On Thu, Jan 9, 2014 at 3:00 PM, xmeng <xm...@cs.wisc.edu> wrote: > > 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?
I doubt it. Please file a bug report with a test case as described at http://gcc.gnu.org/bugs/ . Thanks. Ian
