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

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

[sebastian.huber@embedded-brains.de]

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

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This is done intentionally, so that one gets e.g. usable backtraces from abort.

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

[sebastian.huber@embedded-brains.de]

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

--- Comment #2 from Sebastian Huber <sebastian.huber@embedded-brains.de> ---
(In reply to Jakub Jelinek from comment #1)
> This is done intentionally, so that one gets e.g. usable backtraces from
> abort.

Ok, the stack frame could be a feature.

The extra nop on SPARC hurts a bit in my case since I have to show that my
program has no unreachable code and here I get an unreachable nop:

f:
        save    %sp, -96, %sp
        call    u, 0
         nop
        nop <--------------- unreachable

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

[pinskia at gcc dot gnu.org]

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Is the nop due to alignment?

[Bug middle-end/99151] Missed optimization: Superfluous stack frame and code with noreturn or __builtin_unreachable()

[sebastian.huber@embedded-brains.de]

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

--- Comment #4 from Sebastian Huber <sebastian.huber@embedded-brains.de> ---
(In reply to Andrew Pinski from comment #3)
> Is the nop due to alignment?

With -g and -coverage I get this code:

sparc-rtems7-gcc -O2 -o - -S unreachable.c -fverbose-asm -g -coverage
        .file   "unreachable.c"
! GNU C17 (GCC) version 11.0.0 20210205 (RTEMS 7, RSB
61dcadee0825867ebe51f9f367430ef75b8fe9c0, Newlib d4a756f) (sparc-rtems7)
!       compiled by GNU C version 11.0.0 20201130 (experimental) [revision
b46314c78061a5156bac44a317c87d32b00d4295], GMP version 6.1.0, MPFR version
3.1.4, MPC version 1.0.3, isl version isl-0.18-GMP

! GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
! options passed: -mcpu=v7 -g -O2 -fprofile-arcs -ftest-coverage
        .section        ".text"
.LLtext0:
        .cfi_sections   .debug_frame
        .align 4
        .global g
        .type   g, #function
        .proc   020
g:
.LLFB0:
        .file 1 "unreachable.c"
        .loc 1 5 1 view -0
        .cfi_startproc
        save    %sp, -96, %sp   !
        .cfi_window_save
        .cfi_register 15, 31
        .cfi_def_cfa_register 30
        sethi   %hi(__gcov0.g), %g1     !, tmp112
        ldd     [%g1+%lo(__gcov0.g)], %i4       ! __gcov0.g[0], __gcov0.g[0]
        addcc   %i5, 1, %g3     ! __gcov0.g[0],, tmp115
        addx    %i4, 0, %g2     ! __gcov0.g[0],,
        .loc 1 6 9 view .LLVU1
        call    r, 0    !,
.LLVL0:
         std    %g2, [%g1+%lo(__gcov0.g)]       ! tmp115, __gcov0.g[0]
        nop

In the the no-return function case, this nop has no line debug information and
there is no profiling counter increment.

        .cfi_endproc
.LLFE0:
        .size   g, .-g
        .align 4
        .global f
        .type   f, #function
        .proc   020
f:
.LLFB1:
        .loc 1 11 1 view -0
        .cfi_startproc
        save    %sp, -96, %sp   !
        .cfi_window_save
        .cfi_register 15, 31
        .cfi_def_cfa_register 30
        sethi   %hi(__gcov0.f), %g1     !, tmp114
        ldd     [%g1+%lo(__gcov0.f)], %i2       ! __gcov0.f[0], __gcov0.f[0]
        addcc   %i3, 1, %g3     ! __gcov0.f[0],, tmp117
        addx    %i2, 0, %g2     ! __gcov0.f[0],,
        or      %g1, %lo(__gcov0.f), %i5        ! tmp114,, tmp113
        .loc 1 12 9 view .LLVU3
        call    u, 0    !,
.LLVL1:
         std    %g2, [%g1+%lo(__gcov0.f)]       ! tmp117, __gcov0.f[0]
        ldd     [%i5+8], %i2    ! __gcov0.f[1], __gcov0.f[1]
        addcc   %i3, 1, %g3     ! __gcov0.f[1],, tmp123
        addx    %i2, 0, %g2     ! __gcov0.f[1],,
        std     %g2, [%i5+8]    ! tmp123, __gcov0.f[1]
        .loc 1 13 9 view .LLVU4
        .cfi_endproc
.LLFE1:
        .size   f, .-f

In the __builtin_unreachable() case we don't have a nop, but a profiling
counter increment is there.

