Lift constructor tag allocation out of a loop

Authored by niteria on Jan 5 2018, 9:20 AM.

Description

Lift constructor tag allocation out of a loop

Before this change, for each constructor that we want
to allocate a tag for we would traverse a list of all
the constructors in a datatype to determine which tag
a constructor should get.

This is obviously quadratic and for datatypes with 10k
constructors it actually makes a big difference.

This change implements the plan outlined by @simonpj in
https://mail.haskell.org/pipermail/ghc-devs/2017-October/014974.html
which is basically about using a map and constructing it outside the
loop.

One place where things got a bit awkward was TysWiredIn.hs,
it would have been possible to just assign the tags by hand, but
that seemed error-prone to me, so I decided to go through a map
there as well.

Test Plan:
./validate
On a file with 10k constructors
Before:

 8,130,522,344 bytes allocated in the heap
Total   time    3.682s  (  3.920s elapsed)

After:

 4,133,478,744 bytes allocated in the heap
Total   time    2.509s  (  2.750s elapsed)

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: goldfire, rwbarton, thomie, simonmar, carter, simonpj

GHC Trac Issues: Trac #14657

Differential Revision: https://phabricator.haskell.org/D4289