Hi,
Here is a strange code snippet in gcc.bin in version 4.7.0:
0000000000402e20 <_ZL28if_exists_else_spec_functioniPPKc>:
402e20: 31 c0 xor %eax,%eax
402e22: 83 ff 02 cmp $0x2,%edi
402e25: 75 11 jne 402e38
402e27: 53 push %rbx
402e28: 48 8b 3e mov (%rsi),%rdi
402e2b: 48 89 f3 mov %rsi,%rbx
402e2e: 80 3f 2f cmpb $0x2f,(%rdi)
402e31: 74 0d je 402e40
402e33: 48 8b 43 08 mov 0x8(%rbx),%rax
402e37: 5b pop %rbx
402e38: f3 c3 repz retq
402e3a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
402e40: be 04 00 00 00 mov $0x4,%esi
402e45: e8 3e fa ff ff callq 402888 <access@plt>
402e4a: 85 c0 test %eax,%eax
402e4c: 75 e5 jne 402e33
402e4e: 48 8b 03 mov (%rbx),%rax
402e51: 5b pop %rbx
402e52: eb e4 jmp 402e38
402e54: 66 66 66 2e 0f 1f 84 data32 data32 nopw
%cs:0x0(%rax,%rax,1)
402e5b: 00 00 00 00 00
The last instruction of this function is a two bytes jump "jmp 402e38".
It jumps to a two bytes return "repz retq". Why not just emit a two
bytes return at the end of the function, instead we jump to the return?
I actually find similar "jump to a return" snippets in every version
from 4.7.0 to 4.8.2, but I don't find any such case for 4.6 or prior.
Is there any reason for emitting such code snippets?
Thanks
--Xiaozhu
