better redundant compare elimination

[John Carr <jfc at tiac dot net>]

This patch eliminates redundant compares when a label is only reached
from a jump by setting CC status according to the instruction preceding
the branch.

This has a very small effect on the code I tried it on.  Is it worth
installing?

Any other comments?

*** final.c	1998/08/26 08:11:20	1.55
--- final.c	1998/09/03 01:59:08
*************** final (first, file, optimize, prescan)
*** 1970,1975 ****
--- 1970,1990 ----
  	if (GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) > max_line)
  	  max_line = NOTE_LINE_NUMBER (insn);
      }
+ #ifdef HAVE_cc0
+   /* If CC tracking across branches is enabled, record the insn which
+      jumps to each branch only reached from one place.  */
+   if (flag_expensive_optimizations)
+     for (insn = first; insn; insn = NEXT_INSN (insn))
+       {
+ 	rtx lab;
+ 	if (GET_CODE (insn) == JUMP_INSN
+ 	    && (lab = JUMP_LABEL (insn))
+ 	    && LABEL_NUSES (lab) == 1)
+ 	  {
+ 	    LABEL_REFS (lab) = insn;
+ 	  }
+       }
+ #endif
  
    line_note_exists = (char *) oballoc (max_line + 1);
    bzero (line_note_exists, max_line + 1);
*************** final_scan_insn (insn, file, optimize, p
*** 2283,2289 ****
--- 2298,2327 ----
  	    ASM_OUTPUT_ALIGN (file, align);
  #endif
  	}
+ #ifdef HAVE_cc0
        CC_STATUS_INIT;
+       /* If this label is reached from only one place, set the condition
+ 	 codes from the instruction just before the branch.  */
+       if (flag_expensive_optimizations && LABEL_NUSES (insn) == 1)
+ 	{
+ 	  rtx jump = LABEL_REFS (insn);
+ 	  rtx barrier = prev_nonnote_insn (insn);
+ 	  rtx prev;
+ 	  /* If the LABEL_REFS field of this label has been set to point
+ 	     at a branch, the predecessor of the branch is a regular
+ 	     insn, and that branch is the only way to reach this label,
+ 	     set the condition codes based on the branch and its
+ 	     predecessor.  */
+ 	  if (barrier && GET_CODE (barrier) == BARRIER
+ 	      && jump && GET_CODE (jump) == JUMP_INSN
+ 	      && (prev = prev_nonnote_insn (jump))
+ 	      && GET_CODE (prev) == INSN)
+ 	    {
+ 	      NOTICE_UPDATE_CC (PATTERN (prev), prev);
+ 	      NOTICE_UPDATE_CC (PATTERN (jump), jump);
+ 	    }
+ 	}
+ #endif
        if (prescan > 0)
  	break;
        new_block = 1;