Benchmark passing structs by value

hash.cpp

#include "hash.hpp"

size_t rehash1_by_value(hash256 h) {
  size_t s = 0;
  for (auto c : h.bytes)
	s ^= c + 0x9e3779b9 + (s<<6) + (s>>2);
  return s;
}

size_t rehash1_by_ref(hash256 const& h) {
  size_t s = 0;
  for (auto c : h.bytes)
	s ^= c + 0x9e3779b9 + (s<<6) + (s>>2);
  return s;
}

size_t rehash2_by_value(hash256 h1, hash256 h2) {
  size_t s = 0;
  for (size_t i = 0; i < sizeof(h1.bytes); ++i) {
    s ^= h1.bytes[i] + 0x9e3779b9 + (s<<6) + (s>>2);
    s ^= h2.bytes[i] + 0x9e3779b9 + (s<<6) + (s>>2);
  }
  return s;
}

size_t rehash2_by_ref(hash256 const& h1, hash256 const& h2) {
  size_t s = 0;
  for (size_t i = 0; i < sizeof(h1.bytes); ++i) {
    s ^= h1.bytes[i] + 0x9e3779b9 + (s<<6) + (s>>2);
    s ^= h2.bytes[i] + 0x9e3779b9 + (s<<6) + (s>>2);
  }
  return s;
}

std::string makeCodeId(hash256 codeHash, int mode)
{
	static const auto hexChars = "0123456789abcdef";
	std::string str;
	str.reserve(sizeof(codeHash) * 2 + 1);
	for (auto b: codeHash.bytes)
	{
		str.push_back(hexChars[b & 0xf]);
		str.push_back(hexChars[b >> 4]);
	}
	str.push_back(mode == 0 ? 'F' : 'H');
	return str;
}

std::string makeCodeId_ref(hash256 const& codeHash, int mode)
{
	static const auto hexChars = "0123456789abcdef";
	std::string str;
	str.reserve(sizeof(codeHash) * 2 + 1);
	for (auto b: codeHash.bytes)
	{
		str.push_back(hexChars[b & 0xf]);
		str.push_back(hexChars[b >> 4]);
	}
	str.push_back(mode == 0 ? 'F' : 'H');
	return str;
}

hash.hpp

#include <string>

struct hash256 {
  char bytes[32];
};

size_t rehash1_by_value(hash256 h);

size_t rehash1_by_ref(hash256 const& h);

size_t rehash2_by_value(hash256 h1, hash256 h2);

size_t rehash2_by_ref(hash256 const& h1, hash256 const& h2);

std::string makeCodeId(hash256 codeHash, int mode);
std::string makeCodeId_ref(hash256 const& codeHash, int mode);

results

Run on (8 X 840.984 MHz CPU s)
2016-08-16 22:16:24
***WARNING*** CPU scaling is enabled, the benchmark real time measurements may be noisy and will incur extra overhead.
Benchmark                          Time           CPU Iterations
----------------------------------------------------------------
BM_hash1_by_value_mean            55 ns         55 ns   11888587
BM_hash1_by_value_stddev           0 ns          0 ns          0
BM_hash1_by_ref_mean              53 ns         53 ns   12500000
BM_hash1_by_ref_stddev             0 ns          0 ns          0
BM_hash2_by_value_mean           112 ns        112 ns    6034483
BM_hash2_by_value_stddev           1 ns          1 ns          0
BM_hash2_by_ref_mean             108 ns        108 ns    6481481
BM_hash2_by_ref_stddev             0 ns          0 ns          0
BM_codeId_by_value_mean          189 ns        189 ns    3645833
BM_codeId_by_value_stddev          2 ns          2 ns          0
BM_codeId_by_ref_mean            195 ns        195 ns    3645833
BM_codeId_by_ref_stddev            1 ns          1 ns          0