[Bug tree-optimization/106570] New: DCE sometimes fails with depending if statements

[Bug tree-optimization/106570] New: DCE sometimes fails with depending if statements

[tmayerl at student dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

            Bug ID: 106570
           Summary: DCE sometimes fails with depending if statements
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tmayerl at student dot ethz.ch
  Target Milestone: ---

Sometimes, DCE fails when multiple if statements are used. 

For example, GCC detects that the following if statements always evaluate to
false and thus removes the dead code:

#include <stdio.h>
#include <stdbool.h>

void DCEMarker0_();

void f(bool s, bool c) {
    if (!c == !s) {
        if (s && !c) {
            DCEMarker0_();
        }
    }
}

In the next snippet, the if statements are used to set a variable. This
variable is then used in the next if statement. However, GCC now fails to
detect and eliminate the dead code:

#include <stdio.h>
#include <stdbool.h>

void DCEMarker0_();

void f(bool s, bool c) {
    int intermediate_result = 0;
    if (!c == !s) {
        if (s && !c) {
            intermediate_result = 1;
        }
    }
    if (((!c == !s) && (s && !c)) || intermediate_result) {
        DCEMarker0_();
    }
}

This is actually a regression: It works fine until GCC 11.3.

This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/n9dKMfqsd

[Bug tree-optimization/106570] [12/13 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-08-09
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
            Summary|DCE sometimes fails with    |[12/13 Regression] DCE
                   |depending if statements     |sometimes fails with
                   |                            |depending if statements
                   |                            |since
                   |                            |r12-2305-g398572c1544d8b75
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |amacleod at redhat dot com,
                   |                            |marxin at gcc dot gnu.org

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
Started with r12-2305-g398572c1544d8b75.

[Bug tree-optimization/106570] [12/13 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[amacleod at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
I think this is a duplicate of PR106379 .   At the VRP2 stage I see:

  <bb 2> [local count: 1073741824]:
  if (c_6(D) == s_7(D))
    goto <bb 3>; [34.00%]
  else
    goto <bb 5>; [66.00%]

  <bb 3> [local count: 365072224]:
  _1 = ~c_6(D);
  _2 = _1 & s_7(D);
  if (_2 != 0)
    goto <bb 4>; [75.00%]
  else
    goto <bb 5>; [25.00%]

  <bb 4> [local count: 628138969]:
  DCEMarker0_ ();

  <bb 5> [local count: 1073741824]:
  return;

Which is basically the identical sequence.. it just took longer to get to it
:-)  We aren't removing this yet with ranger as I need to get to integrate
rangers relation oracle with the simplifier so that it will see that  _2 = ~s_7
& s_7.

[Bug tree-optimization/106570] [12/13 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.2

[Bug tree-optimization/106570] [12/13 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
           Priority|P3                          |P2

[Bug tree-optimization/106570] [12/13 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |needs-bisection
      Known to fail|                            |12.1.0
      Known to work|                            |11.1.0

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note on the trunk, now the first example fails at -O2 and not just -O3. It
might be useful to do a bisect there too.

The second example failed for -O2 for GCC 12 too.

[Bug tree-optimization/106570] [12/13 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-bisection             |

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #4)
> Note on the trunk, now the first example fails at -O2 and not just -O3. It
> might be useful to do a bisect there too.

That started with r13-3596-ge7310e24b1c0ca67.

[Bug tree-optimization/106570] [12/13/14 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.3                        |12.4

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 12.3 is being released, retargeting bugs to GCC 12.4.

[Bug tree-optimization/106570] [12/13/14 Regression] DCE sometimes fails with depending if statements since r12-2305-g398572c1544d8b75

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106570

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=107880

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect if we optimize:
 _1 = ~c_6(D);
  _2 = _1 & s_7(D);

to:
c < s;

VRP will just work now.

(that would be PR 107880 ).