Hmm. It’s not clear to me why we have *any* limitation on the size of unboxed
tuples.
For boxed tuples I know: we need an info table, code etc. But not so for
unboxed.
Why do we need a limit at all?
Simon
From: ghc-devs <ghc-devs-boun...@haskell.org> On Behalf Of Ryan Scott
Sent: 25 September 2020 23:21
To: GHC developers <ghc-devs@haskell.org>
Subject: How is GHC.Prim.unpackInt8X64# meant to be used?
GHC imposes an upper bound of 62 on the size of tuples one can use [1].
Currently, this upper bound applies to both boxed and unboxed tuples alike. For
example, if you try to create an unboxed tuple of arity 64, then GHC will throw
an error:
error:
A 64-tuple is too large for GHC
(max size is 62)
Workaround: use nested tuples or define a data type
|
| f = (#,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,#)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
However, I discovered recently that there are places where GHC *does* use
unboxed tuples with arity greater than 62. For example, the
GHC.Prim.unpackInt8X64# [2] function returns an unboxed tuple of size 64. I was
confused for a while about how this was even possible, but I realized later
than GHC only enforces the tuple size limit in expressions and patterns [3].
Simply having a type signature with a large unboxed tuple is fine in and of
itself, and since unpackInt8X64# is implemented as a primop, no large unboxed
tuples are ever used in the "body" of the function. (Indeed, primops don't have
function bodies in the conventional sense.) Other functions in GHC.Prim that
use unboxed tuples of arity 64 include unpackWord8X64# [4], packInt8X64# [5],
and packWord8X64# [6].
But this makes me wonder: how on earth is it even possible to *use*
unpackInt8X64#? The only thing that comes to mind is to pattern-match on the
unboxed tuple that it returns, but if you try to do that, you'll get a
"64-tuple is too large for GHC" error like shown above. What's more, I can't
find any test cases in the GHC test suite that show how unpackInt8X64# or its
cousins are supposed to be used. What am I missing? The comments for these
functions mention "Warning: this is only available on LLVM", so perhaps someone
familiar with the LLVM backend knows what the answer is?
For some context, this came up in !4097, where I am attempting to extend the
"too large for GHC" error message to include types, in addition to expressions
and patterns [7]. However, doing so causes Haddock to error when processing
unpackInt8X64# et al.
Ryan S.
-----
[1]
https://gitlab.haskell.org/ghc/ghc/-/blob/a1f34d37b47826e86343e368a5c00f1a4b1f2bce/compiler/GHC/Settings/Constants.hs#L13-15<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fblob%2Fa1f34d37b47826e86343e368a5c00f1a4b1f2bce%2Fcompiler%2FGHC%2FSettings%2FConstants.hs%23L13-15&data=02%7C01%7Csimonpj%40microsoft.com%7Ca2fa232c729f4943412a08d861a170e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637366699218050286&sdata=LAb06t4YCkyQgSfp3si07unb0sEW3vE7pEHRbzUxKZU%3D&reserved=0>
[2]
https://hackage.haskell.org/package/ghc-prim-0.6.1/docs/GHC-Prim.html#v:unpackInt8X64-35-<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhackage.haskell.org%2Fpackage%2Fghc-prim-0.6.1%2Fdocs%2FGHC-Prim.html%23v%3AunpackInt8X64-35-&data=02%7C01%7Csimonpj%40microsoft.com%7Ca2fa232c729f4943412a08d861a170e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637366699218060277&sdata=eC52wb5dTfzVVAgJLMc428av%2FDgY9NyqyMCROfkD6ro%3D&reserved=0>
[3]
https://gitlab.haskell.org/ghc/ghc/-/issues/18723<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fissues%2F18723&data=02%7C01%7Csimonpj%40microsoft.com%7Ca2fa232c729f4943412a08d861a170e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637366699218070276&sdata=VRucYwSTbGD1iD9KhJst2ZTmsuZ%2BQdwKTyivnTqKBm4%3D&reserved=0>
[4]
https://hackage.haskell.org/package/ghc-prim-0.6.1/docs/GHC-Prim.html#v:unpackWord8X64-35-<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhackage.haskell.org%2Fpackage%2Fghc-prim-0.6.1%2Fdocs%2FGHC-Prim.html%23v%3AunpackWord8X64-35-&data=02%7C01%7Csimonpj%40microsoft.com%7Ca2fa232c729f4943412a08d861a170e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637366699218070276&sdata=3naSYWb5y2SpznOO0y4ip9Pd3hzzmbSuBWViCPbM%2FAU%3D&reserved=0>
[5]
https://hackage.haskell.org/package/ghc-prim-0.6.1/docs/GHC-Prim.html#v:packInt8X64-35-<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhackage.haskell.org%2Fpackage%2Fghc-prim-0.6.1%2Fdocs%2FGHC-Prim.html%23v%3ApackInt8X64-35-&data=02%7C01%7Csimonpj%40microsoft.com%7Ca2fa232c729f4943412a08d861a170e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637366699218080272&sdata=QpdcjMe1gN91NJAPzTjdksFFXR3sibl5bZqhd23f6Hc%3D&reserved=0>
[6]
https://hackage.haskell.org/package/ghc-prim-0.6.1/docs/GHC-Prim.html#v:packWord8X64-35-<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhackage.haskell.org%2Fpackage%2Fghc-prim-0.6.1%2Fdocs%2FGHC-Prim.html%23v%3ApackWord8X64-35-&data=02%7C01%7Csimonpj%40microsoft.com%7Ca2fa232c729f4943412a08d861a170e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637366699218090268&sdata=x9%2FqCgRRALtH4%2B7wTK9hqxk%2FrhIqSs%2BPwIk%2BjQlljsY%3D&reserved=0>
[7]
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4097#note_301191<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fmerge_requests%2F4097%23note_301191&data=02%7C01%7Csimonpj%40microsoft.com%7Ca2fa232c729f4943412a08d861a170e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637366699218090268&sdata=oGUSsN7bwBLV9p7MchpSfQH4dtjWm6apaumIM1IGg2Q%3D&reserved=0>
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