overhead of eBPF JIT

Lets try to estimate overhead of JIT compiler

I wrote simple perl script - it just counts redundant bytes for several cases:

• pair mov reg, rbp/add reg, imm (total length 7 bytes) can be replaced with lea reg, [rbp-imm] which is only 4 bytes
• pair mov reg, imm/add reg,imm can be replaced with just loading of right address so second instruction can be removed
• add reg, 1/sub reg, 1 (length 4 bytes) can be replaced to inc/dec reg (which has length 3 bytes)

etc etc

Results

jitted eBPF code

I add yesterday disasm for jitted eBPF code. To put it mildly this code is very poor

Every function has 7 bytes of nops in prolog. Comment says that this is for BPF trampoline - well, ok

Lots of code like

 mov eax, 0x1

 cmp r14, 0x2

 jnz 0xc0561497

 xor eax, eax

0xc0561497:

 ...

Somebody - tell them about cmovXX instructions

Lots of code like

mov rdi, 0xffff8fd687f3e000

add rdi, 0x110

and related to get addresses of stack var:

mov rdi, rbp

add rdi, 0xffffffffffffffe0 

Perhaps it would be preferable to use lea rdi, [rbp-XX]

Slow inc/dec:

add r8, 0x1

sub rdi, 0x1

Lots of repeated instructions:
and rdi, 0xfff

and rdi, 0xfff

it's obvious bug

And finally