[Bug c/103103] New: Incorrect integer optimization omits x < 0 comparison

[Bug c/103103] New: Incorrect integer optimization omits x < 0 comparison

[k.melekhin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103103

            Bug ID: 103103
           Summary: Incorrect integer optimization omits x < 0 comparison
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: k.melekhin at gmail dot com
  Target Milestone: ---

Created attachment 51739
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51739&action=edit
Simple test program + version info

On higher optimization levels O2,O3 the following code omits the "newsz < 0"
comparison, likely due to invalid optimization / assumtion that value will
never change and always be 0, however this is incorrect.

See attachment, shows very simple test to reproduce. Also my gcc version
included
in that file comment.

Gcc's output with -O0 is correct. Tested with other C compilers and they
generate correct output on any optimization level.

[Bug c/103103] Incorrect integer optimization omits x < 0 comparison

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103103

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

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Signed integer overflow is undefined and gcc is doing a valid transformation.
Use either unsigned integer arthemaric or -fwrapv.

[Bug c/103103] Incorrect integer optimization omits x < 0 comparison

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103103

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Also you can detect this at runtime with -fsanitize=undefined.

[Bug c/103103] Incorrect integer optimization omits x < 0 comparison

[k.melekhin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103103

--- Comment #3 from Kyryl Melekhin <k.melekhin at gmail dot com> ---
Thank you!

I figured that this has something to do with being UB behavior with int
overflow, however it's strange to find this kind of bug only on 1 compiler,
with no possible warnings or idications that something is wrong. Perhaps a
warning of UB into overflow from compiler be nice.

Have a good day.