[Bug tree-optimization/101912] New: -Wmaybe-uninitialized false alarm in tzdb localtime.c

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101912

            Bug ID: 101912
           Summary: -Wmaybe-uninitialized false alarm in tzdb localtime.c
           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 51303
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51303&action=edit
-Wuninitialized false alarm

This is a simplified version of a problem I found while compiling tzdb's
localtime.c with gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1) on x86-64.
Compile the attached program w.i with:

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

The output is:

w.i: In function ‘tzloadbody’:
w.i:17:27: warning: ‘prevcorr’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   16 |                        : (corr == prevcorr
      |                          ~~~~~~~~~~~~~~~~~
   17 |                           || corr == prevcorr - 1)))))
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~


This is a false alarm, since the underlined code is executed only when leapcnt
is nonzero, and leapcnt is set to a nonzero value only after prevcorr is
initialized.