[Bug fortran/108735] New: Wrong line reported on -Wmaybe-uninitialized false positive at -O2 and missing optimizations

[Bug fortran/108735] New: Wrong line reported on -Wmaybe-uninitialized false positive at -O2 and missing optimizations

[rimvydas.jas at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108735

            Bug ID: 108735
           Summary: Wrong line reported on -Wmaybe-uninitialized false
                    positive at -O2 and missing optimizations
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rimvydas.jas at gmail dot com
  Target Milestone: ---

Reduced testcase:
$ cat ilev.f90
subroutine foo(lev,p,r)
implicit none
integer :: lev,i,j
integer :: p,r,z,w
i = 1
lev = max(min(abs(lev),99),2) ! [2,99]
do j=i, lev, +1
  if (j==i) then
    w = 2
  else
    r = w
  endif
  w = r + 1
enddo
do j=lev, i, -1
  if (j==lev) then
    z = 2
  else
    p = z
  endif
  z = p + 1 ! this is comment
enddo
if (loc(p)>loc(r)) continue ! suppress unused
if (loc(z)>loc(w)) continue ! suppress unused
end subroutine

$ gfortran -Wall -Wextra -O2 -c ilev.f90
ilev.f90:21:29:

   21 |   z = p + 1 ! this is comment
      |                             ^
Warning: 'z' may be used uninitialized [-Wmaybe-uninitialized]
ilev.f90:4:16:

    4 | integer :: p,r,z,w
      |                ^
note: 'z' was declared here
$ gfortran -Wall -Wextra -O1 -c ilev.f90
ilev.f90:19:9:

   19 |     p = z
      |         ^
Warning: 'z' may be used uninitialized [-Wmaybe-uninitialized]
ilev.f90:4:16:

    4 | integer :: p,r,z,w
      |                ^
note: 'z' was declared here

The -O2 and above points to wrong line (also end of the comment).

Comparison of -fdump-tree-optimized of first do loop shows that second loop
gets pessimized, the "if (j==lev) then" branch in first iteration is not
considered in initialization analysis while still being store optimized out.

Changing (j==lev) to (j==lev-1) does not promote diagnostic to -Wuninitialized.

Moreover both do loops could be fully optimized down to:
 lev = max(min(abs(lev),99),2)
 r = r + 1*lev - 1
 p = p + 1*lev - 1