To:	mlist-djgpp AT nntp-server DOT caltech DOT edu
Path:	rollins
From:	rollins AT grue DOT caltech DOT edu (hr)
Newsgroups:	mlist.djgpp
Subject:	[bug] ebp optimization problem
Date:	30 Apr 1995 19:31:56 GMT
Organization:	California Institute of Technology, Pasadena
Lines:	119
Nntp-Posting-Host:	grue.caltech.edu

#if 0

hi,
  I think I found an optimization bug in djgpp.  I compiled with:

	gcc -m486 -O6 -ffast-math -fstrength-reduce -funroll-loops 
	-fomit-frame-pointer -finline-functions -fexpensive-optimizations
	 -Wall

Using the -fomit-frame-pointer caused ebp to be used for indexing
an array in the data segment, however, no ds: override was produced:

.data
_shifts2:
	.byte 0
	.byte 0
	.byte 1
	.byte 1
	.byte 1
	.byte 1
	.byte 1
	.byte 1
	.byte 0
	.byte 1
	.byte 1
	.byte 1
	.byte 1
	.byte 1
	.byte 1
	.byte 0
.text
	.align 4
.globl _des_set_key
_des_set_key:

...

L5:
	cmpb $0,_shifts2(%ebp)          ; should have ds: override!!!!
	je L6

...

This causes a stack fault under DPMI but doesn't crash otherwise.
The C code to produce this follows.

-hr

#endif


#include <stdio.h>

extern unsigned long skb[8][64];

#define PERM_OP(a,b,t,n,m) ((t)=((((a)>>(n))^(b))&(m)),\
	(b)^=(t),\
	(a)^=((t)<<(n)))

#define HPERM_OP(a,t,n,m) ((t)=((((a)<<(16-(n)))^(a))&(m)),\
	(a)=(a)^(t)^(t>>(16-(n))))\

static char shifts2[16]={0,0,1,1,1,1,1,1,0,1,1,1,1,1,1,0};

int des_set_key(char *key, char *schedule)
{
    register unsigned long c,d,t,s;
    register unsigned char *in;
    register unsigned long *k;
    register int i;
    
    k=(unsigned long *)schedule;
    in=(unsigned char *)key;
    c = schedule[0];
    d = schedule[1];
    
    /* I now do it in 47 simple operations :-)
     * for the inspiration. :-) */
    PERM_OP (d,c,t,4,0x0f0f0f0f);
    HPERM_OP(c,t,-2,0xcccc0000);
    HPERM_OP(d,t,-2,0xcccc0000);
    PERM_OP (d,c,t,1,0x55555555);
    PERM_OP (c,d,t,8,0x00ff00ff);
    PERM_OP (d,c,t,1,0x55555555);
    d=	(((d&0x000000ff)<<16)| (d&0x0000ff00)     |
	 ((d&0x00ff0000)>>16)|((c&0xf0000000)>>4));
    c&=0x0fffffff;

    for (i=0; i<16; i++)
	{
	    if (shifts2[i])
		{ c=((c>>2)|(c<<26)); d=((d>>2)|(d<<26)); }
	    else
		{ c=((c>>1)|(c<<27)); d=((d>>1)|(d<<27)); }
	    c&=0x0fffffff;
	    d&=0x0fffffff;
	    /* could be a few less shifts but I am to lazy at this
	     * point in time to investigate */
	    
	    s=	skb[0][ (c    )&0x3f                ]|
		skb[1][((c>> 6)&0x03)|((c>> 7)&0x3c)]|
		skb[2][((c>>13)&0x0f)|((c>>14)&0x30)]|
		skb[3][((c>>20)&0x01)|((c>>21)&0x06) |
			       ((c>>22)&0x38)];
	    t=	skb[4][ (d    )&0x3f                ]|
		skb[5][((d>> 7)&0x03)|((d>> 8)&0x3c)]|
		skb[6][ (d>>15)&0x3f                ]|
		skb[7][((d>>21)&0x0f)|((d>>22)&0x30)];
	    
		/* table contained 0213 4657 */
		*(k++)=((t<<16)|(s&0x0000ffff))&0xffffffff;
		s=     ((s>>16)|(t&0xffff0000));
		
		s=(s<<4)|(s>>28);
		*(k++)=s&0xffffffff;
	}
    return(0);
}

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