RE: [sdcc-devel] Re: [ sdcc-Bugs-524685 ] Division by power of 2 fails with neg nu

[Sandeep D. <sa...@wi...>]

Doing it for unsigned only is the way to go ....
Sandeep

> -----Original Message-----
> From: sdc...@li...
> [mailto:sdc...@li...]On Behalf Of Johan Knol
> Sent: Saturday, March 02, 2002 3:52 AM
> To: sdc...@li...
> Subject: [sdcc-devel] Re: [ sdcc-Bugs-524685 ] Division by power of 2
> fails with neg nu
>
>
> Now this is a nice one :). In fact this is two bugs in one.
>
> First the _literal_ shift. The "(SPEC_USIGN(let) ? (unsigned long)
> operandLitValue (left) : (long) operandLitValue (left))" in
> SDCCicode.c:operandOperation() doesn't work because the type
> of the ternary
> operation is always the type of the second operand (it
> seems), so the shift
> will allways be done unsigned. This is fixed in
> SDCCicode.c:130 for RIGHT_OP
> but will probably also be needed for the other ops. Now at
> least "-18 >> 2"
> is 0xfffffffb instead of 0x3ffffffb.
>
> Second the divide. -18 >> 2 = -5 whereas -18 / 4 = -4.
> Perhaps we could
> say -18 / 4 = (-18 >> 2) + (MSBit >> size-1) but I wonder if
> that would be
> more efficient that doing just the divide. If not, this
> optimation should
> only be done for unsigned.
>
> Please comment.
>
> Johan
>
>
> ----- Original Message -----
> From: <no...@so...>
> To: <no...@so...>
> Sent: Saturday, March 02, 2002 6:21 AM
> Subject: [sdcc-devel] [ sdcc-Bugs-524685 ] Division by power
> of 2 fails with
> neg nu
>
>
> > Bugs item #524685, was opened at 2002-03-01 21:21
> > You can respond by visiting:
> >
> http://sourceforge.net/tracker/?func=detail&atid=100599&aid=52
4685&group_id=
599
>
> Category: C-Front End
> Group: None
> Status: Open
> Resolution: None
> Priority: 5
> Submitted By: Michael Hope (michaelh)
> Assigned to: Nobody/Anonymous (nobody)
> Summary: Division by power of 2 fails with neg nu
>
> Initial Comment:
> See the regression tests under this number.
>
> The front end optimises division by a constant power of two as in:
>
>   volatile int left;
>
>   left = -18;
>   ASSERT(left/4 == (-18/4));
>
> However, if left is negative then shifting and division are not
equivalent.
>
> The optimisation still holds if left is unsigned.
>
>
> ----------------------------------------------------------------------
>
> You can respond by visiting:
>
http://sourceforge.net/tracker/?func=detail&atid=100599&aid=524685&group_id=
599
>
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>


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