Re: Bignums and .sleb128

[Daniel Jacobowitz <drow at false dot org>]

On Mon, Jan 31, 2005 at 09:32:46PM +0000, Richard Sandiford wrote:
> Daniel Jacobowitz <drow@false.org> writes:
> > I spent most of this morning chasing a bug in .sleb128 support.  After
> > I finished running around in circles and discovered that Richard Sandiford
> > had fixed it two weeks ago (thanks!) I compared the testcases I'd written
> > with the ones that he committed.  There were a couple of differences,
> > basically related to this comment from bignum.h:
> >
> >  *      Bignums are >= 0.                                               *
> 
> Ugh, didn't notice that, sorry.  But I think that comment fell by the
> wayside the moment we tried to handle '-' for O_bigs.  After all, the
> precision of bignums is completely arbitrary, so if the result of
> -bignum is supposed to be unsigned, there's no obvious cut-off
> point for the sign extension.

Yes.  I tried disabling that entirely, but it breaks the .quad and sleb
tests.

> 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.  So right now 2**64-1 is a negative bignum, and 2**48-1
is a negative bignum, and 2**56-1 is a positive bignum.

> > So generating a sleb128 from one is pretty strange - the sign bit is
> > ambiguously handled.
> 
> Perhaps, but the problem isn't limited to sleb128.  E.g.:
> 
> 	.quad	-0xffffffffffff
> 
> acts in the same way as ".quad 1", not ".quad 0xffff000000000001".
> 
> The direction of recent changes suggests there really is a need
> for negative bignums, so IMO we should try to support them.

I don't much care.  I was working on basically the same testcase you
were; GCC with long long HOST_WIDE_INT will output enumerators as
.sleb128 0xffffffff.  I just encountered all the other bogus bits when
I tried to write tests for .sleb128.

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.

-- 
Daniel Jacobowitz