Can we mark DataCon ptrs only in allocation sites and not generate entry code?

[Ömer Sinan Ağacan]

> StgCmmCon.hs:240 reads
>              ; return (mkRhsInit dflags reg lf_info hp_plus_n) }
>
> what's that got to do with pointer tagging?

If I understand correctly, mkRhsInit returns a tagged pointer:

    mkRhsInit :: DynFlags -> LocalReg -> LambdaFormInfo -> CmmExpr -> CmmAGraph
    mkRhsInit dflags reg lf_info expr
      = mkAssign (CmmLocal reg) (addDynTag dflags expr (lfDynTag
dflags lf_info))

> But also we clearly must do so in the entry code for a data constructor

Why? It's not clear to me. If every pointer to a constructor is tagged then
maybe we don't need to enter constructors at all.

2016-08-25 15:41 GMT+00:00 Simon Peyton Jones <simonpj at microsoft.com>:
> StgCmmCon.hs:240 reads
>              ; return (mkRhsInit dflags reg lf_info hp_plus_n) }
>
> what's that got to do with pointer tagging?
>
>
> But yes we need to do it in both places.  Consider
>
> f x xs = let y = x:xs
>          in g y
>
> We should tag y before passing it to g.  That's the StgCmmCon case.
>
>
> But also we clearly must do so in the entry code for a data constructor
>
>
> Simon
>
> |  -----Original Message-----
> |  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ömer
> |  Sinan Agacan
> |  Sent: 25 August 2016 11:46
> |  To: ghc-devs <ghc-devs at haskell.org>
> |  Subject: Can we mark DataCon ptrs only in allocation sites and not
> |  generate entry code?
> |
> |  As far as I can see in the native code compiler we mark DataCon
> |  pointers in two places:
> |
> |  1. In allocation sites (StgCmmCon.hs:240)
> |  2. In DataCon entry code (StgCmm.hs:244)
> |
> |  I was wondering why we can't get away with just doing (1). Can anyone
> |  give me
> |  an example where an allocation doesn't return a tagged pointer and we
> |  need to
> |  tag it in entry code? If every allocation returns a tagged pointer,
> |  then why do
> |  we need (2) ?
> |
> |  Thanks
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