Fix twolf -funroll-loops -O3 miscompilation (a semi-latent web.c bug)

[Jan Hubicka]

Hi,
I finally tracked down twolf misoptimization triggered by my loop-unroll.c
changes.  It has turned out to be semi-latent wrong code issue in webizer.
What happens is:

1) gcse.c drop REG_EQUAL note on the induction variable
2) loop optimizer unrolls the loop enabling webizer to cleanup
3) webizer do not track reaching refs into REG_EQUAL note because
   the variable is dead and renames it to unused pseudo
   Program is stil valid but the REG_EQUAL is bogus.
4) CSE eventually takes the value and put it back into the code
5) init-regs initializes it to 0

and result is a segfault on the following testcase.
Fixed by making webizer to not prune dead regs.
Will commit it after testing on x86_64-linux.

Honza

/* { dg-do run } */
/* { dg-options "-O3 -funroll-loops" } */
typedef struct rowbox {
    int startx ;
    int endx ;
    int endx1 ;
    int startx2 ;
    int ypos ;
    int desiredL ;
} ROWBOX ;
ROWBOX rowArray1[2] ;
ROWBOX *rowArray = rowArray1;

int numRows = 2;

int row = 1;
int block = 0;
double ckt_size_factor ;

__attribute__ ((noinline))
configure2()
{
  block = 0 ;
  for( row = 1 ; row <= numRows ; row++ ) {
      block++ ;
    if( rowArray[row].endx1 > 0 ) {
      block++ ;
    }
  }
}

main()
{
  configure2();
}

	* web.c (web_main): Do not set DF_RD_PRUNE_DEAD_DEFS flag.
Index: web.c
===================================================================
--- web.c	(revision 192369)
+++ web.c	(working copy)
@@ -313,7 +313,8 @@ web_main (void)
   rtx insn;
 
   df_set_flags (DF_NO_HARD_REGS + DF_EQ_NOTES);
-  df_set_flags (DF_RD_PRUNE_DEAD_DEFS);
+  /* We can not RD_PRUNE_DEAD_DEFS, because we care about REG_EQUAL
+     notes.  */
   df_chain_add_problem (DF_UD_CHAIN);
   df_analyze ();
   df_set_flags (DF_DEFER_INSN_RESCAN);