Re: Bignums and .sleb128

[Daniel Jacobowitz <drow at false dot org>]

On Mon, Jan 31, 2005 at 10:17:56PM +0000, Richard Sandiford wrote:
> Daniel Jacobowitz <drow@false.org> writes:
> >> You said later that:
> >> 
> >> > If we're going to use these semantics, at least the '-' case in
> >> > operand() needs to be fixed.
> >> 
> >> but I wasn't sure what you meant by "these semantics".  Do you mean
> >> treating bignums as signed, or treating them as unsigned?  By my reading,
> >> operand()'s current handling of '-' already assumes they are signed,
> >> just like the sleb128 code does (and did ;).
> >
> > It doesn't work, because sometimes bignums are signed and sometimes
> > they aren't.  Consider -0xffffffffffff; the current code will return 1. 
> > If you want to treat the input as unsigned, then you need to add a new
> > word with the sign bit.  Note that with one less leading 'f', it
> > suddenly works.
> 
> Right, that's exactly the point I made later.  Like you say, if you
> treat the bignum as signed (as the current '-' code does), then
> {0xffff, 0xffff, 0xffff} is an invalid representation of a positive
> number, it should be {0xffff, 0xffff, 0xffff, 0x0000} instead.
> So the '-' code _does_ seem OK if you treat bignums as signed,
> the problem is that the integer parsing code is still assuming
> that bignums are unsigned, and that the extra 0x0000 littlenum
> isn't needed.
> 
> I couldn't tell whether that's what you were saying too, or whether
> you think that opcode() is wrong even if bignums are treated as signed.
> It's quite possible we're in violent agreement here ;)

I suppose that's one way to look at it.  If you assume that this code's
assumptions are correct, then it follows that this code is correct; the
bug is earlier.  There's still a bug, we've just rocked the terrarium a
bit :-)

> 
> > One approach to fix the problem would be to define X_unsigned as a
> > secondary "sign bit" for the bignum.  The core changes for that would
> > be easy.  It's the backends that bother me.
> 
> I guess that's one way, but both existing bits of "bignums are signed"
> code just use the top bit of the bignum as the sign bit.  Maybe that is
> the most natural representation?

Lots of places in the backend check that the bignum has a certain size.
These tests will break down with an additional sign-extension word.
Something's gotta give.

-- 
Daniel Jacobowitz