[Bug tree-optimization/110080] New: [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[Bug tree-optimization/110080] New: [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[theodort at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

            Bug ID: 110080
           Summary: [13/14 Regression] Missed Dead Code Elimination at -Os
                    when using __builtin_unreachable since
                    r13-6945-g429a7a88438
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: theodort at inf dot ethz.ch
  Target Milestone: ---

Given: 

void foo(void);
static unsigned char a = 131;
static int *b;
static int **c = &b;
static void d(int e, unsigned f) {
    int *g;
    if (e){
        for (; a; ++a)
            for (e = 0; 0;)
                ;
        g = &e;
        int **h = &g;
        if (**h) {
            foo();
        }
    }
    *c = &e;
}
int main() { d(4 & a, a); }


gcc trunk/13.1/12.3 at -Os generate:

main:
        testb   $4, a(%rip)
        je      .L3
        movb    $0, a(%rip)
.L3:
        leaq    -4(%rsp), %rax
        movq    %rax, b(%rip)
        xorl    %eax, %eax
        ret
a:
        .byte   -125

If I include a __builtin_unreachable() to help the compiler:

void foo(void);
static unsigned char a = 131;
static int *b;
static int **c = &b;
static void d(int e, unsigned f) {
    int *g;
    if (f != 131) {
        __builtin_unreachable(); // <- THIS
    }
    if (!e){
        for (; a; ++a)
            for (e = 0; 0;)
                ;
        g = &e;
        int **h = &g;
        if (**h) {
            foo();
        }
    }
    *c = &e;
}
int main() { d(4 & a, a); }

gcc-12.3 at -Os generates better code:

main:
        leaq    -4(%rsp), %rax
        movb    $0, a(%rip)
        movq    %rax, b(%rip)
        xorl    %eax, %eax
        ret
a:
        .byte   -125

But gcc-13.1/trunk at -Os generate worse code:

main:
        subq    $24, %rsp
        movb    a(%rip), %al
        movl    %eax, %edx
        andl    $4, %edx
        movl    %edx, 12(%rsp)
        xorl    %edx, %edx
.L2:
        testb   %al, %al
        je      .L8
        incl    %eax
        movb    $1, %dl
        jmp     .L2
.L8:
        testb   %dl, %dl
        je      .L4
        movb    $0, a(%rip)
        jmp     .L5
.L4:
        cmpl    $0, 12(%rsp)
        je      .L5
        call    foo
.L5:
        leaq    12(%rsp), %rax
        movq    %rax, b(%rip)
        xorl    %eax, %eax
        addq    $24, %rsp
        ret
a:
        .byte   -125

https://godbolt.org/z/zvqshjEj3


Started with r13-6945-g429a7a88438

[Bug tree-optimization/110080] [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[theodort at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

--- Comment #1 from Theodoros Theodoridis <theodort at inf dot ethz.ch> ---
Oops, the first code snippet is wrong in the original post: 

void foo(void);
static unsigned char a = 131;
static int *b;
static int **c = &b;
static void d(int e, unsigned f) {
    int *g;
    if (!e){ // <------------------------ if (!e) is correct
        for (; a; ++a)
            for (e = 0; 0;)
                ;
        g = &e;
        int **h = &g;
        if (**h) {
            foo();
        }
    }
    *c = &e;
}
int main() { d(4 & a, a); }

this is also the correct assembly for gcc trunk/13.1/12.3 at -Os:

main:
        movb    a(%rip), %al
        testb   $4, %al
        jne     .L5
        xorl    %edx, %edx
.L2:
        testb   %al, %al
        je      .L12
        incl    %eax
        movb    $1, %dl
        jmp     .L2
.L12:
        testb   %dl, %dl
        je      .L5
        movb    $0, a(%rip)
.L5:
        leaq    -4(%rsp), %rax
        movq    %rax, b(%rip)
        xorl    %eax, %eax
        ret
a:
        .byte   -125


the code with the _builtin_unreachable and the rest are correct

[Bug tree-optimization/110080] [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.2
           Keywords|                            |missed-optimization

[Bug tree-optimization/110080] [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.2                        |13.3

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 13.2 is being released, retargeting bugs to GCC 13.3.

[Bug tree-optimization/110080] [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
  a.0_1 = aD.2830;
  # RANGE [irange] unsigned int [0, 255] MASK 0xff VALUE 0x0
  _2 = (unsigned intD.9) a.0_1;
  # RANGE [irange] unsigned char [0, 0][4, 4] MASK 0x4 VALUE 0x0
  _6 = a.0_1 & 4;
  # RANGE [irange] int [0, 0][4, 4] MASK 0x4 VALUE 0x0
  _3 = (intD.6) _6;

Those ranges should have been:
a.0_1 [131,131]
_2 [131,131]
_6 [0,0]
_3 [0,0]

Like we got from the (not) unreachable branch:
=========== BB 4 ============
Imports: a.0_1  _3  
Exports: a.0_1  _3  _6  
a.0_1   [irange] unsigned char [131, 131]
_3      [irange] int [0, 0] MASK 0x4 VALUE 0x0


The store to e was not 131 because it was before the unreachable though:
  e = _3;
  if (a.0_1 != 131)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

  <bb 3> [count: 0]:
  __builtin_unreachable ();

[Bug tree-optimization/110080] [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-08-21

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug tree-optimization/110080] [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Andrew Macleod <amacleod@gcc.gnu.org>:

https://gcc.gnu.org/g:bf6b107e2a342319b3787ec960fc8014ef3aff91

commit r14-4141-gbf6b107e2a342319b3787ec960fc8014ef3aff91
Author: Andrew MacLeod <amacleod@redhat.com>
Date:   Wed Sep 13 11:52:15 2023 -0400

    New early __builtin_unreachable processing.

    in VRP passes before __builtin_unreachable MUST be removed, only remove it
    if all exports affected by the unreachable can have global values updated,
and
    do not involve loads from memory.

            PR tree-optimization/110080
            PR tree-optimization/110249
            gcc/
            * tree-vrp.cc (remove_unreachable::final_p): New.
            (remove_unreachable::maybe_register): Rename from
            maybe_register_block and call early or final routine.
            (fully_replaceable): New.
            (remove_unreachable::handle_early): New.
            (remove_unreachable::remove_and_update_globals): Remove
            non-final processing.
            (rvrp_folder::rvrp_folder): Add final flag to constructor.
            (rvrp_folder::post_fold_bb): Remove unreachable registration.
            (rvrp_folder::pre_fold_stmt): Move unreachable processing to here.
            (execute_ranger_vrp): Adjust some call parameters.

            gcc/testsuite/
            * g++.dg/pr110249.C: New.
            * gcc.dg/pr110080.c: New.
            * gcc.dg/pr93917.c: Adjust.

[Bug tree-optimization/110080] [13/14 Regression] Missed Dead Code Elimination at -Os when using __builtin_unreachable since r13-6945-g429a7a88438

[amacleod at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080

Andrew Macleod <amacleod at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from Andrew Macleod <amacleod at redhat dot com> ---
fixed.