[Bug tree-optimization/102920] New: [12 Regression] Wrong code with -O3

[Bug tree-optimization/102920] New: [12 Regression] Wrong code with -O3

[vsevolod.livinskij at frtk dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102920

            Bug ID: 102920
           Summary: [12 Regression] Wrong code with -O3
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vsevolod.livinskij at frtk dot ru
  Target Milestone: ---

https://godbolt.org/z/qn3dsxsW8

Reproducer:
#include <stdio.h>

unsigned short a = 42;
unsigned short b = 1;
long long int c = 1;
unsigned char var_120;
unsigned char var_123;

void test(unsigned short a, unsigned short b, long long int c)
__attribute__((noipa));

void test(unsigned short a, unsigned short b, long long c) {
    for (char i = 0; i < (char)c; i += 5)
        if (!b)
            var_120 = a;
        else
            var_123 = a;
}

int main() {
    test(a, b, c);
    printf("%hhu\n", var_123);
    //if (var_123 != 42)
    //    __builtin_abort();
}

Error:
>$ g++ -O3 small.cpp && ./a.out
0
>$ g++ -O2 small.cpp && ./a.out
42

GCC version:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/testing/gcc/bin_master/libexec/gcc/x86_64-pc-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /testing/gcc/gcc_src_master/configure --enable-multilib
--prefix=/testing/gcc/bin_master --disable-bootstrap
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.0 20211023 (experimental) (GCC) 
root@abe71269b867:/testing/result/S_1635079215/reduce# cat
/testing/gcc/gcc_rev.txt 
git://gcc.gnu.org/git/gcc.git:master
e3725624ec0735996a18e1a90317e230bef899ac

[Bug tree-optimization/102920] [12 Regression] Wrong code with -O3

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102920

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Target Milestone|---                         |12.0
                 CC|                            |rguenth at gcc dot gnu.org
   Last reconfirmed|                            |2021-10-25

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
  <bb 6> [local count: 70079549]:
  var_123 = _27(D);
  goto <bb 3>; [100.00%]


Fre messed up:
Before:
  # var_120_lsm.8_36 = PHI <_1(5), _27(D)(4)>
  # var_120_lsm_flag.9_37 = PHI <1(5), 0(4)>
  # var_123_lsm.10_38 = PHI <_24(D)(5), _43(4)>
  # var_123_lsm_flag.11_39 = PHI <0(5), 1(4)>

After:
  # var_120_lsm.8_36 = PHI <_1(5), _27(D)(4)>
  # var_120_lsm_flag.9_37 = PHI <1(5), 0(4)>
  # var_123_lsm_flag.11_39 = PHI <0(5), 1(4)>

This would mean this was caused by r12-4625-gfe8475c500.

[Bug tree-optimization/102920] [12 Regression] Wrong code with -O3

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102920

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Mine then.

[Bug tree-optimization/102920] [12 Regression] Wrong code with -O3

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102920

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
So the "logic" is that

   # var_120_lsm.9_7 = PHI <_42(6), _29(D)(5)>
   # var_123_lsm.11_13 = PHI <_26(D)(6), _1(5)>

can be considered equal since _42 and _26(D) are optimistically equal and
_29(D) and _1 are optimistically equal as well.  But we may only use
optimistic equality when merging values on different edges, not when
merging values on the same edge - in particular we may not choose the
undef value on any edge when there's a not undef value as well.

Hmm, we're also treating a[i][undef] and a[undef][i] as the same but there
I guess both undefs are "used" and thus both expressions invoke undefined
behavior.  While with the PHIs we don't know which edge will be taken
and thus which copy will result in undefined behavior.

[Bug tree-optimization/102920] [12 Regression] Wrong code with -O3

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102920

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:aa15952d646fd5dd569fce287b719a737ae66e4f

commit r12-4650-gaa15952d646fd5dd569fce287b719a737ae66e4f
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Oct 25 09:33:15 2021 +0200

    tree-optimization/102920 - fix PHI VN with undefined args

    This fixes a latent issue exposed by now allowing VN_TOP in PHI
    arguments.  We may only use optimistic equality when merging values on
    different edges, not when merging values on the same edge - in particular
    we may not choose the undef value on any edge when there's a not undef
    value as well.

    2021-10-25  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/102920
            * tree-ssa-sccvn.h (expressions_equal_p): Add argument
            controlling VN_TOP matching behavior.
            * tree-ssa-sccvn.c (expressions_equal_p): Likewise.
            (vn_phi_eq): Do not optimistically match VN_TOP.

            * gcc.dg/torture/pr102920.c: New testcase.

[Bug tree-optimization/102920] [12 Regression] Wrong code with -O3

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102920

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

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

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed.

[Bug tree-optimization/102920] [12 Regression] Wrong code with -O3

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102920

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |zhendong.su at inf dot ethz.ch

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
*** Bug 102902 has been marked as a duplicate of this bug. ***