Interesting. The highly optimized unrolled implementation of CRC32C on intel is about the same speed as a tight loop around ‘crc32q’. Both do like 30G/s on a 4KiB block with many iterations. Also compared to an older code of the tight loop with one extra instruction (probably causing register dependency with crc32q) is about 11G/s instead.

45d350:  ┌─> 48 8b 08                     movq   (%rax),%rcx
45d353:  │   f2 48 0f 38 f1 f1            crc32q %rcx┌%rsi
45d359:  │   48 83 c0 08                  addq   $0x8┌%rax
45d35d:  │   48 39 d0                     cmpq   %rdx┌%rax
45d360:  └── 75 ee                        jne    45d350 <crc32c_sse42+0x30>

vs:

455000:  ┌─> 48 8b 08                     movq   (%rax),%rcx
455003:  │   89 fe                        movl   %edi┌%esi
455005:  │   f2 48 0f 38 f1 f1            crc32q %rcx┌%rsi
45500b:  │   89 f7                        movl   %esi┌%edi
45500d:  │   48 83 c0 08                  addq   $0x8┌%rax
455011:  │   48 39 d0                     cmpq   %rdx┌%rax
455014:  └── 75 ea                        jne    455000 <crc32c_intel+0x20>

The optimized linux starts at https://elixir.bootlin.com/linux/latest/source/lib/crc/x86/crc-pclmul-template.S . This also has the AVX versions that could be faster than plain ‘crc32’ instruction.