A microbenchmark replicating some overhead I measured in an application that leverages std::variant across library boundaries, thus preventing inlining.
Results on my machine with a Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz,
compiled in RelWithDebInfo mode (i.e. -O2 -g):
using Var0 = uint64_t (Baseline / no std::variant):
Performance counter stats for './variant 0' (5 runs):
341,67 msec task-clock # 0,998 CPUs utilized ( +- 0,72% )
11 context-switches # 0,033 K/sec ( +- 30,59% )
1 cpu-migrations # 0,004 K/sec ( +- 31,18% )
382 page-faults # 0,001 M/sec ( +- 0,16% )
1.065.848.451 cycles # 3,120 GHz ( +- 0,36% ) (61,93%)
2.102.158.173 instructions # 1,97 insn per cycle ( +- 0,05% ) (74,61%)
656.675.208 branches # 1921,950 M/sec ( +- 0,14% ) (74,79%)
37.706 branch-misses # 0,01% of all branches ( +- 1,51% ) (75,27%)
524.873.150 L1-dcache-loads # 1536,193 M/sec ( +- 0,07% ) (75,43%)
16.486.841 L1-dcache-load-misses # 3,14% of all L1-dcache accesses ( +- 0,02% ) (75,43%)
68.399 LLC-loads # 0,200 M/sec ( +- 11,18% ) (49,78%)
4.058 LLC-load-misses # 5,93% of all LL-cache accesses ( +- 61,06% ) (49,30%)
0,34247 +- 0,00257 seconds time elapsed ( +- 0,75% )
using Var1 = std::variant<uint64_t>:
Performance counter stats for './variant 1' (5 runs):
2.491,14 msec task-clock # 0,999 CPUs utilized ( +- 0,10% )
38 context-switches # 0,015 K/sec ( +- 40,81% )
4 cpu-migrations # 0,001 K/sec ( +- 16,67% )
383 page-faults # 0,154 K/sec ( +- 0,30% )
7.773.391.551 cycles # 3,120 GHz ( +- 0,03% ) (62,42%)
4.073.914.612 instructions # 0,52 insn per cycle ( +- 0,00% ) (74,97%)
657.336.683 branches # 263,870 M/sec ( +- 0,01% ) (74,97%)
84.887 branch-misses # 0,01% of all branches ( +- 3,03% ) (74,97%)
1.706.806.939 L1-dcache-loads # 685,152 M/sec ( +- 0,01% ) (75,03%)
16.709.392 L1-dcache-load-misses # 0,98% of all L1-dcache accesses ( +- 0,03% ) (75,10%)
158.525 LLC-loads # 0,064 M/sec ( +- 3,40% ) (50,00%)
17.597 LLC-load-misses # 11,10% of all LL-cache accesses ( +- 9,53% ) (49,93%)
2,49302 +- 0,00260 seconds time elapsed ( +- 0,10% )
using Var2 = std::variant<uint64_t, uint32_t>:
Performance counter stats for './variant 2' (5 runs):
2.865,06 msec task-clock # 0,998 CPUs utilized ( +- 0,16% )
280 context-switches # 0,098 K/sec ( +- 60,75% )
14 cpu-migrations # 0,005 K/sec ( +- 27,83% )
381 page-faults # 0,133 K/sec ( +- 0,13% )
8.958.269.283 cycles # 3,127 GHz ( +- 0,03% ) (62,39%)
7.221.058.673 instructions # 0,81 insn per cycle ( +- 0,02% ) (74,93%)
1.051.224.728 branches # 366,912 M/sec ( +- 0,03% ) (74,97%)
117.336 branch-misses # 0,01% of all branches ( +- 15,68% ) (75,04%)
2.887.449.354 L1-dcache-loads # 1007,816 M/sec ( +- 0,01% ) (75,01%)
16.833.544 L1-dcache-load-misses # 0,58% of all L1-dcache accesses ( +- 0,39% ) (75,03%)
231.853 LLC-loads # 0,081 M/sec ( +- 15,34% ) (50,02%)
