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

total: 105374 odd 4528 4.3%

other samples shows similar overhead - between 4.3 and 5.2%

of course lots of code like

 mov eax, 0x1

 cmp r14, 0x2

 jnz 0xc05674ab

 xor eax, eax

c05674ab:

 ...

 leave

 ret

can be replaced with something like:
xor eax, eax

cmp r14, 0x2

setnz al

but it matters only in big IP filters

Unfortunately that's not all - we can see lots of repeated code like

mov [r13+0x58], bl

mov [r13+0x57], bl

...

mov [r13+0x39], bl

but this is big questions to LLVM eBPF backend