When does GHC produce unlifted `let` bindings?

[Sebastian Graf]

Thanks both of you, that makes sense.

Re: let-bindings floating more easily, would a check for
`exprIsOkForSpeculation scrut` in theory suffice to float out cases?

On Mon, Oct 30, 2017 at 10:56 AM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> See Note [CoreSyn let/app invariant] in CoreSyn.
>
>
>
> Briefly, you can write
>
>             let x::Int# = e in …
>
> if e is “ok-for-speculation”.  See extensive comments in CoreUtils on what
> that means.
>
>
>
> You could also use case, but let-bindings “float” more easily than cases,
> because they are not worried about preserving exception behaviour.
>
>
>
> Simon
>
>
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Sebastian
> Graf
> *Sent:* 29 October 2017 21:07
> *To:* ghc-devs <ghc-devs at haskell.org>
> *Subject:* When does GHC produce unlifted `let` bindings?
>
>
>
> Hi folks,
>
>
>
> I was debugging a Core-to-Core transform for TEST=spec-inline
> <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> and
> was wondering (yet again) why GHC produces unlifted `let` bindings in Core
> like it seems supposed to be doing
> <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>
> .
>
>    - Why doesn't this use `case` instead?
>    - Is there a semantic difference?
>    - Can `case` be used with unlifted types?
>    - And if not, why can `let`?
>
> Unlifted `let` seems much close to `case` than to `let`. Some GHC passes
> seem to agree.
> <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>
>
>
>
> Cheers,
>
> Sebastian
>
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