Bogus compare coming from `simplify_condition()'

Thread View: gnu.gcc.bug

1 messages

[T] Back to Threads [V] Tree View [G] Group Articles

1 total messages Started by will Wed, 11 Nov 1992 16:45

#1360

Author: will
Date: Wed, 11 Nov 1992 16:45

50 lines
2404 bytes

The following suggestion allows something useful to be done 
with a bit field produced by a signed extraction compared against zero.

The RTL pattern

    (set (reg:SI n)
         (ashiftrt:SI (ashift (reg:SI)     ;; from a bit field extraction
                              (const_int))
                      (const_int)))
    (set (cc0)
         (compare (reg:SI n)
                  (const_int 0)))

was being munged by `simplify_condition()' into 

   (set (cc0)
        (subreg:BLK (reg:SI n) 0))

by adding the line denoted by /* WRA */ in the following 
test in `simplify_condition()'

       case ASHIFTRT:
	  /* If OP0 is a sign extension and CODE is not an unsigned comparison,
	     do the comparison in a narrower mode.  */
	  if (! unsigned_comparison_p
	      && GET_CODE (XEXP (op0, 1)) == CONST_INT
	      && GET_CODE (XEXP (op0, 0)) == ASHIFT
	      && XEXP (op0, 1) == XEXP (XEXP (op0, 0), 1)
	      && (tmode = mode_for_size (mode_width - INTVAL (XEXP (op0, 1)),
					 MODE_INT, 1)) != VOIDmode
/* WRA */     && tmode != BLKmode  
	      && ((unsigned) const_op <= GET_MODE_MASK (tmode)
		  || (unsigned) - const_op <= GET_MODE_MASK (tmode)))
	    {
	      op0 = gen_lowpart_for_combine (tmode, XEXP (XEXP (op0, 0), 0));
	      continue;
	    }

the RTL was reduced to the "correct" zero_extraction pattern.

    (set (cc0)
         (zero_extract (reg:SI n) ..))
                       
----------------------------------------------------------------------
	     ___________     William Athanasiou, Software Engineer
	    /  ________/__   ...!{harvard,decvax,uunet}!masscomp!will
	   /__/_______/  /   will@westford.ccur.com
  Concurrent /__________/    Phone:  (508) 392-2647
Computer Corporation	     FAX: Fax? We don't need no stinkin Faxes!
----------------------------------------------------------------------

Thread Navigation

This is a paginated view of messages in the thread with full content displayed inline.

Messages are displayed in chronological order, with the original post highlighted in green.

Use pagination controls to navigate through all messages in large threads.

Back to All Threads