Improving the Int/Word story inside GHC

[Edward Z. Yang]

Yes, in particular if the offset is zero.  Morally, however, we're
just doing this to clear the tag bit.

Edward

Excerpts from Johan Tibell's message of 2014-08-07 14:45:43 +0100:
> I'm hacking on this now. I'm not 100% sure that ByteOff isn't used for
> negative values though, see for example
> 
> mkTaggedObjectLoad
>   :: DynFlags -> LocalReg -> LocalReg -> ByteOff -> DynTag -> CmmAGraph
> -- (loadTaggedObjectField reg base off tag) generates assignment
> --      reg = bitsK[ base + off - tag ]
> -- where K is fixed by 'reg'
> mkTaggedObjectLoad dflags reg base offset tag
>   = mkAssign (CmmLocal reg)
>              (CmmLoad (cmmOffsetB dflags
>                                   (CmmReg (CmmLocal base))
>                                   (offset - tag))
>                       (localRegType reg))
> 
> from StgCmmUtils.
> 
> Wouldn't it be possible that the offset in cmmOffsetB (which is of type
> ByteOff) could be negative?
> 
> 
> 
> On Thu, Aug 7, 2014 at 1:49 PM, Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
> 
> >  I’m all for it!
> >
> >
> >
> > I believe that ByteOff/WordOff are always 0 or positive.   At least, they
> > were when I introduced them!
> >
> >
> >
> > SImon
> >
> >
> >
> > *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Johan
> > Tibell
> > *Sent:* 07 August 2014 12:21
> > *To:* Simon Marlow
> > *Cc:* ghc-devs at haskell.org
> > *Subject:* Re: Improving the Int/Word story inside GHC
> >
> >
> >
> > Simon M, is the intention of ByteOff and WordOff that they should be able
> > to represent negative quantities as well? If so we might need to split it
> > into ByteOff (still an Int) and ByteIndex (a Word) to have a type for
> > indexing into arrays.
> >
> >
> >
> > On Thu, Aug 7, 2014 at 1:16 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> >
> >  If it's strictly just in the codegen (and not affecting user code),
> > seems fine to me.
> >
> > Edward
> >
> > Excerpts from Johan Tibell's message of 2014-08-07 12:10:37 +0100:
> >
> > > Inside GHC we mostly use Int instead of Word, even when we want to
> > > represent non-negative values, such as sizes of things or indices into
> > > things. This is now causing some grief in
> > > https://ghc.haskell.org/trac/ghc/ticket/9416, where an allocation
> > boundary
> > > case test fails with a segfault because a n < m Int comparison overflows.
> > >
> > > I tried to fix the issue by changing the type of maxInlineAllocSize,
> > which
> > > is used on one side of the above comparison, to Word. However, that
> > > unravels a bunch of other issues, such as wordsToBytes, ByteOff, etc are
> > > all Int-valued quantities.
> > >
> > > I could perhaps work around these problems by judicious use of
> > fromIntegral
> > > in StgCmmPrim, but I'm a bit unhappy about it because it 1) makes the
> > code
> > > uglier and 2) needs to be done in quite a few places.
> > >
> > > How much work would it be to try to switch the codegen to use Word for
> > most
> > > of these quantities instead?
> > >
> > > -- Johan
> >
> >
> >