[Bug c/99151] New: Missed optimization: Superfluous stack frame and code with noreturn or __builtin_unreachable()

["sebastian.huber@embedded-brains.de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99151

            Bug ID: 99151
           Summary: Missed optimization: Superfluous stack frame and code
                    with noreturn or __builtin_unreachable()
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sebastian.huber@embedded-brains.de
  Target Milestone: ---

The following test code:

_Noreturn void r(void);
void u(void);

void g(void)
{
        r();
}


void f(void)
{
        u();
        __builtin_unreachable();
}

Produces code the following code on a sample set of architectures. There should
be no stack frame set up. There should be no instructions after the no-return
function calls (for example sparc).

sparc-rtems6-gcc -O2 -o - -S unreachable.c 
        .file   "unreachable.c"
        .section        ".text"
        .align 4
        .global g
        .type   g, #function
        .proc   020
g:
        save    %sp, -96, %sp
        call    r, 0
         nop
        nop
        .size   g, .-g
        .align 4
        .global f
        .type   f, #function
        .proc   020
f:
        save    %sp, -96, %sp
        call    u, 0
         nop
        nop
        .size   f, .-f
        .ident  "GCC: (GNU) 10.2.1 20210205 (RTEMS 6, RSB
61dcadee0825867ebe51f9f367430ef75b8fe9c0, Newlib d4a756f)"

arm-rtems6-gcc -O2 -o - -S unreachable.c 
        .cpu arm7tdmi
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 2
        .eabi_attribute 34, 0
        .eabi_attribute 18, 4
        .file   "unreachable.c"
        .text
        .align  2
        .global g
        .arch armv4t
        .syntax unified
        .arm
        .fpu softvfp
        .type   g, %function
g:
        @ Function supports interworking.
        @ Volatile: function does not return.
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        push    {r4, lr}
        bl      r
        .size   g, .-g
        .align  2
        .global f
        .syntax unified
        .arm
        .fpu softvfp
        .type   f, %function
f:
        @ Function supports interworking.
        @ Volatile: function does not return.
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        push    {r4, lr}
        bl      u
        .size   f, .-f
        .ident  "GCC: (GNU) 10.2.1 20210205 (RTEMS 6, RSB
61dcadee0825867ebe51f9f367430ef75b8fe9c0, Newlib d4a756f)"
powerpc-rtems6-gcc -O2 -o - -S unreachable.c 
        .file   "unreachable.c"
        .machine ppc
        .section        ".text"
        .align 2
        .globl g
        .type   g, @function
g:
.LFB0:
        .cfi_startproc
        stwu 1,-16(1)
        .cfi_def_cfa_offset 16
        mflr 0
        stw 0,20(1)
        .cfi_offset 65, 4
        bl r
        .cfi_endproc
.LFE0:
        .size   g,.-g
        .align 2
        .globl f
        .type   f, @function
f:
.LFB1:
        .cfi_startproc
        stwu 1,-16(1)
        .cfi_def_cfa_offset 16
        mflr 0
        stw 0,20(1)
        .cfi_offset 65, 4
        bl u
        .cfi_endproc
.LFE1:
        .size   f,.-f
        .ident  "GCC: (GNU) 10.2.1 20210205 (RTEMS 6, RSB
61dcadee0825867ebe51f9f367430ef75b8fe9c0, Newlib d4a756f)"
        .section        .note.GNU-stack,"",@progbits