Re: Bignums and .sleb128

[Paul Schlie <schlie at comcast dot net>]

> 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.

Strongly suspect that the proper idiom to is to treat all non-explicitly
negative constants as being unsigned values; where the point of confusion
is that with the exception of decimal numbers; binary, octal, and hex
digits directly correspond to N-bit patters which were likely specified
as such with the implicit intent they be preserved, the only remaining
ambiguity is whether the most-significant specified set-bit is intended
to be sign-extended if the value is stored with greater precision than
than the otherwise required as determined by the most-significant non-0
bit position i.e.:

  -0x1 == [1...]1, where [1...] represents the variable precision
  sign-extension of the most significant bit explicitly specified, which
  would otherwise only require a signed-bit-field:1, but would need to
  be sign extended to fill the remaining most-significant bits if stored
  with greater precision as may be required.

Thereby all constant values may be treated uniformly:

  +1 == +0b01 == +0x1 ...

  -1 == -0b01 == -0x1 ...

  +2 == +0b10 == +0x2 ...

  -2 == -0b10 == -0x2 ...


Which seems quite sensible.