Improving the Int/Word story inside GHC

[Simon Peyton Jones]

When I introduced them in the first place they were used for positive offsets within StackAreas and heap objects.  Both are organised with the zeroth byte of the stack area or heap object being at the lowest address.

It's true that a positive offset from the beginning of a block of contiguous freshly-allocated heap objects will turn into a negative displacement from the actual, physical heap pointer.  If ByteOff is used for both purpose then yes there will be negative ones.

More than that I cannot say.  They may well be being used for other purposes by now.

One thought is that the profiling word appears just *before* the start of a heap object, so that might need a negative offset, but it seems like a rather special case.

Simon

| -----Original Message-----
| From: Simon Marlow [mailto:marlowsd at gmail.com]
| Sent: 07 August 2014 15:26
| To: Simon Peyton Jones; Johan Tibell
| Cc: ghc-devs at haskell.org
| Subject: Re: Improving the Int/Word story inside GHC
| 
| Hmm, surely these are used for negative offsets a lot?  All Hp-relative
| indices are negative (but virtual Hp offsets are positive), and Sp-
| relative indices can be both negative and positive.
| 
| On 07/08/2014 12:49, Simon Peyton Jones 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
| > <mailto: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
| >