libfortran: Fix up _gfortran_{,m,s}findloc2_s{1,4} [PR120196]

As mentioned in the PR, _gfortran_{,m,s}findloc2_s{1,4} iterate too many
times in the back case if nothing is found.
For !back, the loops are for (i = 1; i <= extent; i++) so i is in the
body [1, extent] if nothing is found, but for back it is
for (i = extent; i >= 0; i--) so i is in the body [0, extent] and compares
one element before the start of the array.
Note, findloc1_s{1,4} uses
          for (n = len; n > 0; n--, src -= delta * len_array)
for the back loop and
          for (n = 1; n <= len; n++, src += delta * len_array)
for !back.  This patch fixes that.
The testcase fails under valgrind without the libgfortran changes and
succeeds with those.

2025-05-13  Jakub Jelinek  <jakub@redhat.com>

	PR libfortran/120196
	* m4/ifindloc2.m4 (header1, header2): For back use i > 0 rather than
	i >= 0 as for condition.
	* generated/findloc2_s1.c: Regenerate.
	* generated/findloc2_s4.c: Regenerate.

	* gfortran.dg/pr120196.f90: New test.

(cherry picked from commit 748a7bc462)

gcc/testsuite/gfortran.dg/pr120196.f90

@@ -0,0 +1,26 @@
+! PR libfortran/120196
+! { dg-do run }
+
+program pr120196
+  character(len=:, kind=1), allocatable :: a(:), s
+  character(len=:, kind=4), allocatable :: b(:), t
+  logical, allocatable :: l(:)
+  logical :: m
+  allocate (character(len=16, kind=1) :: a(10), s)
+  allocate (l(10))
+  a(:) = ""
+  s = "*"
+  l = .true.
+  m = .true.
+  if (findloc (a, s, dim=1, back=.true.) .ne. 0) stop 1
+  if (findloc (a, s, mask=l, dim=1, back=.true.) .ne. 0) stop 2
+  if (findloc (a, s, mask=m, dim=1, back=.true.) .ne. 0) stop 3
+  deallocate (a, s)
+  allocate (character(len=16, kind=4) :: b(10), t)
+  b(:) = ""
+  t = "*"
+  if (findloc (b, t, dim=1, back=.true.) .ne. 0) stop 4
+  if (findloc (b, t, mask=l, dim=1, back=.true.) .ne. 0) stop 5
+  if (findloc (b, t, mask=m, dim=1, back=.true.) .ne. 0) stop 6
+  deallocate (b, t, l)
+end program pr120196

libgfortran/generated/findloc2_s1.c

@@ -49,7 +49,7 @@ findloc2_s1 (gfc_array_s1 * const restrict array, const GFC_UINTEGER_1 * restric
   if (back)
     {
       src = array->base_addr + (extent - 1) * sstride;
-      for (i = extent; i >= 0; i--)
+      for (i = extent; i > 0; i--)
 	{
 	  if (compare_string (len_array, (char *) src, len_value, (char *) value) == 0)
 	    return i;
@@ -112,7 +112,7 @@ mfindloc2_s1 (gfc_array_s1 * const restrict array,
     {
       src = array->base_addr + (extent - 1) * sstride;
       mbase += (extent - 1) * mstride;
-      for (i = extent; i >= 0; i--)
+      for (i = extent; i > 0; i--)
 	{
 	  if (*mbase && (compare_string (len_array, (char *) src, len_value, (char *) value) == 0))
 	    return i;

libgfortran/generated/findloc2_s4.c

@@ -49,7 +49,7 @@ findloc2_s4 (gfc_array_s4 * const restrict array, const GFC_UINTEGER_4 * restric
   if (back)
     {
       src = array->base_addr + (extent - 1) * sstride;
-      for (i = extent; i >= 0; i--)
+      for (i = extent; i > 0; i--)
 	{
 	  if (compare_string_char4 (len_array, src, len_value, value) == 0)
 	    return i;
@@ -112,7 +112,7 @@ mfindloc2_s4 (gfc_array_s4 * const restrict array,
     {
       src = array->base_addr + (extent - 1) * sstride;
       mbase += (extent - 1) * mstride;
-      for (i = extent; i >= 0; i--)
+      for (i = extent; i > 0; i--)
 	{
 	  if (*mbase && (compare_string_char4 (len_array, src, len_value, value) == 0))
 	    return i;

libgfortran/m4/ifindloc2.m4

@@ -41,7 +41,7 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
   if (back)
     {
       src = array->base_addr + (extent - 1) * sstride;
-      for (i = extent; i >= 0; i--)
+      for (i = extent; i > 0; i--)
 	{
 	  if ('comparison`'`)
 	    return i;
@@ -94,7 +94,7 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
     {
       src = array->base_addr + (extent - 1) * sstride;
       mbase += (extent - 1) * mstride;
-      for (i = extent; i >= 0; i--)
+      for (i = extent; i > 0; i--)
 	{
 	  if (*mbase && ('comparison`'`))
 	    return i;