[Bug tree-optimization/101508] New: Possible undefined behaviour in cpp program using "unsigned" type starting from GCC 9

[Bug tree-optimization/101508] New: Possible undefined behaviour in cpp program using "unsigned" type starting from GCC 9

[pnarsing at mathworks dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101508

            Bug ID: 101508
           Summary: Possible undefined behaviour in cpp program using
                    "unsigned" type starting from GCC 9
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pnarsing at mathworks dot com
  Target Milestone: ---

Hi Team,

I'm observing an optimization issue while compiling with GCC 10.3 .
On investigating figured that issue is reproducible in GCC 9 as well .

This can be reproduced by simple test code mentioned below :

------------------------------------------------------

using int_type = unsigned int; // works without "unsigned"
int_type constexpr start{0};   // works if >= 1
int_type constexpr stop{3};    // works if <= 2
int_type constexpr mult{2};    // works if != 2

int main()
{
    int result = 0;
    for(int_type aa = start; aa < stop; ++aa)
    {
        int_type const bb = aa * mult; // works if volatile
        if (aa > bb)
        {
            return 99;
        }
        ++result;
    }
    return result; // should be == stop
}

----------------------------------------------------------------

This code snippet returns 99 instead of 3 .

You can check the following link to see the issue reproducibility:
https://godbolt.org/z/zd79PPKfn

Modifying compiler option to -O1 and -Og avoids the issue.

[Bug tree-optimization/101508] [9/10/11/12 Regression] Possible undefined behaviour in cpp program using "unsigned" type starting with r9-4145

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101508

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amker at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-07-19
            Summary|Possible undefined          |[9/10/11/12 Regression]
                   |behaviour in cpp program    |Possible undefined
                   |using "unsigned" type       |behaviour in cpp program
                   |starting from GCC 9         |using "unsigned" type
                   |                            |starting with r9-4145
   Target Milestone|---                         |9.5
     Ever confirmed|0                           |1

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Started with r9-4145-ga81e2c6240655f60a49c16e0d8bbfd2ba40bba51
Simplified C testcase:
int
main ()
{
  unsigned i;
  for (i = 0; i < 3; ++i)
    {
      if (i > i * 2)
        __builtin_abort ();
    }
  return 0;
}

[Bug tree-optimization/101508] [9/10/11/12 Regression] Possible undefined behaviour in cpp program using "unsigned" type starting with r9-4145

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101508

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |100740
           Keywords|                            |wrong-code

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
This one is resolved with the (bogus) fix for PR100740


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100740
[Bug 100740] [9/10/11/12 Regression] wrong code at -O1 and above on
x86_64-linux-gnu since r9-4145

[Bug tree-optimization/101508] [9/10/11/12 Regression] Possible undefined behaviour in cpp program using "unsigned" type starting with r9-4145

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101508

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug tree-optimization/101508] [9/10/11/12 Regression] Possible undefined behaviour in cpp program using "unsigned" type starting with r9-4145

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101508

--- Comment #3 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:f1af8528d34418bc874ae9d993ee0dc3559972d2

commit r12-6844-gf1af8528d34418bc874ae9d993ee0dc3559972d2
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Jan 24 11:50:06 2022 +0100

    tree-optimization/102131 - fix niter analysis wrt overflow

    This fixes the overflow issues seen with analyzing
    BASE0 + STEP0 cmp BASE1 + STEP1 as BASE0 + STEP0 - STEP1 cmp BASE1
    by following the logic we have when simplifying comparisons.

    2022-01-24  Richard Biener  <rguenther@suse.de>
                Jiufu Guo  <guojiufu@linux.ibm.com>

            PR tree-optimization/100740
            PR tree-optimization/101508
            PR tree-optimization/101972
            PR tree-optimization/102131
            * tree-ssa-loop-niter.cc (number_of_iterations_cond): Properly
            constrain BASE0 + STEP0 cmp BASE1 + STEP1 to
            BASE0 + STEP0 - STEP1 cmp BASE1 transform.

            * gcc.dg/torture/pr100740.c: New testcase.
            * gcc.dg/torture/pr101508.c: Likewise.
            * gcc.dg/torture/pr101972.c: Likewise.
            * gcc.dg/torture/pr102131-1.c: Likewise.
            * gcc.dg/torture/pr102131-2.c: Likewise.
            * gcc.dg/torture/pr102131-3.c: Likewise.
            * gcc.dg/torture/pr102131-4.c: Likewise.

[Bug tree-optimization/101508] [9/10/11 Regression] Possible undefined behaviour in cpp program using "unsigned" type starting with r9-4145

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101508

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11/12 Regression]     |[9/10/11 Regression]
                   |Possible undefined          |Possible undefined
                   |behaviour in cpp program    |behaviour in cpp program
                   |using "unsigned" type       |using "unsigned" type
                   |starting with r9-4145       |starting with r9-4145
             Status|NEW                         |ASSIGNED
      Known to work|                            |12.0
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org

[Bug tree-optimization/101508] [9/10/11 Regression] Possible undefined behaviour in cpp program using "unsigned" type starting with r9-4145

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101508

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

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

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Dup.

*** This bug has been marked as a duplicate of bug 100740 ***