[Bug c/109507] New: Optimizer creates incorrect program

[Bug c/109507] New: Optimizer creates incorrect program

[aran at 100acres dot us]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109507

            Bug ID: 109507
           Summary: Optimizer creates incorrect program
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aran at 100acres dot us
  Target Milestone: ---

Created attachment 54854
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54854&action=edit
Example program.

The attached program should print 0 and does with optimization level zero (0). 
However, with optimization at any non-zero value (e.g., g, s, 1, 2) it prints
1.  This is on an Intel I7-2600.

The code that the optimizer writes reduces to a single test instruction that
checks if x is less or equal to zero.  However, this is incorrect for the most
negative number.  

To reproduce compile the attached file with -O0 and with -O1.

[Bug c/109507] Optimizer creates incorrect program

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109507

Sam James <sjames at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sjames at gcc dot gnu.org

--- Comment #1 from Sam James <sjames at gcc dot gnu.org> ---
UBSAN with GCC:
```
$ /tmp/foo
/tmp/foo.c:4:7: runtime error: negation of -2147483648 cannot be represented in
type 'int'; cast to an unsigned type to negate this value to itself
0
```

UBSAN with Clang:
```
/tmp/foo.c:4:15: runtime error: signed integer overflow: 2147483647 + 1 cannot
be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /tmp/foo.c:4:15 in
0
```

[Bug c/109507] Optimizer creates incorrect program

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109507

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

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
-fsanitize=undefined, at runtime gives:

/app/example.cpp:4:7: runtime error: negation of -2147483648 cannot be
represented in type 'int'; cast to an unsigned type to negate this value to
itself

Note the message is slightly wrong but points you to the right reason.
There is an overflow. because ~0x80000000 is 2147483648 and then you add 1 to
it giving you an overflow which is undefined. Note ~a+1 is the same as -a as
both cases will overflow at the same time with 0x80000000 which is why GCC is
providing that error message rather then on dealing with +1 like clang does:

Note clang gives:
/app/example.cpp:4:15: runtime error: signed integer overflow: 2147483647 + 1
cannot be represented in type 'int'

[Bug c/109507] Optimizer creates incorrect program

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109507

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
When you created this bug report there was a red banner at the top of the page
that begins by asking you to read https://gcc.gnu.org/bugs/ and that tells you
to try -fsanitize=undefined

[Bug c/109507] Optimizer creates incorrect program

[aran at 100acres dot us]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109507

Aran Clauson <aran at 100acres dot us> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #4 from Aran Clauson <aran at 100acres dot us> ---
Disregard.