29.185 LLC-load-misses # 12,59% of all LL-cache accesses ( +- 18,40% ) (49,93%)
2,87072 +- 0,00276 seconds time elapsed ( +- 0,10% )
using Var0 = uint64_t (Baseline / no std::variant):
Performance counter stats for './variant 0' (5 runs):
346,50 msec task-clock # 0,997 CPUs utilized ( +- 1,38% )
34 context-switches # 0,097 K/sec ( +- 48,58% )
1 cpu-migrations # 0,002 K/sec ( +- 66,67% )
382 page-faults # 0,001 M/sec ( +- 0,24% )
1.079.167.246 cycles # 3,114 GHz ( +- 0,77% ) (62,33%)
2.103.531.907 instructions # 1,95 insn per cycle ( +- 0,11% ) (74,99%)
657.171.926 branches # 1896,579 M/sec ( +- 0,09% ) (74,95%)
42.756 branch-misses # 0,01% of all branches ( +- 3,25% ) (74,95%)
524.893.755 L1-dcache-loads # 1514,828 M/sec ( +- 0,06% ) (75,15%)
16.497.132 L1-dcache-load-misses # 3,14% of all L1-dcache accesses ( +- 0,12% ) (75,31%)
47.037 LLC-loads # 0,136 M/sec ( +- 8,09% ) (49,90%)
7.534 LLC-load-misses # 16,02% of all LL-cache accesses ( +- 35,78% ) (49,73%)
0,34770 +- 0,00450 seconds time elapsed ( +- 1,29% )
using Var1 = std::variant<uint64_t>:
Performance counter stats for './variant 1' (5 runs):
395,06 msec task-clock # 0,991 CPUs utilized ( +- 0,50% )
58 context-switches # 0,146 K/sec ( +- 25,40% )
3 cpu-migrations # 0,007 K/sec ( +- 13,36% )
382 page-faults # 0,966 K/sec ( +- 0,18% )
1.237.260.633 cycles # 3,132 GHz ( +- 0,28% ) (61,76%)
2.891.221.196 instructions # 2,34 insn per cycle ( +- 0,12% ) (74,58%)
657.485.185 branches # 1664,250 M/sec ( +- 0,07% ) (74,90%)
45.695 branch-misses # 0,01% of all branches ( +- 5,31% ) (75,02%)
525.225.462 L1-dcache-loads # 1329,469 M/sec ( +- 0,08% ) (75,13%)
16.500.768 L1-dcache-load-misses # 3,14% of all L1-dcache accesses ( +- 0,05% ) (75,59%)
57.421 LLC-loads # 0,145 M/sec ( +- 9,26% ) (49,96%)
9.089 LLC-load-misses # 15,83% of all LL-cache accesses ( +- 30,70% ) (49,38%)
0,39866 +- 0,00262 seconds time elapsed ( +- 0,66% )
using Var2 = std::variant<uint64_t, uint32_t>:
Performance counter stats for './variant 2' (5 runs):
638,65 msec task-clock # 0,996 CPUs utilized ( +- 0,85% )
45 context-switches # 0,071 K/sec ( +- 63,14% )
4 cpu-migrations # 0,006 K/sec ( +- 69,82% )
382 page-faults # 0,598 K/sec ( +- 0,13% )
1.998.567.369 cycles # 3,129 GHz ( +- 0,51% ) (62,16%)
6.297.649.870 instructions # 3,15 insn per cycle ( +- 0,05% ) (74,82%)
1.050.921.005 branches # 1645,530 M/sec ( +- 0,05% ) (75,07%)
49.234 branch-misses # 0,00% of all branches ( +- 11,14% ) (75,28%)
1.442.218.800 L1-dcache-loads # 2258,223 M/sec ( +- 0,04% ) (75,28%)
16.537.896 L1-dcache-load-misses # 1,15% of all L1-dcache accesses ( +- 0,10% ) (75,27%)
79.831 LLC-loads # 0,125 M/sec ( +- 8,38% ) (49,65%)
8.658 LLC-load-misses # 10,84% of all LL-cache accesses ( +- 52,74% ) (49,45%)
0,64147 +- 0,00657 seconds time elapsed ( +- 1,02% )
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