[Bug tree-optimization/113224] New: Warning "is used uninitialized" raised for an initialized variable

[Bug tree-optimization/113224] New: Warning "is used uninitialized" raised for an initialized variable

[gandalf at winds dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113224

            Bug ID: 113224
           Summary: Warning "is used uninitialized" raised for an
                    initialized variable
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gandalf at winds dot org
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

In an attempt to convert a float value bytewise to an integer, this function
returns the following unexpected GCC warnings at optimization level -O2 and -O3
on x86_64:

unsigned int myfunc() {
  float _f=2.5;
  unsigned int *_x=(unsigned int *)&_f;
  return *_x;
}

gcc -O2 -Wall test12.c -c

test12.c: In function 'myfunc':
test12.c:4:10: warning: '_f' is used uninitialized
test12.c:2:9: note: '_f' declared here

It compiles correctly, at least. The warning goes away when
-fno-strict-aliasing is applied.

[Bug tree-optimization/113224] Warning "is used uninitialized" raised for an initialized variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113224

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

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This code is undefined for violating C/C++ aliasing rules.
The warning is because of the undefinedness of the code.

[Bug tree-optimization/113224] Warning "is used uninitialized" raised for an initialized variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113224

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>In an attempt to convert a float value bytewise to an integer,

You should use memcpy instead (or an union which itself a GCC extension).

[Bug tree-optimization/113224] Warning "is used uninitialized" raised for an initialized variable

[gandalf at winds dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113224

--- Comment #3 from gandalf at winds dot org ---
Thank you for the suggestion. I'll try the union.