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Comment by nynyny7 | Hacker News Reader

In all fairness it needs to be said that the libc's implementation has to consider portability to more "exotic" architectures. For example, not every CPU allows to make unaligned 32-bit or 64-bit writes, or it takes a huge penalty for such writes.

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What do I mean by "not every CPU allows to make unaligned 32-bit or 64-bit writes"? Let's test the code (as of commit eac67b6) on a Raspberry Pi 400:

  pi@rasppi400:~/memset_benchmark $ uname -a
  Linux rasppi400 5.10.63-v8+ #1459 SMP PREEMPT Wed Oct 6 16:42:49 BST 2021 aarch64 GNU/Linux
  
  pi@rasppi400:~/memset_benchmark $ ./bench_memset
  size, alignment, offset, libc, local
  0, 16, 0, 1237452, 834116, 1.483549,
  1, 16, 0, 1612697, 945325, 1.705971,
  2, 16, 0, 1779538, 945320, 1.882472,
  3, 16, 0, 1557081, 945324, 1.647140,
  4, 16, 0, 1779527, 889736, 2.000062,
  5, 16, 0, 1557103, 1000940, 1.555641,
  6, 16, 0, 1779551, 1000944, 1.777873,
  7, 16, 0, 1557111, 1000945, 1.555641,
  8, 16, 0, 1334654, 889723, 1.500078,
  Bus error
  
  pi@rasppi400:~/memset_benchmark $ gdb ./bench_memset
  [...]
  (gdb) run
  Starting program: /home/pi/memset_benchmark/bench_memset
  size, alignment, offset, libc, local
  0, 16, 0, 1557105, 722928, 2.153887,
  1, 16, 0, 1557103, 889797, 1.749953,
  2, 16, 0, 1557107, 889849, 1.749855,
  3, 16, 0, 1557108, 889759, 1.750033,
  4, 16, 0, 1557117, 889789, 1.749985,
  5, 16, 0, 1557110, 889745, 1.750063,
  6, 16, 0, 1557116, 889754, 1.750052,
  7, 16, 0, 1557110, 889758, 1.750038,
  8, 16, 0, 1557109, 889803, 1.749948,
  
  Program received signal SIGBUS, Bus error.
  
  small_memset (n=<optimized out>, c=<optimized out>, s=0x29690)
      at /home/pi/memset_benchmark/src/lib.c:33
  33          *((uint64_t *)last) = val8;

Is that 32 bit ARM code on 64 bit kernel? I thought ARM (since v6) allows unaligned access, although it might have to be emulated through the kernel which is going to be super-slow.

On SPARC you have no choice, align or die!

Yes it is 32 bit code on a 64 bit kernel. I didn't debug what the instruction is that ultimately causes the bus error.

  pi@rasppi400:~/memset_benchmark $ uname -a
  Linux rasppi400 5.10.63-v8+ #1459 SMP PREEMPT Wed Oct 6 16:42:49 BST 2021 aarch64 GNU/Linux
  pi@rasppi400:~/memset_benchmark $
  pi@rasppi400:~/memset_benchmark $ file ./bench_memset
  ./bench_memset: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 3.2.0, BuildID[sha1]=ebeb69b6cb9664d78c1256a2c862f3d28f11e15e, with debug_info, not stripped

PS: It's STRD, which as far as I understand the Arm Architecture Reference Manual always requires word alignment.

  Program received signal SIGBUS, Bus error.
  small_memset (n=<optimized out>, c=<optimized out>, s=0x29690)
      at /home/pi/memset_benchmark/src/lib.c:33
  33          *((uint64_t *)last) = val8;
  1: x/i $pc
  => 0x11c8c <local_memset+1560>: strd    r0, [r7, #-8]
  (gdb) info registers
  r0             0x0                 0
  r1             0x0                 0
  r2             0x0                 0
  r3             0x1475              5237
  r4             0x5f5e100           100000000
  r5             0x11674             71284
  r6             0x29690             169616
  r7             0x29699             169625

Can't look right now, but you might not have benchmarked against libc, but against an optimized version included in Raspbian (https://github.com/simonjhall/copies-and-fills). I'm not sure if that's still active in the latest Raspberry Pi OS releases.

In all fairness we need the fastest memset on every architecture. Whatever the cost of maintenance.

glibc has hand crafted assembler implementations of memcpy (often specialized for specific size ranges) for many architectures.

Does glibc not have feature detection and conditional compilation for cases like this? That is surprising to me.

It does. Each subdirectory of sysdeps/ can contain specific implementations per platform, arch, etc. eg: the aarch64 assembler memset is:

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/aar...

There's also the "ifunc" mechanism which can be used to make the choice at runtime, eg:

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/aar...

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