libc 可以比 syscall 快吗？

问：

我正在努力大量优化一些代码（谢天谢地，没有平台独立性 - 仅限 Linux）。我创建了一个非常简单的框架来测量经过的时间（以时钟周期为单位）。我早期的想法是抓住用 Linux 系统调用替换 libc 函数的唾手可得的果实（重要提示：我一次最多打印一个字符，并且没有任何格式化）。然而，我的测试始终表明，它的速度大约是（时钟周期是用 64 位十六进制整数获得的，并且是 64 位十六进制整数;如果足够小，它们的差异显示为十进制）：putcharsyscallrdtsc

注意：测试不包含重复：以下代码仅通过执行一次来衡量。

syscall:

# %rdi contains a string
    movq $1 , %rax # system call 1: write
    movq %rdi, %rsi # what: char pointer
    movq $1 , %rdi # where: stdout
    movq $1 , %rdx # how many bytes: 1
    syscall

__________________________________
RDTSC   Post:   0x0002c22fbac1c7b5
RDTSC    Pre:   0x0002c22fbabf4d1f
Clock cycles:   0x0000000000027a96
Clock cycles:   162454

putchar:

# %rdi contains a string
    mov %rdi, %rsi
    xor %edi, %edi
    movb (%rsi), %dil
    call putchar

__________________________________
RDTSC   Post:   0x0002c221713726cd
RDTSC    Pre:   0x0002c2217136469f
Clock cycles:   0x000000000000e02e
Clock cycles:   57390

注意：即使我只包含一个结果，该方法的结果也始终慢约 ~2.5 倍。syscall

考虑到通过使用 GDB 逐步执行此函数可以看出其所有内部工作原理，这尤其奇怪：putchar

IO_validate_vtable (vtable=0x7ffff7e16600 <_IO_file_jumps>) at ./libio/libioP.h:943
943     ./libio/libioP.h: No such file or directory.
(gdb) 
__GI__IO_file_doallocate (fp=0x7ffff7e1a780 <_IO_2_1_stdout_>) at ./libio/libioP.h:947
947     in ./libio/libioP.h
(gdb) 
__GI__IO_file_stat (fp=0x7ffff7e1a780 <_IO_2_1_stdout_>, st=0x7fffffffde90) at ./libio/fileops.c:1146
1146    ./libio/fileops.c: No such file or directory.
(gdb) 
1147    in ./libio/fileops.c
(gdb) 
__GI___fstat64 (fd=1, buf=0x7fffffffde90) at ../sysdeps/unix/sysv/linux/fstat64.c:29
29      ../sysdeps/unix/sysv/linux/fstat64.c: No such file or directory.
(gdb) 
30      in ../sysdeps/unix/sysv/linux/fstat64.c
(gdb) 
35      in ../sysdeps/unix/sysv/linux/fstat64.c
(gdb) 
__GI___fstatat64 (fd=1, file=0x7ffff7dd846f "", buf=0x7fffffffde90, flag=4096) at ../sysdeps/unix/sysv/linux/fstatat64.c:153
153     ../sysdeps/unix/sysv/linux/fstatat64.c: No such file or directory.
(gdb) 
163     in ../sysdeps/unix/sysv/linux/fstatat64.c
(gdb) 
fstatat64_time64_stat (flag=4096, buf=0x7fffffffde90, file=0x7ffff7dd846f "", fd=1) at ../sysdeps/unix/sysv/linux/fstatat64.c:98
98      in ../sysdeps/unix/sysv/linux/fstatat64.c
(gdb) 
__GI___fstatat64 (fd=1, file=0x7ffff7dd846f "", buf=0x7fffffffde90, flag=4096) at ../sysdeps/unix/sysv/linux/fstatat64.c:166
166     in ../sysdeps/unix/sysv/linux/fstatat64.c
(gdb) 
__GI__IO_file_doallocate (fp=0x7ffff7e1a780 <_IO_2_1_stdout_>) at ./libio/filedoalloc.c:86
86      ./libio/filedoalloc.c: No such file or directory.
(gdb) 
91      in ./libio/filedoalloc.c
(gdb) 
__gnu_dev_major (__dev=34817) at ../include/sys/sysmacros.h:47
47      ../include/sys/sysmacros.h: No such file or directory.
(gdb) 
91      ./libio/filedoalloc.c: No such file or directory.
(gdb) 
94      in ./libio/filedoalloc.c
(gdb) 
97      in ./libio/filedoalloc.c
(gdb) 
101     in ./libio/filedoalloc.c
(gdb) 
__GI___libc_malloc (bytes=bytes@entry=1024) at ./malloc/malloc.c:3287
3287    ./malloc/malloc.c: No such file or directory.
(gdb) 
3294    in ./malloc/malloc.c
(gdb) 
3295    in ./malloc/malloc.c
(gdb) 
ptmalloc_init () at ./malloc/arena.c:315
315     ./malloc/arena.c: No such file or directory.
(gdb) 
ptmalloc_init () at ./malloc/arena.c:313
313     in ./malloc/arena.c
(gdb) 
321     in ./malloc/arena.c
(gdb) 
0x00007ffff7ca1a31 in tcache_key_initialize () at ./malloc/malloc.c:3162
3162    ./malloc/malloc.c: No such file or directory.
(gdb) 
318     ./malloc/arena.c: No such file or directory.
(gdb) 
321     in ./malloc/arena.c
(gdb) 
tcache_key_initialize () at ./malloc/malloc.c:3162
3162    ./malloc/malloc.c: No such file or directory.
(gdb) 
__GI___getrandom (buffer=buffer@entry=0x7ffff7e204d8 <tcache_key>, length=length@entry=8, flags=flags@entry=1) at ../sysdeps/unix/sysv/linux/getrandom.c:28