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From: "foreese at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/94978] [8/9/10/11 Regression] Bogus warning "Array
 reference at (1) out of bounds in loop beginning at (2)"
Date: Wed, 06 May 2020 23:26:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94978

--- Comment #1 from Fritz Reese <foreese at gcc dot gnu.org> ---
The regression is caused by r253156, which introduces the warning in the fi=
rst
place. The relevant code is in frontend-passes.c (do_subscript). Apparently,
the FE is aware that when there is a conditional it cannot correctly simpli=
fy
the subscript bounds. However, in the testcase there is an inner DO-loop wh=
ich,
when the bounds are reduced to the empty set, prevents the code from becomi=
ng
invalid. Therefore no warning should be issued.

The warning can be bypassed by guarding the inner DO-loop with any conditio=
nal,
including a vacuously true one:

$ diff -auw pascal.f03 pascal2.f03
--- pascal.f03  2020-05-06 19:14:50.966646632 -0400
+++ pascal2.f03 2020-05-06 19:23:48.209569659 -0400
@@ -9,9 +9,11 @@

 do i =3D 0, 8
   pascal(i,0) =3D 1
+  if (.true.) then
   do j =3D 1, i-1
     pascal(i,j) =3D pascal(i-1,j) + pascal(i-1,j-1)
   enddo
+  endif
   do j =3D i, 8
     pascal(i,j) =3D 0
   enddo
$ gfortran pascal2.f03


Normally the warning can be suppressed with -Wno-do-subscript, but the code=
 in
do_subscript() determines that this is "definitely" an issue and therefore
moves the warning to category 0.

(An interesting note: moving the code in the testcase into a subroutine, ra=
ther
than the main program, suppresses the warning for GCC 8, but not GCC 9, 10,=
 or
11.)=