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ORC-1041: Use memcpy during LZO decompression #958

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merged 2 commits into from
Nov 4, 2021
Merged

ORC-1041: Use memcpy during LZO decompression #958

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e3c7d16
Fix TestDecompression.testLzoLong on Debian 11
guiyanakuang Nov 2, 2021
3d4cc3b
Additional Fix
guiyanakuang Nov 3, 2021
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15 changes: 5 additions & 10 deletions c++/src/LzoDecompressor.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -312,13 +312,11 @@ namespace orc {
output += SIZE_OF_INT;
matchAddress += increment32;

*reinterpret_cast<int32_t*>(output) =
*reinterpret_cast<int32_t*>(matchAddress);
memcpy(output, matchAddress, SIZE_OF_INT);
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The combination of reinterpret_cast + assignment looks cheaper than memcpy function invocation. I'm wondering if we need to pay some performance penalty here.

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The combination of reinterpret_cast + assignment looks cheaper than memcpy function invocation. I'm wondering if we need to pay some performance penalty here.

I'll do some performance tests later, repeat_cast + assignment makes direct use of registers, memcpy is usually used for larger copies of data, I'm not sure if it's lossy yet

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The compiler may optimize the memcpy call. BTW, should we wrap a bit_cast function which uses memcpy before C++20 and uses the native one if C++20 is available?

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@wgtmac you are right, the compiler does optimize memcpy, the performance of both ways is similar in different compilers, in older versions of the compiler expand the assignment will be faster.
I agree to wrap a bit_cast function for binary copy between different types.

@dongjoon-hyun, So I don't think there's any performance loss here compared to the original.

WX20211104-104627
WX20211104-105010
WX20211104-105348

#include <string.h>

static void use_memcpy(benchmark::State& state) {
  auto size = state.range(0);
  char buf[size];
  for (int i = 0; i < 8; ++i) {
    buf[i] = 'a';
  }
  for (auto _ : state) {
    char *output = buf + 8;
    char *matchAddress = buf;
    char *matchOutputLimit = buf + size;
    while (output < matchOutputLimit) {
      memcpy(output, matchAddress, 8);
      matchAddress += 8;
      output += 8;
    }
  }
}

static void use_expanded_assignment(benchmark::State& state) {
  auto size = state.range(0);
  char buf[size];
  for (int i = 0; i < 8; ++i) {
    buf[i] = 'a';
  }
  for (auto _ : state) {
    char *output = buf + 8;
    char *matchAddress = buf;
    char *matchOutputLimit = buf + size;
    while (output < matchOutputLimit) {
      output[0] = *matchOutputLimit;
      output[1] = *(matchOutputLimit + 1);
      output[2] = *(matchOutputLimit + 2);
      output[3] = *(matchOutputLimit + 3);
      output[4] = *(matchOutputLimit + 4);
      output[5] = *(matchOutputLimit + 5);
      output[6] = *(matchOutputLimit + 6);
      output[7] = *(matchOutputLimit + 7);
      matchAddress += 8;
      output += 8;
    }
  }
}

static void use_reinterpret_assignment(benchmark::State& state) {
  auto size = state.range(0);
  char buf[size];
  for (int i = 0; i < 8; ++i) {
    buf[i] = 'a';
  }
  for (auto _ : state) {
    char *output = buf + 8;
    char *matchAddress = buf;
    char *matchOutputLimit = buf + size;
    while (output < matchOutputLimit) {
      *reinterpret_cast<int64_t*>(output) =
                *reinterpret_cast<int64_t*>(matchAddress);
      matchAddress += 8;
      output += 8;
    }
  }
}

BENCHMARK(use_memcpy)->Arg(100000);

BENCHMARK(use_expanded_assignment)->Arg(100000);

BENCHMARK(use_reinterpret_assignment)->Arg(100000);

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Thanks. Could you put this investigation result into the PR description?

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No problem, I've updated the pr description

output += SIZE_OF_INT;
matchAddress -= decrement64;
} else {
*reinterpret_cast<int64_t*>(output) =
*reinterpret_cast<int64_t*>(matchAddress);
memcpy(output, matchAddress, SIZE_OF_LONG);
matchAddress += SIZE_OF_LONG;
output += SIZE_OF_LONG;
}
Expand All @@ -329,8 +327,7 @@ namespace orc {
}

while (output < fastOutputLimit) {
*reinterpret_cast<int64_t*>(output) =
*reinterpret_cast<int64_t*>(matchAddress);
memcpy(output, matchAddress, SIZE_OF_LONG);
matchAddress += SIZE_OF_LONG;
output += SIZE_OF_LONG;
}
Expand All @@ -340,8 +337,7 @@ namespace orc {
}
} else {
while (output < matchOutputLimit) {
*reinterpret_cast<int64_t*>(output) =
*reinterpret_cast<int64_t*>(matchAddress);
memcpy(output, matchAddress, SIZE_OF_LONG);
matchAddress += SIZE_OF_LONG;
output += SIZE_OF_LONG;
}
Expand All @@ -366,8 +362,7 @@ namespace orc {
// fast copy. We may over-copy but there's enough room in input
// and output to not overrun them
do {
*reinterpret_cast<int64_t*>(output) =
*reinterpret_cast<const int64_t*>(input);
memcpy(output, input, SIZE_OF_LONG);
input += SIZE_OF_LONG;
output += SIZE_OF_LONG;
} while (output < literalOutputLimit);
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