c++ - g++ 和 clang++ 上的 std::normal_distribution 性能差异

Question

#include <random>

int main() {
  std::vector<double> norms;
  norms.reserve(1000000);

  std::mt19937_64 mtEngine(42);
  std::normal_distribution<> nd;
  for (int i = 0; i != 1000000; ++i) {
    norms.push_back(nd(mtEngine));
  }
}

g++ -std=c++17 -O3（10.2.0 版）和clang++ -std=c++17 -O3（11.0.0 版）生成的二进制文件在性能上有显着差异。

$ time ./random_clang
./random_clang  0.11s user 0.00s system 99% cpu 0.113 total

$ time ./random_gcc
./random_gcc  0.03s user 0.00s system 99% cpu 0.032 total

以下是Compiler Explorer和valgrind --tool=callgrind.

./random_clang
--------------------------------------------------------------------------------
Ir          
--------------------------------------------------------------------------------
278,231,181  PROGRAM TOTALS

--------------------------------------------------------------------------------
Ir           file:function
--------------------------------------------------------------------------------
135,606,558  ???:double std::generate_canonical<double, 53ul, std::mersenne_twister_engine<unsigned long, 64ul, 312ul, 156ul, 31ul, 13043109905998158313ul, 29ul, 6148914691236517205ul, 17ul, 8202884508482404352ul, 37ul, 18444473444759240704ul, 43ul, 6364136223846793005ul> >(std::mersenne_twister_engine<unsigned long, 64ul, 312ul, 156ul, 31ul, 13043109905998158313ul, 29ul, 6148914691236517205ul, 17ul, 8202884508482404352ul, 37ul, 18444473444759240704ul, 43ul, 6364136223846793005ul>&) [/home/xxx/EffectiveCpp/test/random_clang]
 53,449,536  /build/glibc-eX1tMB/glibc-2.31/math/../sysdeps/x86_64/fpu/e_logl.S:__ieee754_logl [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
 32,096,514  ???:main [/home/xxx/EffectiveCpp/test/random_clang]
 27,997,376  /build/glibc-eX1tMB/glibc-2.31/math/w_logl_compat.c:logl [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
 22,905,902  /build/glibc-eX1tMB/glibc-2.31/math/../sysdeps/ieee754/dbl-64/e_log.c:__ieee754_log_fma [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
  2,500,000  /build/glibc-eX1tMB/glibc-2.31/math/./w_log_template.c:log@@GLIBC_2.29 [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
  1,000,000  ???:0x0000000004a322f0 [???]


./random_gcc
--------------------------------------------------------------------------------
Ir          
--------------------------------------------------------------------------------
125,607,194  PROGRAM TOTALS

--------------------------------------------------------------------------------
Ir          file:function
--------------------------------------------------------------------------------
75,746,682  ???:main [/home/xxx/EffectiveCpp/test/random_gcc]
22,905,902  /build/glibc-eX1tMB/glibc-2.31/math/../sysdeps/ieee754/dbl-64/e_log.c:__ieee754_log_fma [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
19,769,747  ???:std::mersenne_twister_engine<unsigned long, 64ul, 312ul, 156ul, 31ul, 13043109905998158313ul, 29ul, 6148914691236517205ul, 17ul, 8202884508482404352ul, 37ul, 18444473444759240704ul, 43ul, 6364136223846793005ul>::_M_gen_rand() [/home/xxx/EffectiveCpp/test/random_gcc]
 2,500,000  /build/glibc-eX1tMB/glibc-2.31/math/./w_log_template.c:log@@GLIBC_2.29 [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
 1,000,000  ???:0x00000000001090f0 [???]
 1,000,000  ???:0x0000000004a322f0 [???]
   916,425  /build/glibc-eX1tMB/glibc-2.31/elf/dl-lookup.c:_dl_lookup_symbol_x [/usr/lib/x86_64-linux-gnu/ld-2.31.so]
   544,815  /build/glibc-eX1tMB/glibc-2.31/elf/dl-lookup.c:do_lookup_x [/usr/lib/x86_64-linux-gnu/ld-2.31.so]

为什么clang++版本花这么多时间在调用std::generate_canonical？我见过有人声称g++内联更积极，但在我的情况下更改选项clang++并没有真正帮助（-mllvm -inline-threshold=10000）。

这是一个错误还是我错过了一些其他重要的编译器选项？我知道还有其他方法可以更快地生成正态分布的随机变量，但我认为常用标准库函数的这种速度不一致是不正常的。

更新：似乎在我将clang++版本链接到libc++with-stdlib=libc++ -lc++abi后，性能与原始g++版本相当。

$ time ./random_perf
./random_perf  0.03s user 0.00s system 98% cpu 0.027 total


./random_perf
--------------------------------------------------------------------------------
Ir          
--------------------------------------------------------------------------------
147,608,621  PROGRAM TOTALS

--------------------------------------------------------------------------------
Ir           file:function
--------------------------------------------------------------------------------
106,311,924  /usr/lib/llvm-10/bin/../include/c++/v1/random:double std::__1::normal_distribution<double>::operator()<std::__1::mersenne_twister_engine<unsigned long, 64ul, 312ul, 156ul, 31ul, 13043109905998158313ul, 29ul, 6148914691236517205ul, 17ul, 8202884508482404352ul, 37ul, 18444473444759240704ul, 43ul, 6364136223846793005ul> >(std::__1::mersenne_twister_engine<unsigned long, 64ul, 312ul, 156ul, 31ul, 13043109905998158313ul, 29ul, 6148914691236517205ul, 17ul, 8202884508482404352ul, 37ul, 18444473444759240704ul, 43ul, 6364136223846793005ul>&, std::__1::normal_distribution<double>::param_type const&) [/home/xxx/EffectiveCpp/bin/random_perf]
 22,905,902  /build/glibc-eX1tMB/glibc-2.31/math/../sysdeps/ieee754/dbl-64/e_log.c:__ieee754_log_fma [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
  6,000,007  /usr/lib/llvm-10/bin/../include/c++/v1/vector:main
  3,003,122  /usr/lib/llvm-10/bin/../include/c++/v1/random:main
  3,000,016  /home/xxx/EffectiveCpp/src/random_perf.cpp:main [/home/xxx/EffectiveCpp/bin/random_perf]
  2,500,000  /build/glibc-eX1tMB/glibc-2.31/math/./w_log_template.c:log@@GLIBC_2.29 [/usr/lib/x86_64-linux-gnu/libm-2.31.so]
  1,000,002  /usr/lib/llvm-10/bin/../include/c++/v1/memory:main
  1,000,000  ???:0x000000000494b2f0 [???]
    507,749  /build/glibc-eX1tMB/glibc-2.31/elf/dl-lookup.c:_dl_lookup_symbol_x [/usr/lib/x86_64-linux-gnu/ld-2.31.so]

标签： c++