Improving the Int/Word story inside GHC

[Johan Tibell]

I guess this example, from mk_switch in StgCmmUtils, is the same

    return (mkSwitch (cmmOffset dflags tag_expr (- real_lo_tag)) arms)

?

(This is clearly a negative offset and I don't know the implications of the
Cmm code we output if we switch to ByteOff = Word)


On Thu, Aug 7, 2014 at 3:49 PM, Edward Z. Yang <ezyang at mit.edu> wrote:

> 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
> > >
> > >
> > >
>
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