aux/disksim: use even less memory
 [rsc] --rw-rw-r-- M 1812954 rsc sys 11178 Feb 26 09:40 sys/src/cmd/aux/disksim.c
	/n/sourcesdump/2006/0226/plan9/sys/src/cmd/aux/disksim.c:23,28 - 
	/n/sourcesdump/2006/0227/plan9/sys/src/cmd/aux/disksim.c:23,29
	  	LOGNPTR = LOGBLKSZ-2,	/* assume sizeof(void*) == 4 */
	  	NPTR = 1<<LOGNPTR,
	  };
	+ static uchar zero[BLKSZ];
	  
	  struct Trip
	  {
	/n/sourcesdump/2006/0226/plan9/sys/src/cmd/aux/disksim.c:239,245 - 
	/n/sourcesdump/2006/0227/plan9/sys/src/cmd/aux/disksim.c:240,245
	  uchar*
	  getblock(vlong addr, int alloc)
	  {
	- 	static uchar zero[BLKSZ];
	   	Dbl *p2;
	  	Ind *p1;
	  	uchar *p0;
	/n/sourcesdump/2006/0226/plan9/sys/src/cmd/aux/disksim.c:356,364 - 
	/n/sourcesdump/2006/0227/plan9/sys/src/cmd/aux/disksim.c:356,377
	  }
	  
	  int
	+ isnonzero(void *v, ulong n)
	+ {
	+ 	uchar *a, *ea;
	+ 	
	+ 	a = v;
	+ 	ea = a+n;
	+ 	for(; a<ea; a++)
	+ 		if(*a)
	+ 			return 1;
	+ 	return 0;
	+ }
	+ 
	+ int
	  rdwrpart(Req *r)
	  {
	- 	int q;
	+ 	int q, nonzero;
	  	Part *p;
	  	vlong offset;
	  	long count, tot, n, o;
	/n/sourcesdump/2006/0226/plan9/sys/src/cmd/aux/disksim.c:396,429 - 
	/n/sourcesdump/2006/0227/plan9/sys/src/cmd/aux/disksim.c:409,444
	  		move = evommem;
	  
	  	tot = 0;
	+ 	nonzero = 1;
	  	if(r->ifcall.type == Tread)
	  		dat = (uchar*)r->ofcall.data;
	- 	else
	+ 	else{
	  		dat = (uchar*)r->ifcall.data;
	+ 		nonzero = isnonzero(dat, r->ifcall.count);
	+ 	}
	  	o = offset & (BLKSZ-1);
	  
	  	/* left fringe block */
	- 	if(o && count){	
	- 		blk = getblock(offset, r->ifcall.type==Twrite);
	- 		if(blk == nil)
	- 			abort();
	+ 	if(o && count){
	+ 		blk = getblock(offset, r->ifcall.type==Twrite && nonzero);
	  		n = BLKSZ - o;
	  		if(n > count)
	  			n = count;
	- 		(*move)(dat, blk+o, n);
	+ 		if(r->ifcall.type != Twrite || blk != zero)
	+ 			(*move)(dat, blk+o, n);
	  		if(r->ifcall.type == Twrite)
	  			dirty(offset, blk);
	  		tot += n;
	  	}
	+ next:
	  	/* full and right fringe blocks */
	  	while(tot < count){
	- 		blk = getblock(offset+tot, r->ifcall.type==Twrite);
	- 		if(blk == nil)
	- 			abort();
	+ 		blk = getblock(offset+tot, r->ifcall.type==Twrite && nonzero);
	  		n = BLKSZ;
	  		if(n > count-tot)
	  			n = count-tot;
	- 		(*move)(dat+tot, blk, n);
	+ 		if(r->ifcall.type != Twrite || blk != zero)
	+ 			(*move)(dat+tot, blk, n);
	  		if(r->ifcall.type == Twrite)
	  			dirty(offset+tot, blk);
	  		tot += n;
 [rsc] --rw-rw-r-- M 1812954 rsc sys 1452 Feb 26 22:08 sys/man/8/disksim