[Bug tree-optimization/101768] New: -Wmaybe-uninitialized false alarm with 'switch' instead of 'if'

[eggert at cs dot ucla.edu via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101768

            Bug ID: 101768
           Summary: -Wmaybe-uninitialized false alarm with 'switch'
                    instead of 'if'
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at cs dot ucla.edu
  Target Milestone: ---

Created attachment 51255
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51255&action=edit
False alarm with gcc -O2 -Wmaybe-uninitialized

The attached program is a greatly-simplified version of GNU diffutils' cmp
program. Compiling it this way:

gcc -O2 -Wmaybe-uninitialized -S u.i

causes gcc to emit the bogus diagnostic:

u.i: In function ‘cmp’:
u.i:21:11: warning: ‘offset_width’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   21 |           foo (offset_width);
      |           ^~~~~~~~~~~~~~~~~~

I tried simplifying it further (removing the 'main' function, turning the
switch into an if, etc.) but the warning went away with the further
simplifications.

This is with gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1) on x86-64.