Improving the Int/Word story inside GHC
johan.tibell at gmail.com
Thu Aug 7 13:45:43 UTC 2014
I'm hacking on this now. I'm not 100% sure that ByteOff isn't used for
negative values though, see for example
:: 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))
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>
> I’m all for it!
> I believe that ByteOff/WordOff are always 0 or positive. At least, they
> were when I introduced them!
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Johan
> *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.
> 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
> > 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,
> > 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
> > in StgCmmPrim, but I'm a bit unhappy about it because it 1) makes the
> > 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
> > of these quantities instead?
> > -- Johan
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