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Path: utzoo!linus!philabs!cmcl2!seismo!umcp-cs!chris
From: chris@umcp-cs.UUCP (Chris Torek)
Newsgroups: net.bugs.4bsd
Subject: missed optimizations in pcc
Message-ID: <1213@umcp-cs.UUCP>
Date: Sun, 18-Nov-84 07:24:33 EST
Article-I.D.: umcp-cs.1213
Posted: Sun Nov 18 07:24:33 1984
Date-Received: Tue, 20-Nov-84 06:44:07 EST
Distribution: net
Organization: U of Maryland, Computer Science Dept., College Park, MD
Lines: 71

Index: lib/pcc/optim.c 4.2 Fix

Description:
	The machine independent expression optimizer doesn't handle
	"/=", "%", or "%=" by the constants 1 and 0.

Repeat-By:
	Feed the following through /lib/ccom; look at the assembly:

		f () {
			register i,j;

			i = j / 1;	/* nice, the division goes away */
			i /= 1;		/* but not this time! */
			i = j % 1;	/* result is always 0, but talk
					   about doing things the hard way */
			i %= 1;		/* same */
		}

Fix:
	The following changes to optim.c make it like /= as well as /,
	and make it handle % 1 and %= 1 in a reasonable way.  We also
	added checks for division and mod by zero (they used to just
	generate divisions by zero).

RCS file: RCS/optim.c,v
retrieving revision 1.1
diff -c1 -r1.1 optim.c
*** /tmp/,RCSt1008671	Sun Nov 18 07:16:07 1984
--- optim.c	Sun Nov 18 07:15:44 1984
***************
*** 142,143
  
  	case DIV:

--- 142,157 -----
  
+ 	case ASG MOD:
+ 	case MOD:
+ 		if( nncon( p->in.right ) ){
+ 			if( p->in.right->tn.lval == 1 ){
+ 				/* mod by one gives zero */
+ 				p->in.op = p->in.op==MOD ? COMOP : ASSIGN;
+ 				p->in.right->tn.lval = 0;
+ 				break;
+ 				}
+ 			if( p->in.right->tn.lval == 0 ) uerror("mod by zero");
+ 			}
+ 		break;
+ 
+ 	case ASG DIV:
  	case DIV:
***************
*** 143,145
  	case DIV:
! 		if( nncon( p->in.right ) && p->in.right->tn.lval == 1 ) goto zapright;
  		break;

--- 157,162 -----
  	case DIV:
! 		if( nncon( p->in.right ) ){
! 			if( p->in.right->tn.lval == 1 ) goto zapright;
! 			if( p->in.right->tn.lval == 0 ) uerror("division by zero");
! 			}
  		break;
-- 
(This line accidently left nonblank.)

In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (301) 454-7690
UUCP:	{seismo,allegra,brl-bmd}!umcp-cs!chris
CSNet:	chris@umcp-cs		ARPA:	chris@maryland