<html>
<head>
<style><!--
.hmmessage P
{
margin:0px;
padding:0px
}
body.hmmessage
{
font-size: 10pt;
font-family:Verdana
}
--></style>
</head>
<body class='hmmessage'>
1) Tony, I claim that if Target.Int is really opaque, then values of it should only be formed in Target.i3/Target.m3.<BR>
Or TInt/TWord/.i3/.m3, since they know deeply about it.<BR>
Having m3back know about them seems a bit off.<BR>
Granted, pushing constants that only m3back uses into Target/TInt/TWord isn't great either, pollutes them some.<BR>
This is a minor dilemna though.<BR>
<BR>
<BR>
2) Are you sure having a fixed precision of 8 bytes is correct here?<BR>
More generally are you sure TInt has the right interface?<BR>
How does one check for overflow within the precision one cares about?<BR>I think I know.<BR>
I think Chop should return a boolean, as to if the value fits.<BR>
For that matter, have SignedChop and UnsignedChop.<BR>
Or TInt.Chop and TWord.Chop.<BR>
TInt.Chop is already SignedChop.<BR>
<BR>
Do the math in 64bits, if that overflows, then overflow.<BR>
And then truncate to whatever, and that can overflow too.<BR>
This is related to "you all have convinced me that overflow isn't<BR>
all that important really, but range checking is".<BR>
So you in sense defer the overflow. You don't do the complete<BR>
overflow check at Add or whatever, but then you do a range<BR>
check converting to a small size.<BR>
<BR>
But hm. This will skip "intermediate overflow". Does that matter?<BR>
1 billion * 8 / 8<BR>
<BR>
<BR>
Should that overflow in 32bits?<BR>
<BR>
<BR>
Well, easy enough, do the chop after each operation.<BR>
I said "defer", but it is brief.<BR>
1B * 8 / 8<BR>
isn't written as<BR>
1* 8 / 8<BR>
convert to 32bits<BR>
<BR>
but rather<BR>
1B * 8<BR>
convert to 32bits<BR>
=> failure here <BR>
/ 8<BR>
convert to 32bits<BR>
<BR>
?<BR>
<BR>
<BR>
3) Related to #2.<BR>
Where should RightShift insert zeros?<BR>
I guess you chop first, and then it works.<BR>
<BR>
<BR>
Proposed something like:<BR>
<BR>
<BR>TInt:<BR>
PROCEDURE Chop (VAR r: Int; n: CARDINAL): BOOLEAN =<BR>
VAR extension := Mask * ORD(And (r [n-1], SignMask) = 0);<BR>
result := TRUE;<BR> BEGIN<BR> FOR i := n TO LAST(Int) DO<BR>
result := (result AND (r[i] # extension));<BR>
r[i] := extension;<BR>
END;<BR>
return result;<BR> END Chop;<BR><BR>
TWord:<BR>
PROCEDURE Chop (VAR r: Int; n: CARDINAL): BOOLEAN =<BR>
CONST extension = 0;<BR>
VAR result := TRUE;<BR> BEGIN<BR> FOR i := n TO LAST(Int) DO<BR>
result := (result AND (r[i] # extension));<BR>
r[i] := extension;<BR>
END;<BR>
return result;<BR> END Chop;<BR><BR>
<BR>
<BR>
But I have to back to your version of m3back and try these out.<BR>
Very speculative at the moment.<BR>
<BR>
Could also leave the data alone upon failure, trivially related to previous:<BR>
<BR>
TInt:<BR>
PROCEDURE Chop (VAR r: Int; n: CARDINAL): BOOLEAN =<BR>
VAR extension := Mask * ORD(And (r [n-1], SignMask) = 0);<BR>
BEGIN<BR> FOR i := n TO LAST(Int) DO<BR>
IF r[i] # extension THEN<BR>
RETURN FALSE;<BR>
END;<BR>
END;<BR>
FOR i := n TO LAST(Int) DO<BR>
r[i] := extension;<BR>
END;<BR>
RETURN TRUE;<BR>
END Chop;<BR><BR>
<BR>
TWord:<BR>
PROCEDURE Chop (VAR r: Int; n: CARDINAL): BOOLEAN =<BR>
VAR result := TRUE;<BR> BEGIN<BR> FOR i := n TO LAST(Int) DO<BR>
IF r[i] # 0 THEN<BR>
RETURN FALSE;<BR>
END;<BR>
END;<BR>
FOR i := n TO LAST(Int) DO<BR>
r[i] := 0;<BR>
END;<BR>
return TRUE;<BR> END Chop;<BR><BR>
<BR>
I'm not sure I like the subtlety of TInt.Chop vs. TWord.Chop.<BR>
I'd prefer TInt.UnsignedChop and TInt.SignedChop.<BR>
Granted, "Un" doesn't exactly stand out, but at least "signed" does and makes you wonder about and discover the existance of signed vs. unsigned.<BR>
<BR>
<BR>
Alternatively Chop and UChop. More subtle, but I think plenty programmars are used to the names "uchar", "ushort", "uint", "ulong". "u" is pretty widely recognized as meaning "unsigned". More so I believe than "Word".<BR>
(I still don't like the name "interface Long"!, it doesn't imply unsigned at all!)<BR>
<BR>
<BR>
- Jay<BR> </body>
</html>