[Bug ipa/99373] New: [missed optimization] dead code not eliminated from short circuiting

[Bug ipa/99373] New: [missed optimization] dead code not eliminated from short circuiting

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99373

            Bug ID: 99373
           Summary: [missed optimization] dead code not eliminated from
                    short circuiting
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhendong.su at inf dot ethz.ch
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

[538] % gcctk -v
Using built-in specs.
COLLECT_GCC=gcctk
COLLECT_LTO_WRAPPER=/local/suz-local/software/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/11.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--prefix=/local/suz-local/software/local/gcc-trunk --enable-languages=c,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.1 20210303 (experimental) [master revision
d97a92dca90:bc15d73426f:9b2084db9f9917eb9b19b1eb5ec03cdcb05f349e] (GCC) 
[539] % 
[539] % gcctk -O3 -S -o small.s small.c
[540] % 
[540] % cat small.c
extern void foo();

struct a {
  int b;
  long c;
} g;
static int d = 1;
int e, *f, *h;
long *i;

static long j() {
  struct a k;
  struct a *m = &g;
  struct a **n[] = {&m};
  long **l = 0;
  int o[42];
  foo();
  int p[] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
  int *d[] = {&p[3]};
  i = &k.c;
  long ***a[] = {&l};
  int b[5];
  f = &b[3];
  if (e)
    while (1)
      h = &o[7];
  return 0;
}
int main() {
  d || j();
  return 0;
}
[541] % 
[541] % cat small.s
        .file   "small.c"
        .text
        .p2align 4
        .type   j.isra.0, @function
j.isra.0:
.LFB2:
        .cfi_startproc
        subq    $56, %rsp
        .cfi_def_cfa_offset 64
        xorl    %eax, %eax
        call    foo
        leaq    8(%rsp), %rax
        movq    %rax, i(%rip)
        leaq    28(%rsp), %rax
        movq    %rax, f(%rip)
        movl    e(%rip), %eax
        testl   %eax, %eax
        jne     .L3
        addq    $56, %rsp
        .cfi_remember_state
        .cfi_def_cfa_offset 8
        ret
.L3:
        .cfi_restore_state
        jmp     .L3
        .cfi_endproc
.LFE2:
        .size   j.isra.0, .-j.isra.0
        .section        .text.startup,"ax",@progbits
        .p2align 4
        .globl  main
        .type   main, @function
main:
.LFB1:
        .cfi_startproc
        xorl    %eax, %eax
        ret
        .cfi_endproc
.LFE1:
        .size   main, .-main
        .globl  i
        .bss
        .align 8
        .type   i, @object
        .size   i, 8
i:
        .zero   8
        .globl  h
        .align 8
        .type   h, @object
        .size   h, 8
h:
        .zero   8
        .globl  f
        .align 8
        .type   f, @object
        .size   f, 8
f:
        .zero   8
        .globl  e
        .align 4
        .type   e, @object
        .size   e, 4
e:
        .zero   4
        .globl  g
        .align 16
        .type   g, @object
        .size   g, 16
g:
        .zero   16
        .ident  "GCC: (GNU) 11.0.1 20210303 (experimental) [master revision
d97a92dca90:bc15d73426f:9b2084db9f9917eb9b19b1eb5ec03cdcb05f349e]"
        .section        .note.GNU-stack,"",@progbits

[Bug ipa/99373] unused static function not being removed in some cases after optimization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99373

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
            Summary|[missed optimization] dead  |unused static function not
                   |code not eliminated from    |being removed in some cases
                   |short circuiting            |after optimization
           Keywords|                            |missed-optimization

[Bug ipa/99373] unused static function not being removed in some cases after optimization

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99373

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-03-04
                 CC|                            |hubicka at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is only IPA reference promotes 'd' constant and thus only late
optimization elides the call to 'j'.  That's too late to eliminate the
function.
Note we process 'j' first duing late opts (to make the late local IPA
pure-const
useful).

We'd need another IPA phase before RTL expansion to collect unreachable
functions again (IIRC the original parallel compilation GSoC project added
one).

I'm also quite sure we have a duplicate of this PR.

[Bug ipa/99373] unused static function not being removed in some cases after optimization

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99373

--- Comment #2 from Zhendong Su <zhendong.su at inf dot ethz.ch> ---
The following might be the same/related. The dead call is eliminated if b is
changed to:

static int b(int *c) {
    foo();
    d++;
}

[623] % gcctk -O3 -S small.c
[624] %
[624] % cat small.c
extern void foo(void);

static int a, d;

static int b(int *c) {
  while (1) {
    foo();
    d++;
  }
}

int main () {
  a && b(&a);
  return 0;
}
[625] %
[625] % cat small.s
        .file   "small.c"
        .text
        .section        .text.startup,"ax",@progbits
        .p2align 4
        .globl  main
        .type   main, @function
main:
.LFB1:  
        .cfi_startproc
        movl    a(%rip), %edx
        testl   %edx, %edx
        jne     .L7
        xorl    %eax, %eax
        ret
.L7:
        pushq   %rax
        .cfi_def_cfa_offset 16
.L3:
        call    foo
        addl    $1, d(%rip)
        jmp     .L3
        .cfi_endproc
.LFE1:  
        .size   main, .-main
        .local  d
        .comm   d,4,4
        .local  a
        .comm   a,4,4
        .ident  "GCC: (GNU) 11.0.1 20210304 (experimental) [master revision
cdfc2f6a6dc:ab3cea6cccd:f3641ac70eb0ae9f8983b7ddb1660c92439565de]"
        .section        .note.GNU-stack,"",@progbits

[Bug ipa/99373] unused static function not being removed in some cases after optimization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99373

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eric-bugs at omnifarious dot org

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 108361 has been marked as a duplicate of this bug. ***