FW: noreturn atribute and its optimization

[boris smidt <borissmidt@xxxxxxxxxxx>]

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. 


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