[Bug fortran/105691] Incorrect calculation of INDEX(str1,str2) at compile time

[kargl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105691

kargl at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-05-22
                 CC|                            |kargl at gcc dot gnu.org
     Ever confirmed|0                           |1
           Priority|P3                          |P4
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from kargl at gcc dot gnu.org ---
There is some torched logic looking for the substring, which can never find it.

diff --git a/gcc/fortran/simplify.cc b/gcc/fortran/simplify.cc
index 233cc42137f..16b231f5707 100644
--- a/gcc/fortran/simplify.cc
+++ b/gcc/fortran/simplify.cc
@@ -3606,32 +3606,29 @@ gfc_simplify_index (gfc_expr *x, gfc_expr *y, gfc_expr
*b, gfc_expr *kind)
        }
       else
        {
-         for (i = 0; i < len; i++)
+         /* Start at the tail of the string, offset by the length of the
+            substring, and search for a match moving towards the head of
+            string.  */
+         for (i = len - lensub; i >= 0; i--)
            {
-             for (j = 0; j < lensub; j++)
+             /* If the first character does not match, then the rest of the
+                string cannot match.  */
+             if (x->value.character.string[i]
+                 == y->value.character.string[0])
                {
-                 if (y->value.character.string[j]
-                     == x->value.character.string[len - i])
+
+                 /* Compare substring to starting location in string.  */      
+                 for (j = 0, count = 0; j < lensub; j++, count++)
                    {
-                     start = len - i;
-                     if (start <= len - lensub)
-                       {
-                         count = 0;
-                         for (k = 0; k < lensub; k++)
-                           if (y->value.character.string[k]
-                               == x->value.character.string[k + start])
-                             count++;
-
-                         if (count == lensub)
-                           {
-                             index = start + 1;
-                             goto done;
-                           }
-                       }
-                     else
-                       {
-                         continue;
-                       }
+                     if (x->value.character.string[i + j]
+                         != y->value.character.string[j])
+                       break;
+                   }
+
+                 if (count == lensub)
+                   {
+                     index = i + 1;
+                     goto done;
                    }
                }
            }