[Patch] fix for loop optimizer (strength reduction)

["Jose Luu" <jluu at mainsoft dot com>]

Please someone integrate, since I dont have write access.

Bug report and test case:
http://gcc.gnu.org/ml/gcc-bugs/2000-01/msg00526.html


When a giv (global induction variable) is used before its initial assignment
is is not correct to allow replacement by the strength reduced variable, I
added the code to detect this condition in function record_giv.

Previously, this condition was caught later in the case of for loops (or
when the biv is incremented before the giv) by the code in
check_final_value, I think that's why this problem went undetected so far.

ChangeLog

2000-01-20   Jose Luu   (jluu@mainsoft.com)

    gcc/loop.c (record_giv): detect if the giv is used before its
assignment, assert not_replaceable in this case.



Index: loop.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/loop.c,v
retrieving revision 1.216
diff -c -3 -p -r1.216 loop.c
*** loop.c      2000/01/05 06:55:01     1.216
--- loop.c      2000/01/20 18:14:21
*************** record_giv (v, insn, src_reg, dest_reg,
*** 5764,5769 ****
--- 5764,5786 ----
        }
      }

+   /* giv is not repleaceable if its initial value is used in the loop
before
+      the assignment */
+   if (!v->not_replaceable) {
+       rtx p = loop_start;
+       while (p != v->insn) {
+         if ((GET_CODE (p) == INSN || GET_CODE (p) == JUMP_INSN
+             || GET_CODE (p) == CALL_INSN) &&
+             (reg_mentioned_p (v->dest_reg, PATTERN (p))))
+         {
+             v->replaceable = 0;
+             v->not_replaceable = 1;
+             break;
+         }
+         p = NEXT_INSN (p);
+       }
+   }
+
    /* Record whether the add_val contains a const_int, for later use by
       combine_givs.  */
    {