平均而言，现代 x64 CPU cmpxchg16b 比 64 位或 32 位 CPU 慢得多？

Question

我相信 Windows 已经在内部使用该指令很长时间了，所以 CPU 制造商会花精力去优化吗？

当然，假设适当对齐的内存和不共享缓存线等。

Answer 1

出于好奇，我写了一个小的基准进行比较的4和8字节的成本cmpxchg有cmpxchg16b：

#include <cstdint>
#include <benchmark/benchmark.h>

alignas(16) char input[16 * 1024] = {};

template<class T>
void do_benchmark(benchmark::State& state) {
    unsigned n = 0;
    T* p = reinterpret_cast<T*>(input);
    constexpr unsigned count = sizeof input / sizeof(T);
    unsigned i = 0;
    for(auto _ : state) {
        T v{0};
        n += __sync_bool_compare_and_swap(p + i++ % count, v, v);
    }
    benchmark::DoNotOptimize(n);
}

BENCHMARK_TEMPLATE(do_benchmark, std::int32_t);
BENCHMARK_TEMPLATE(do_benchmark, std::int64_t);
BENCHMARK_TEMPLATE(do_benchmark, __int128);
BENCHMARK_MAIN();

并在 Coffee Lake i9-9900KS CPU 上运行它。

结果与gcc-8.3.0：

$ make -rC ~/src/test -j8 BUILD=release run_cmpxchg16b_benchmark
g++ -o /home/max/src/test/release/gcc/cmpxchg16b_benchmark.o -c -pthread -march=native -std=gnu++17 -W{all,extra,error,no-{maybe-uninitialized,unused-function}} -g -fmessage-length=0 -O3 -mtune=native -ffast-math -falign-{functions,loops}=64 -DNDEBUG -mcx16 -MD -MP /home/max/src/test/cmpxchg16b_benchmark.cc
g++ -o /home/max/src/test/release/gcc/cmpxchg16b_benchmark -fuse-ld=gold -pthread -g /home/max/src/test/release/gcc/cmpxchg16b_benchmark.o -lrt /usr/local/lib/libbenchmark.a
sudo cpupower frequency-set --related --governor performance >/dev/null
/home/max/src/test/release/gcc/cmpxchg16b_benchmark
2020-03-15 20:18:48
Running /home/max/src/test/release/gcc/cmpxchg16b_benchmark
Run on (16 X 5100 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x8)
  L1 Instruction 32 KiB (x8)
  L2 Unified 256 KiB (x8)
  L3 Unified 16384 KiB (x1)
Load Average: 0.43, 0.40, 0.34
---------------------------------------------------------------------
Benchmark                           Time             CPU   Iterations
---------------------------------------------------------------------
do_benchmark<std::int32_t>       3.53 ns         3.53 ns    198281069
do_benchmark<std::int64_t>       3.53 ns         3.53 ns    198256710
do_benchmark<__int128>           6.35 ns         6.35 ns    110215116

结果与clang-8.0.0：

$ make -rC ~/src/test -j8 BUILD=release TOOLSET=clang run_cmpxchg16b_benchmark
clang++ -o /home/max/src/test/release/clang/cmpxchg16b_benchmark.o -c -pthread -march=native -std=gnu++17 -W{all,extra,error,no-unused-function} -g -fmessage-length=0 -O3 -mtune=native -ffast-math -falign-functions=64 -DNDEBUG -mcx16 -MD -MP /home/max/src/test/cmpxchg16b_benchmark.cc
clang++ -o /home/max/src/test/release/clang/cmpxchg16b_benchmark -fuse-ld=gold -pthread -g /home/max/src/test/release/clang/cmpxchg16b_benchmark.o -lrt /usr/local/lib/libbenchmark.a
sudo cpupower frequency-set --related --governor performance >/dev/null
/home/max/src/test/release/clang/cmpxchg16b_benchmark
2020-03-15 20:19:00
Running /home/max/src/test/release/clang/cmpxchg16b_benchmark
Run on (16 X 5100 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x8)
  L1 Instruction 32 KiB (x8)
  L2 Unified 256 KiB (x8)
  L3 Unified 16384 KiB (x1)
Load Average: 0.36, 0.39, 0.33
---------------------------------------------------------------------
Benchmark                           Time             CPU   Iterations
---------------------------------------------------------------------
do_benchmark<std::int32_t>       3.84 ns         3.84 ns    182461520
do_benchmark<std::int64_t>       3.84 ns         3.84 ns    182160259
do_benchmark<__int128>           5.99 ns         5.99 ns    116972653

它看起来cmpxchg16b比cmpxchgIntel Coffee Lake 上的8 字节贵约 1.6-1.8 倍。

Ryzen 9 5950X 上的相同基准测试以及gcc-9.3.0：

Running /home/max/src/test/release/gcc/cmpxchg16b_benchmark
Run on (32 X 4889.51 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x16)
  L1 Instruction 32 KiB (x16)
  L2 Unified 512 KiB (x16)
  L3 Unified 32768 KiB (x2)
Load Average: 1.11, 0.52, 0.33
---------------------------------------------------------------------
Benchmark                           Time             CPU   Iterations
---------------------------------------------------------------------
do_benchmark<std::int32_t>       1.58 ns         1.58 ns    436624535
do_benchmark<std::int64_t>       1.58 ns         1.58 ns    443977862
do_benchmark<__int128>           2.22 ns         2.22 ns    316143309

cmpxchg16b比cmpxchgAMD Ryzen 9 上的8 字节贵约 1.4 倍。