Hi Mike
| The right-hand argument of <+ leads to a dictionary construction that is a
| proof of a certain property, but the dictionary itself ends up being dead,
| like so:
|
| case $w$dOpCon_r2kGJ ...
| of
| { (# ww1_s27L3 #) -> ... }
| ^^^^^^^^^
| never used
GHC needs to figure out that $w$dOpCon_r2kGJ always terminates, without
throwing an exception etc. That can't be too hard, and Sebastian Graf has
thought about it quite a bit.
Could you offer a small repro case, with a pointer to evidence that it matters
in practice, and open a ticket?
Thanks
Simon
| -----Original Message-----
| From: Glasgow-haskell-users <glasgow-haskell-users-boun...@haskell.org> On
| Behalf Of Michael Sperber
| Sent: 06 August 2021 14:06
| To: glasgow-haskell-users@haskell.org
| Subject: Avoiding construction of dead dictionaries
|
| [You don't often get email from sper...@deinprogramm.de. Learn why this is
| important at http://aka.ms/LearnAboutSenderIdentification.]
|
| I have another optimization problem. ConCat includes this definition:
|
| (<+) :: Con a => (Con b => r) -> (a |- b) -> r r <+ Entail (Sub Dict) = r
|
| The right-hand argument of <+ leads to a dictionary construction that is a
| proof of a certain property, but the dictionary itself ends up being dead,
| like so:
|
| case $w$dOpCon_r2kGJ ...
| of
| { (# ww1_s27L3 #) -> ... }
| ^^^^^^^^^
| never used
|
| Yet, ghc (8.10.4) never elides this code. (I'm naively assuming because of
| the unboxed tuple, but actually have no clue.)
|
| Is there any chance of convincing ghc of erasing the dictionary
| construction?
|
| Help would be much appreciated!
|
| --
| Regards,
| Mike
|
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