Marcos Paulo de Souza@mpdesouza@floss.social
@ptesarik@infosec.exchange /me is grabbing popcorn and waiting for more info
Petr Tesařík@ptesarik@infosec.exchange
@mpdesouza@floss.social How's popcorn? Here's more info:
The issue is not really a difference between the two kernel versions, but the compiler used to build them. SLE15 SP6 kernels (6.4) are built with GCC7, but SLE16 kernels (6.12) are built with GCC13. I rebuilt the SLE15 SP6 kernel with GCC13, and performance drops by more than 4%!
GCC7 produces this:
Samples │
│ ffffffff81610ee0 :
│ mov %rdx,%rax
│ xor %ecx,%ecx
│ add %rsi,%rax
156 │ setb %cl
│ test %rax,%rax
│ ↓ js 26
│ test %rcx,%rcx
│ ↓ jne 26
932 │ stac
│ mov %rdx,%rcx
2092 │ rep movsb %ds:(%rsi),%es:(%rdi)
│ NOP3
2 │ mov %rcx,%rdx
1049 │ clac
│26: mov %edx,%eax
524 │ → ret
GCC13 produces this:
Samples │
│ ffffffff81606120 :
│ mov %rdx,%rax
│ mov %rdx,%rcx
│ xor %edx,%edx
78 │ add %rsi,%rax
│ setb %dl
│ test %rax,%rax
│ ↓ js 23
│ test %rdx,%rdx
│ ↓ jne 23
1059 │ stac
2651 │ rep movsb %ds:(%rsi),%es:(%rdi)
│ NOP3
1509 │ clac
│23: mov %ecx,%eax
779 │ → ret
However, it's not quite clear to me why the latter code is slower.
Cc: @gnutools@fosstodon.org
