Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!wuarchive!udel!eplrx7!cristy
From: cristy@eplrx7.uucp (John Cristy)
Newsgroups: comp.sys.mips
Subject: CC Generates Incorrect Code
Message-ID: <1990Sep7.174758.10424@eplrx7.uucp>
Date: 7 Sep 90 17:47:58 GMT
Sender: usenet@ee.udel.edu
Organization: DuPont Engineering Physics Lab
Lines: 83
Nntp-Posting-Host: louie.udel.edu

Is this a legitimate compiler bug?

This code:

    register u_char ch, ch1;
    register u_char *ptr, *ptr1;

    ptr1 = Raster;
    do {
	ch = ch1 = NEXTBYTE;
	while (ch--) *ptr1++ = NEXTBYTE;
	if ((Raster - ptr1) > filesize)
	    error("%s: %s is a corrupt GIF file (unblock).\n", inf);
    } while(ch1);

using this cc command:

    cc -systype bsd43 -I.. -I/usr/include/bsd43 -I/usr/include -O -c file.c

produces this assembly code:

     # 346	    ptr1 = Raster;
    	lw	$16, Raster
    	la	$18, $$194
    	li	$19, 255
    $65:
    	.loc	2 348
     # 347	    do {
     # 348		ch = ch1 = NEXTBYTE;
    	lbu	$3, 0($8)
    	addu	$8, $8, 1
    	.loc	2 349
     # 349		while (ch--) *ptr1++ = NEXTBYTE;
    	addu	$17, $3, -1
    	and	$2, $17, 255
    	and	$2, $2, 255
    	beq	$3, 0, $67
    $66:
    	lbu	$12, 0($8)
    	sb	$12, 0($16)
    	addu	$8, $8, 1
    	addu	$16, $16, 1
    	move	$3, $2
    	addu	$2, $2, -1
    	and	$2, $2, 255
    	and	$2, $2, 255
    	bne	$3, 0, $66
    $67:
    	.loc	2 350
     # 350		if ((Raster - ptr1) > filesize)
    	lw	$24, Raster
    	subu	$15, $24, $16
    	ble	$15, $20, $68
    	.loc	2 351
     # 351		    error("%s: %s is a corrupt GIF file (unblock).\n", inf);
    	move	$4, $18
    	move	$5, $22
    	sw	$8, 64($sp)
    	.livereg	0xc00ff0e,0xfff
    	jal	error
    	lw	$8, 64($sp)
    $68:
    	.loc	2 352
     # 352	    } while(ch1);
    	.loc	2 352
    	bne	$17, $19, $65
    	.loc	2 354

Incorrect code is generated.  Register $19 should be 0 but instead has
been initilized to 255 (li $19, 255).  This causes the program to fail.  
Compiling without the systype option produces the correct code and
changing the statement

    } while(ch1);

to
   
    } while(ch1 > 0);

fixes the problem.

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