Re: [Squeak-VMdev] Versiojn 4 changes

[Ned K. <ne...@bi...>]

On Tuesday 06 April 2004 3:48 pm, Ian Piumarta wrote:
> On 06 Apr 2004, at 23:50, Yoshiki Ohshima wrote:
> > how about using "01" for OOP
> > and "00" for SmallInteger?  Some processor's addressing mode let us
> > access the word-aligned memory with such pointer, while "no-tag" for
> > SmallInteger may save some bit-operations.
>
> Just another data point: some Smalltalk implementations put the
> SmallInteger tag in the topmost bit.  This makes SI tag and overflow
> checks after arithmetic simpler: addition and subtraction work
> in-place, plus you can just look at the sign flag after the operation
> instead of "mask + test-zero" or "shift + xor + sign-test".
>
> On architectures where you can set the sign flag during move this can
> also often eliminate any need to mask and test on the tag bit; after a
> move you can "trap" immediately on (non-)SI oops.

On a related note, does it seem wasteful to anyone but me that we do the 
following in primBytecodeAdd:

int rcvr;
int arg;

rcvr = longAt(localSP - (1 * 4));
arg = longAt(localSP - (0 * 4));
if (((rcvr & arg) & 1) != 0)  /* areIntegers: rcvr and: arg */
{
    result = ((rcvr >> 1)) + ((arg >> 1));
    if ((result ^ (result << 1)) >= 0) 	/* isIntegerValue: result */
	{
        /* begin internalPop:thenPush: */
        longAtput(localSP -= (2 - 1) * 4, ((result << 1) | 1));
        /* begin fetchNextBytecode */
        currentBytecode = byteAt(++localIP);
        goto l9;
    }
}
else
{
    /* Try to add them as a float */
    /* If success, we're done. Get the next bytecode and loop */
}

    /* otherwise, do a normal send */


For a total operation count (not counting the stack load/store) of:
	1 AND
	2 bit tests
	2 right shifts
	2 left shifts
	1 OR
	1 XOR
	1 ADD

when for the majority of additions (those that don't overflow 31 bits) we only 
have to add the top two values (resetting one low bit first so that we don't 
get a carry from B0 to B1).

Seems like we could save the shifts in most cases by looking at the top two 
bits of the receiver and argument; if the sign bits are different or the high 
bits (B30) are both the same as the sign bits we aren't going to get any 
overflow.

-- 
Ned Konz
http://bike-nomad.com
GPG key ID: BEEA7EFE