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Re: let-bindings floating more easily, would a check for `exprIsOkForSpeculation scrut` in theory suffice to float out cases?<o:p></o:p></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">Yes; but instead we simply turn such cases into lets.<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"> Sebastian Graf [mailto:sgraf1337@gmail.com]
<br>
<b>Sent:</b> 02 November 2017 08:33<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: 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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Thanks both of you, that makes sense.<o:p></o:p></p>
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<o:p> </o:p></p>
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Re: let-bindings floating more easily, would a check for `exprIsOkForSpeculation scrut` in theory suffice to float out cases?<o:p></o:p></p>
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<o:p> </o:p></p>
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On Mon, Oct 30, 2017 at 10:56 AM, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">See Note [CoreSyn let/app invariant] in CoreSyn.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Briefly, you can write</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">            let x::Int# = e in …</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">if e is “ok-for-speculation”.  See extensive comments in CoreUtils on what that means.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><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.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Simon</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>]
<b>On Behalf Of </b>Sebastian Graf<br>
<b>Sent:</b> 29 October 2017 21:07<br>
<b>To:</b> ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> When does GHC produce unlifted `let` bindings?</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">Hi folks,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">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" target="_blank">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" target="_blank">
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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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" target="_blank">
Some GHC passes seem to agree.</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Cheers,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Sebastian<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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