On 11/01/2015 01:11 PM, boris smidt wrote:
Dear,
I have been playing around with the noreturn atribute and created the following program (i know this is not thr right way to write C):
#include <stdio.h>
static void loop()__attribute__((noreturn,nothrow));
//int main()__attribute__((noreturn,nothrow));
static void loop(int a)
{
int b= a+1;
printf("Hello World %u!\n",b);
loop(b);
}
int main()
{
int b= 1;
printf("Hello World %u!\n",b);
loop(b);
}
I would have expected this program to run forever. But when i looked at the generated assembly code with -O3 it calls the function loop and allocates stack in the loop function and thus crashes.
But i wonder why this function doesn't clear/reset the stack, since it is a "no return function" and thus can never return to older stack references.
The noreturn attribute is an indication to the caller that the callee
does not return, and thus it can optimize the caller accordingly, and
avoid warnings about missing return values in the caller.
It does not direct the compiler to optimize the callee, and even if it
did, with optimizations disabled, it wouldn't.
When i compiled the program it with -O3, without the atributes, it generates the expected result. i.e. a jump to the function label and no allocating on the stack.
Here is the generated assembly:
with atrributes
.file "main.c"
.section .rdata,"dr"
.LC0:
.ascii "Hello World %u!\12\0"
.text
.p2align 4,,15
.def loop; .scl 3; .type 32; .endef
.seh_proc loop
loop:
pushq %rbx
.seh_pushreg %rbx
subq $32, %rsp
.seh_stackalloc 32
.seh_endprologue
leal 1(%rcx), %ebx
leaq .LC0(%rip), %rcx
movl %ebx, %edx
call printf
movl %ebx, %ecx
call loop
nop
.seh_endproc
.def __main; .scl 2; .type 32; .endef
.section .text.startup,"x"
.p2align 4,,15
.globl main
.def main; .scl 2; .type 32; .endef
.seh_proc main
main:
subq $40, %rsp
.seh_stackalloc 40
.seh_endprologue
call __main
leaq .LC0(%rip), %rcx
movl $1, %edx
call printf
movl $1, %ecx
call loop
nop
.seh_endproc
.def printf; .scl 2; .type 32; .endef
without attributs
.file "main.c"
.def __main; .scl 2; .type 32; .endef
.section .rdata,"dr"
.LC0:
.ascii "Hello World %u!\12\0"
.section .text.startup,"x"
.p2align 4,,15
.globl main
.def main; .scl 2; .type 32; .endef
.seh_proc main
main:
pushq %rbx
.seh_pushreg %rbx
subq $32, %rsp
.seh_stackalloc 32
.seh_endprologue
movl $1, %ebx
call __main
leaq .LC0(%rip), %rcx
movl $1, %edx
call printf
.p2align 4,,10
.L2:
addl $1, %ebx
leaq .LC0(%rip), %rcx
movl %ebx, %edx
call printf
jmp .L2
.seh_endproc
.def printf; .scl 2; .type 32; .endef
