_value - 0.05)
{
bits_per_value = bpv;
bits_per_group = num_bits_per_group;
elts_per_group = num_elts_per_group;
}
}
}
double bitpack_bits = static_cast<double>(32 - __builtin_clz(num_values - 1));
double pct_savings = ((bitpack_bits - bits_per_value) / bitpack_bits) * 100.0;
#if 0
std::printf(
"%d: %d bits / %d values = %.4f bpv (vs %.4f bpv, %.2f%% savings)\n",
num_values,
bits_per_group,
elts_per_group,
bits_per_value,
bitpack_bits,
pct_savings);
#endif
groupings[num_values] = { elts_per_group, bits_per_group };
std::printf("%d, %.4f, %.4f, %.4f\n", num_values, bits_per_value, bitpack_bits, pct_savings);
std::fprintf(
file,
"%s{ %2d, %2d },%s",
num_values % 8 == 1 ? " " : "",
elts_per_group,
bits_per_group,
num_values % 8 == 0 ? "\n" : " ");
}
void WriteCompressFunction(std::FILE *file, int num_values)
{
int num_bits = LCM(groupings[num_values].bits_per_group, 8);
int num_groups = num_bits / groupings[num_values].bits_per_group;
int num_bytes = num_bits / 8;
std::fprintf(file, R"(// Pack %d groups using %d bytes (%d values total)
void Ultrapack_Compress%d(const uint32_t **input, uint8_t **output)
{
)", num_groups, num_bytes, groupings[num_values].elements_per_group * num_groups, num_values);
for(int igroup = 0; igroup < num_groups; igroup++)
{
std::fprintf(file, " uint64_t group%d = 0;", igroup);
for(int ival = 0; ival < groupings[num_values].elements_per_group; ival++)
{
std::fprintf(file, R"(
group%d *= %d;
group%d += (*input)[%d];
)", igroup, num_values, igroup, ival);
}
int idx = igroup * groupings[num_values].bits_per_group;
int byte_idx = idx / 8;
int bit_idx = idx % 8;
std::fprintf(file, R"(
*((uint64_t *)((*output) + %d)) |= (group%d << %d);
*input += %d;
)", byte_idx, igroup, bit_idx, groupings[num_values].elements_per_group);
}
std::fprintf(file, R"( *output += %d;
}
)", num_bytes);
}
void WriteDecompressFunction(std::FILE *file, int num_values)
{
int num_bits = LCM(groupings[num_values].bits_per_group, 8);
int num_groups = num_bits / groupings[num_values].bits_per_group;
int num_bytes = num_bits / 8;
std::fprintf(file, R"(// Unpack %d bytes into %d values
void Ultrapack_Decompress%d(const uint8_t **input, uint32_t **output)
{
)", num_bytes, groupings[num_values].elements_per_group * num_groups, num_values);
for(int igroup = 0; igroup < num_groups; igroup++)
{
int idx = igroup * groupings[num_values].bits_per_group;
int byte_idx = idx / 8;
int bit_idx = idx % 8;
std::fprintf(file, " uint64_t group%d = (*(uint64_t *)((*input) + %d)) >> %d;", igroup, byte_idx, bit_idx);
std::fprintf(file, " group%d &= (1ull << %d) - 1;", igroup, groupings[num_values].bits_per_group);
for(int ival = groupings[num_values].elements_per_group - 1; ival >= 0; ival--)
{
std::fprintf(file, R"(
(*output)[%d] = (uint32_t)(group%d %% %d);
group%d /= %d;
)", ival, igroup, num_values, igroup, num_values);
}
std::fprintf(file, " *output += %d;\n\n", groupings[num_values].elements_per_group);
}
std::fprintf(file, R"(
*input += %d;
}
)", num_bytes);
}
void AddCompressFunctionToTable(FILE *file, int i)
{
std::fprintf(file, " Ultrapack_Compress%d,\n", i);
}
void AddDecompressFunctionToTable(FILE *file, int i)
{
std::fprintf(file, " Ultrapack_Decompress%d,\n", i);
}
int main()
{
std::FILE *file = std::fopen("ultrapack.h", "w");
std::fprintf(file,
R"(#pragma once
#include <cstdint>
struct UltrapackGrouping
{
int elements_per_group;
int bits_per_group;
};
UltrapackGrouping UltrapackGroupings[] =
{
{ 0, 0 }, )");
for(int i = 2; i <= 256; i++)
ComputeMinBitsPerValue(file, i);
std::fprintf(file,
R"(};
int ElementsPerGroup(int num_distinct_values)
{
return UltrapackGroupings[num_distinct_values - 1].elements_per_group;
}
int BitsPerGroup(int num_distinct_values)
{
return UltrapackGroupings[num_distinct_values - 1].bits_per_group;
}
double BitsPerValue(int num_distinct_values)
{
return static_cast<double>(BitsPerGroup(num_distinct_values)) / static_cast<double>(ElementsPerGroup(num_distinct_values));
}
)");
for(int i = 2; i <= NumValuesToCompute; i++)
WriteCompressFunction(file, i);
std::fprintf(file, R"(
typedef void (*CompressFn)(const uint32_t **, uint8_t **);
CompressFn UltrapackCompress[] =
{
0,
0,
)");
for(int i = 2; i <= NumValuesToCompute; i++)
AddCompressFunctionToTable(file, i);
std::fprintf(file, "};\n");
for(int i = 2; i <= NumValuesToCompute; i++)
WriteDecompressFunction(file, i);
std::fprintf(file, R"(
typedef void (*DecompressFn)(const uint8_t **, uint32_t **);
DecompressFn UltrapackDecompress[] =
{
0,
0,
)");
for(int i = 2; i <= NumValuesToCompute; i++)
AddDecompressFunctionToTable(file, i);
std::fprintf(file, "};\n");
std::fclose(file);
return 0;
}