clang 9 generates this code:

clang -O2 -o - -S unreachable.c -fverbose-asm -g -coverage --target=sparc
        .text
        .file   "unreachable.c"
        .globl  g                       ! -- Begin function g
        .p2align        2
        .type   g,@function
g:                                      ! @g
.Lfunc_begin0:
        .file   1 "/home/EB/sebastian_h/src/rtems/unreachable.c"
        .loc    1 5 0                   ! unreachable.c:5:0
        .cfi_startproc
! %bb.0:
        save %sp, -96, %sp
        .cfi_def_cfa_register %fp
        .cfi_window_save
        .cfi_register 15, 31
.Ltmp0:
        .loc    1 6 9 prologue_end      ! unreachable.c:6:9
        sethi %hi(__llvm_gcov_ctr), %i0
        ldd [%i0+%lo(__llvm_gcov_ctr)], %i2
        addcc %i3, 1, %i5
        addxcc %i2, 0, %i4
        call r
        std %i4, [%i0+%lo(__llvm_gcov_ctr)]
.Ltmp1:
.Lfunc_end0:
        .size   g, .Lfunc_end0-g
        .cfi_endproc
                                        ! -- End function
        .globl  f                       ! -- Begin function f
        .p2align        2
        .type   f,@function
f:                                      ! @f
.Lfunc_begin1:
        .loc    1 11 0                  ! unreachable.c:11:0
        .cfi_startproc
! %bb.0:
        save %sp, -96, %sp
        .cfi_def_cfa_register %fp
        .cfi_window_save
        .cfi_register 15, 31
.Ltmp2:
        .loc    1 12 9 prologue_end     ! unreachable.c:12:9
        sethi %hi(__llvm_gcov_ctr.1), %i0
        ldd [%i0+%lo(__llvm_gcov_ctr.1)], %i2
        addcc %i3, 1, %i5
        addxcc %i2, 0, %i4
        call u
        std %i4, [%i0+%lo(__llvm_gcov_ctr.1)]
.Ltmp3:
.Lfunc_end1:
        .size   f, .Lfunc_end1-f
        .cfi_endproc
                                        ! -- End function

There are no nops and no unreachable profiling counter increments.

[Bug middle-end/99151] Missed optimization: Superfluous stack frame and code with noreturn or __builtin_unreachable()

[ebotcazou at gcc dot gnu.org]

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

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #5 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
static void
sparc_asm_function_epilogue (FILE *file)
{
  /* If the last two instructions of a function are "call foo; dslot;"
     the return address might point to the first instruction in the next
     function and we have to output a dummy nop for the sake of sane
     backtraces in such cases.  This is pointless for sibling calls since
     the return address is explicitly adjusted.  */

[Bug middle-end/99151] Missed optimization: Superfluous stack frame and code with noreturn or __builtin_unreachable()

[sebastian.huber@embedded-brains.de]

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

--- Comment #6 from Sebastian Huber <sebastian.huber@embedded-brains.de> ---
(In reply to Eric Botcazou from comment #5)
> static void
> sparc_asm_function_epilogue (FILE *file)
> {
>   /* If the last two instructions of a function are "call foo; dslot;"
>      the return address might point to the first instruction in the next
>      function and we have to output a dummy nop for the sake of sane
>      backtraces in such cases.  This is pointless for sibling calls since
>      the return address is explicitly adjusted.  */

This nop behaviour could be a bit inconsistent across architectures. For
example, arm and powerpc don't generate a nop here.

I still think that the profiling counter increment in the
__builtin_unreachable() path is a bug.

[Bug middle-end/99151] Missed optimization: Superfluous stack frame and code with noreturn or __builtin_unreachable()

[ebotcazou at gcc dot gnu.org]

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

--- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> This nop behaviour could be a bit inconsistent across architectures. For
> example, arm and powerpc don't generate a nop here.

Well, it's low-level trickery so architecture-dependent by definition.

> I still think that the profiling counter increment in the
> __builtin_unreachable() path is a bug.

How so?  I only see a missed optimization, but with -fprofile-arcs
-ftest-coverage you're splitting hairs IMO.

[Bug middle-end/99151] Missed optimization: Superfluous stack frame and code with noreturn or __builtin_unreachable()

[sebastian.huber@embedded-brains.de]

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

--- Comment #8 from Sebastian Huber <sebastian.huber@embedded-brains.de> ---
(In reply to Eric Botcazou from comment #7)
> > I still think that the profiling counter increment in the
> > __builtin_unreachable() path is a bug.
> 
> How so?  I only see a missed optimization, but with -fprofile-arcs
> -ftest-coverage you're splitting hairs IMO.

The __builtin_unreachable() explicitly mentions the use case with a function
which doesn't return in the documentation:

https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

I would expect from the compiler that it generates then similar code. Having a
profiling counter increment in one case and not in the other is not really a
nice behaviour.

[Bug middle-end/99151] Missed optimization: Superfluous stack frame and code with noreturn or __builtin_unreachable()

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
We certainly don't claim it is the exact same thing as using the noreturn
attribute, it is not.
We could in profile.c ignore edges that lead to basic blocks that start with
__builtin_unreachable () call as the first non-debug stmt after labels if it
was really important, but I'm not convinced it is.