<div dir="ltr">I'll give that a try. The main use of the stg types in the stg-to-cmm pass is to call idPrimRep (which call typePrimRep) to figure out which register type we need to use. I guess as long as I rewrite the stg types so they give me the typePrimRep I want in the end I should be fine.</div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Sep 10, 2015 at 2:37 AM, Simon Peyton Jones <span dir="ltr"><<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal" style="margin-left:36.0pt">The problem is that stg is too strongly typed<span style="font-family:"Calibri",sans-serif">
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</span><p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">It’s not really typed, or at least only in a very half-hearted way. To be concrete I think you can just use Any for any Pointer arg. All STG needs to know, really,
is which things are pointers. Detailed type info like “are you a Char or a Bool” is strictly jam; indeed never used I think. (I could be wrong but I’m pretty sure I’m not wrong in a fundamental way.<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">SImon<u></u><u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Johan Tibell [mailto:<a href="mailto:johan.tibell@gmail.com" target="_blank">johan.tibell@gmail.com</a>]
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<b>Sent:</b> 09 September 2015 23:22<br>
<b>To:</b> Simon Peyton Jones; Simon Marlow; <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Converting unboxed sum types in StgCmm<u></u><u></u></span></p>
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Hi!<u></u><u></u></p>
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The original idea for implementing the backend part of the unboxed sums proposal was to convert from the core representation to the actual data representation (i.e. a tag followed by some pointer and non-pointer fields) in the
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unarise stg-to-stg</a> pass.<u></u><u></u></p>
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I have now realized that this won't work. The problem is that stg is too strongly typed. When we "desugar" sum types we need to convert functions receiving a value e.g. from<u></u><u></u></p>
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f :: (# Bool | Char #) -> ...<u></u><u></u></p>
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to<u></u><u></u></p>
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f :: NonPointer {-# tag#-} -> Pointer {-# Bool or Char #-} -> ...<u></u><u></u></p>
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Since stg is still typed with normal Haskell types (e.g. Bool, Char, etc), this is not possible, as we cannot represent an argument which has two different types.<u></u><u></u></p>
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It seems to me that we will have to do the conversion in the <a href="https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fgithub.com%2fghc%2fghc%2fblob%2fmaster%2fcompiler%2fcodeGen%2fStgCmm.hs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7cca7beffb01494517d75108d2b9652973%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=aXKZ78eGNKbJ6eZkxZgyJHgsAXpgOBjg3Zvqj%2bq7pk0%3d" target="_blank">
stg-to-cmm</a> pass, which is quite a bit more involved. For example, StgCmmEnv.idToReg function will have to change from<u></u><u></u></p>
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idToReg :: DynFlags -> NonVoid Id -> LocalReg<u></u><u></u></p>
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to<u></u><u></u></p>
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idToReg :: DynFlags -> NonVoid Id -> [LocalReg]<u></u><u></u></p>
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to accommodate the fact that we might need more than one register to store a binder.<u></u><u></u></p>
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Any ideas for a better solution?<u></u><u></u></p>
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-- Johan<u></u><u></u></p>
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