[Bug inline-asm/32811] Cannot flag inline assembler code "noreturn"

[adam at consulting dot net dot nz]

------- Comment #2 from adam at consulting dot net dot nz  2010-07-13 23:19 -------
This issue needs to be revisited with the advent of asm goto:

__attribute__ ((noreturn)) void noreturn_via_asm_goto() {
 loop:
  asm goto ("jmp %l[loop]\n" : : : : loop);
}

int main() {}

$ gcc-4.5 -std=gnu99 -O3 noreturn.c && objdump -d -m i386:x86-64:intel|less
noreturn.c: In function ‘noreturn_via_asm_goto’:
noreturn.c:4:1: warning: ‘noreturn’ function does return

00000000004004c0 <noreturn_via_asm_goto>:
  4004c0:       eb fe                   jmp    4004c0 <noreturn_via_asm_goto>
  4004c2:       c3                      ret    
  4004c3:       66 66 66 66 2e 0f 1f    nop    WORD PTR cs:[rax+rax*1+0x0]
  4004ca:       84 00 00 00 00 00 

The warning is spurious and the return instruction is not elided. This can
matter with threaded interpreters jumping to/from code with a native code
length equal to a multiple of 16 bytes. In the worse case this extra byte can
double the space consumed by hundreds of tiny functions.

Best approach would be to actually trust the programmer's use of the noreturn
explicit compiler directive in all circumstances. Do not emit a return
instruction when the compiler has been explicitly told that a particular
function does not return. By all means warn that it appears to the compiler
that the code path is going to drop through to the next address in memory.

Thanks,
Adam


-- 

adam at consulting dot net dot nz changed:

           What    |Removed                     |Added
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                 CC|                            |adam at consulting dot net
                   |                            |dot nz


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32811