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<p class="MsoNormal"><span style="font-size:12.0pt">See Note [CoreSyn let/app invariant] in CoreSyn.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Briefly, you can write<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> let x::Int# = e in …<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">if e is “ok-for-speculation”. See extensive comments in CoreUtils on what that means.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">You could also use case, but let-bindings “float” more easily than cases, because they are not worried about preserving exception behaviour.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Sebastian Graf<br>
<b>Sent:</b> 29 October 2017 21:07<br>
<b>To:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> When does GHC produce unlifted `let` bindings?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Hi folks,<o:p></o:p></p>
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<o:p> </o:p></p>
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I was debugging a Core-to-Core transform for TEST=<a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Fblob%2Ffe04f3783b662c52c4a0ff36b2d62a7a575998a5%2Ftestsuite%2Ftests%2FsimplCore%2Fshould_compile%2Fspec-inline.hs&data=02%7C01%7Csimonpj%40microsoft.com%7C77699e26801a4d458b9a08d51f11085f%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636449080385955811&sdata=t%2BAXQszqKfFRYNjjLJtMHpLsS2RzwTaBFA42JuLBN4o%3D&reserved=0">spec-inline</a> and
was wondering (yet again) why GHC produces unlifted `let` bindings in Core like <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Fblob%2Ffe04f3783b662c52c4a0ff36b2d62a7a575998a5%2Ftestsuite%2Ftests%2FsimplCore%2Fshould_compile%2Fspec-inline.stderr%23L68-L76&data=02%7C01%7Csimonpj%40microsoft.com%7C77699e26801a4d458b9a08d51f11085f%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636449080385955811&sdata=pVNrzDY%2ByfK%2FVl%2Fk8B5P0HuGC6PG5kD7mHiwa0Vck9o%3D&reserved=0">
it seems supposed to be doing</a>.<o:p></o:p></p>
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Why doesn't this use `case` instead? <o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l0 level1 lfo1">
Is there a semantic difference? <o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l0 level1 lfo1">
Can `case` be used with unlifted types? <o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l0 level1 lfo1">
And if not, why can `let`?<o:p></o:p></li></ul>
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<p class="MsoNormal">Unlifted `let` seems much close to `case` than to `let`. <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Fsearch%3Fl%3DHaskell%26q%3D%2522strict%2Blet%2522%26type%3D%26utf8%3D%25E2%259C%2593&data=02%7C01%7Csimonpj%40microsoft.com%7C77699e26801a4d458b9a08d51f11085f%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636449080385955811&sdata=ksdZGbGyw6%2FLEIMiJpCaHlDh4iipdkKcSCEDdTySExk%3D&reserved=0">
Some GHC passes seem to agree.</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Cheers,<o:p></o:p></p>
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<p class="MsoNormal">Sebastian<o:p></o:p></p>
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