fix opt/4311

[Richard Henderson]

The problem is that we tried to unroll the loop twice and got confused.
Fixed by adding a flag telling the loop optimizer that it'll get another
chance to auto-unroll the loop.

Not sure how to render the test for the testsuite, since even success
runs the loop a very large number of times, and the bug requires that
we see integer overflow.  Setting the start of the iteration large
seems to defeat the bug as well.  Ho hum.


r~


        * loop.h (LOOP_FIRST_PASS): New.
        * loop.c (strength_reduce): Mind it when deciding to unroll.
        * toplev.c (rest_of_compilation): Set it.

Index: loop.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/loop.c,v
retrieving revision 1.389.2.2
diff -c -p -d -r1.389.2.2 loop.c
*** loop.c	2002/03/13 07:34:55	1.389.2.2
--- loop.c	2002/04/03 07:49:50
*************** strength_reduce (loop, flags)
*** 5213,5219 ****
       collected.  Always unroll loops that would be as small or smaller
       unrolled than when rolled.  */
    if ((flags & LOOP_UNROLL)
!       || (loop_info->n_iterations > 0
  	  && unrolled_insn_copies <= insn_count))
      unroll_loop (loop, insn_count, 1);
  
--- 5213,5220 ----
       collected.  Always unroll loops that would be as small or smaller
       unrolled than when rolled.  */
    if ((flags & LOOP_UNROLL)
!       || (!(flags & LOOP_FIRST_PASS)
! 	  && loop_info->n_iterations > 0
  	  && unrolled_insn_copies <= insn_count))
      unroll_loop (loop, insn_count, 1);
  
Index: loop.h
===================================================================
RCS file: /cvs/gcc/gcc/gcc/loop.h,v
retrieving revision 1.58
diff -c -p -d -r1.58 loop.h
*** loop.h	2002/01/03 04:59:07	1.58
--- loop.h	2002/04/03 07:49:50
*************** Software Foundation, 59 Temple Place - S
*** 28,33 ****
--- 28,34 ----
  #define LOOP_UNROLL 1
  #define LOOP_BCT 2
  #define LOOP_PREFETCH 4
+ #define LOOP_FIRST_PASS 8
  
  /* Get the loop info pointer of a loop.  */
  #define LOOP_INFO(LOOP) ((struct loop_info *) (LOOP)->aux)
Index: toplev.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/toplev.c,v
retrieving revision 1.574.2.6
diff -c -p -d -r1.574.2.6 toplev.c
*** toplev.c	2002/03/27 18:49:06	1.574.2.6
--- toplev.c	2002/04/03 07:49:50
*************** rest_of_compilation (decl)
*** 2877,2884 ****
  	  cleanup_barriers ();
  
  	  /* We only want to perform unrolling once.  */
! 
! 	  loop_optimize (insns, rtl_dump_file, 0);
  
  	  /* The first call to loop_optimize makes some instructions
  	     trivially dead.  We delete those instructions now in the
--- 2877,2883 ----
  	  cleanup_barriers ();
  
  	  /* We only want to perform unrolling once.  */
! 	  loop_optimize (insns, rtl_dump_file, LOOP_FIRST_PASS);
  
  	  /* The first call to loop_optimize makes some instructions
  	     trivially dead.  We delete those instructions now in the