Improving the Int/Word story inside GHC

[Johan Tibell]

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
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/ghc-devs/attachments/20140807/0e15190a/attachment.html>