[Bug ipa/100220] New: missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2)

[Bug ipa/100220] New: missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2)

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100220

            Bug ID: 100220
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O1, -Os, -O2)
           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: ---

[638] % 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/12.0.0/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 12.0.0 20210422 (experimental) [master revision
3cf04d1afa8:0e51007a40c:d42088e453042f4f8ba9190a7e29efd937ea2181] (GCC) 
[639] % 
[639] % gcctk -O1 -S -o O1.s small.c
[640] % gcctk -O3 -S -o O3.s small.c
[641] % 
[641] % wc O1.s O3.s
  62  135  857 O1.s
  93  200 1337 O3.s
 155  335 2194 total
[642] % 
[642] % grep foo O1.s
[643] % grep foo O3.s
        call    foo
[644] % 
[644] % cat small.c
extern void foo(void);
int b, c, d, e, *h;
static int *f = &e;
static int a() { return 1; }
static void g() {
  if (!*f)
    for (; 1; d++)
      ;
  foo();
}
static void i() {
  int j, l = 0, k[24] = {0}, *m[2] = {&k[4], &l}, n[27];
  h = n;
  if (a() & n[0])
    for (; c; c--)
      ;
  int p[8];
  h = p;
  p[0] && (h = &j);
  e = 0;
}
static void o() {
  int *q, **r = &q, ***s[1];
  s[0] = &r;
  i();
  g();
}
int main() {
  if (b)
    o();
  return 0;
}

[Bug ipa/100220] missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100220

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-09-25
                 CC|                            |hubicka at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
-O3 changes inlining just so slightly and not inlining as much any more;
    /* -O3 parameters.  */
    { OPT_LEVELS_3_PLUS, OPT__param_max_inline_insns_auto_, NULL, 30 },
    { OPT_LEVELS_3_PLUS, OPT__param_early_inlining_insns_, NULL, 14 },
    { OPT_LEVELS_3_PLUS, OPT__param_inline_heuristics_hint_percent_, NULL, 600
},
    { OPT_LEVELS_3_PLUS, OPT__param_inline_min_speedup_, NULL, 15 },
    { OPT_LEVELS_3_PLUS, OPT__param_max_inline_insns_single_, NULL, 200 },

[Bug ipa/100220] missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2) (inlining differences)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100220

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Indeed similar interaction between inlining, static var const promotion and IPA
CP / inline heuristics

[Bug ipa/100220] missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2) (inlining differences)

[hubicka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100220

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |100314

--- Comment #3 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
Here the stack frame size of i is stimated to 244 bytes
void i ()
{
  int p[8];
  int n[27];
  int k[24];
  int l;
  int j;
  int _1;
  int _2;
  int _3;
  int c.2_4;
  int _5;

  <bb 2> [local count: 236223200]:
  l = 0;
  k = {};
  h = &n;
  _1 = n[0];
  _2 = _1 & 1;
  if (_2 != 0)
    goto <bb 8>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 8> [local count: 118111600]:
  goto <bb 4>; [100.00%]

  <bb 3> [local count: 955630225]:
  _3 = c.2_4 + -1;
  c = _3;

  <bb 4> [local count: 1073741824]:
  c.2_4 = c;
  if (c.2_4 != 0)
    goto <bb 3>; [89.00%]
  else
    goto <bb 5>; [11.00%]

  <bb 5> [local count: 236223200]:
  h = &p;
  _5 = p[0];
  if (_5 != 0)
    goto <bb 6>; [50.00%]
  else
    goto <bb 7>; [50.00%]

  <bb 6> [local count: 118111600]:
  h = &j;

  <bb 7> [local count: 236223200]:
  e = 0;
  j ={v} {CLOBBER};
  l ={v} {CLOBBER};
  k ={v} {CLOBBER};
  n ={v} {CLOBBER};
  p ={v} {CLOBBER};
  return;

}

so it indeed has larger arrays. k is initialized but never used (so it is
missed DSE). n is used in stupid way:

  h = &n;                                                                       
  _1 = n[0];                                                                    

where h is write only static var, but we do not know that during early opts (we
could try our luck and schedule one extra writeonly detection before early
optimization passes, but I am not sure it is worth).

I would say that main issue is also missed DSE


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100314
[Bug 100314] missed optimization for dead code elimination at -O3 (vs. -O1) 
(inlining differences due to missed dse)

[Bug ipa/100220] missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2) (inlining differences)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100220

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |needs-bisection
      Known to work|                            |13.1.0

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Seems to be fixed in GCC 13.1.0.

[Bug ipa/100220] missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2) (inlining differences)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100220
Bug 100220 depends on bug 100314, which changed state.

Bug 100314 Summary: missed optimization for dead code elimination at -O3 (vs. -O1)  (inlining differences due to missed dse)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100314

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