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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;mso-fareast-language:EN-US">
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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">SImon<o:p></o:p></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:johan.tibell@gmail.com]
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<b>Sent:</b> 09 September 2015 23:22<br>
<b>To:</b> Simon Peyton Jones; Simon Marlow; ghc-devs@haskell.org<br>
<b>Subject:</b> Converting unboxed sum types in StgCmm<o:p></o:p></span></p>
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Hi!<o:p></o:p></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
<a href="https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fgithub.com%2fghc%2fghc%2fblob%2fmaster%2fcompiler%2fsimplStg%2fUnariseStg.hs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7cca7beffb01494517d75108d2b9652973%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=U%2bFUNsL87iEemajTnAW9SxD9N5b4%2bG8QB1q19%2fX%2bBI4%3d">
unarise stg-to-stg</a> pass.<o:p></o:p></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<o:p></o:p></p>
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f :: (# Bool | Char #) -> ...<o:p></o:p></p>
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to<o:p></o:p></p>
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f :: NonPointer {-# tag#-} -> Pointer {-# Bool or Char #-} -> ...<o:p></o:p></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.<o:p></o:p></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">
stg-to-cmm</a> pass, which is quite a bit more involved. For example, StgCmmEnv.idToReg function will have to change from<o:p></o:p></p>
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idToReg :: DynFlags -> NonVoid Id -> LocalReg<o:p></o:p></p>
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to<o:p></o:p></p>
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idToReg :: DynFlags -> NonVoid Id -> [LocalReg]<o:p></o:p></p>
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to accommodate the fact that we might need more than one register to store a binder.<o:p></o:p></p>
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Any ideas for a better solution?<o:p></o:p></p>
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-- Johan<o:p></o:p></p>
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