[Bug c/108989] New: Two small almost identical programs give different results

[Bug c/108989] New: Two small almost identical programs give different results

[d.j.allerton at sheffield dot ac.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108989

            Bug ID: 108989
           Summary: Two small almost identical programs give different
                    results
           Product: gcc
           Version: og10 (devel/omp/gcc-10)
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: d.j.allerton at sheffield dot ac.uk
  Target Milestone: ---

Created attachment 54570
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54570&action=edit
A small program doing 2D geometry. No math calls, just addition, subtraction,
mutiplication and division of floats.

In a small program, the following lines give different reults:

    x = x0 + ((r1 * r1 - r2 * r2 + x2 * x2) / (2.0 * x2));
    y = y0 + ((r1 * r1 - r3 * r3 + x3 * x3 + y3 * y3 - 2.0 * x3 * x) / (2.0 *
y3));
    printf("x=%f y=%f\n", x, y);

and 
    x = ((r1 * r1 - r2 * r2 + x2 * x2) / (2.0 * x2));
    y = ((r1 * r1 - r3 * r3 + x3 * x3 + y3 * y3 - 2.0 * x3 * x) / (2.0 * y3));
    printf("x=%f y=%f\n", x + x0, y + y0);

the first example gives: x=-0.000000 y=33.070801 (which is wrong)
the second example gives: x=0.000000 y=7.315198  (which is correct)

The only difference is the addition of x0 and y0 in the sets of expressions.

The two programs are attached. They don't use any libraries other than stdlib.h
and stdio.h. I use gcc version 10.2.0 on msys2 on a PC and version 10.2.1 on a
Raspberry Pi 4B and get the same errors with both compilers. A colleague gets
the same errors with version 12.1.0 on a PC.

[Bug c/108989] Two small almost identical programs give different results

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108989

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
x is used to compute y.

[Bug c/108989] Two small almost identical programs give different results

[d.j.allerton at sheffield dot ac.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108989

--- Comment #2 from Dave Allerton <d.j.allerton at sheffield dot ac.uk> ---
Apologies - should have spotted it but thanks for sorting it out.

Best regards
Dave Allerton
------------------------------------------------------------------------
On 02/03/2023 13:32, schwab@linux-m68k.org wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108989
>
> Andreas Schwab<schwab@linux-m68k.org>  changed:
>
>             What    |Removed                     |Added
> ----------------------------------------------------------------------------
>           Resolution|---                         |INVALID
>               Status|UNCONFIRMED                 |RESOLVED
>
> --- Comment #1 from Andreas Schwab<schwab@linux-m68k.org>  ---
> x is used to compute y.
>