An overhaul of the SRT representation

Summary:
- Previously we would hvae a single big table of pointers per module,
  with a set of bitmaps to reference entries within it. The new
  representation is identical to a static constructor, which is much
  simpler for the GC to traverse, and we get to remove the complicated
  bitmap-traversal code from the GC.

- Rewrite all the code to generate SRTs in CmmBuildInfoTables, and
  document it much better (see Note [SRTs]). This has been something
  I've wanted to do since we moved to the new code generator, I
  finally had the opportunity to finish it while on a transatlantic
  flight recently :)

There are a series of 4 diffs:

1. D4632 (this one), which does the bulk of the changes

2. D4633 which adds support for smaller `CmmLabelDiffOff` constants

3. D4634 which takes advantage of D4632 and D4633 to save a word in
   info tables that have an SRT on x86_64. This is where most of the
   binary size improvement comes from.

4. D4637 which makes a further optimisation to merge some SRTs with
   static FUN closures.  This adds some complexity and the benefits
   are fairly modest, so it's not clear yet whether we should do this.

Results (after (3), on x86_64)

- GHC itself (staticaly linked) is 5.2% smaller

- -1.7% binary sizes in nofib, -2.9% module sizes. Full nofib results: P176

- I measured the overhead of traversing all the static objects in a
  major GC in GHC itself by doing `replicateM_ 1000 performGC` as the
  first thing in `Main.main`.  The new version was 5-10% faster, but
  the results did vary quite a bit.

- I'm not sure if there's a compile-time difference, the results are
  too unreliable.

Test Plan: validate

Reviewers: bgamari, michalt, niteria, simonpj, erikd, osa1

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4632

1 files changed, 0 insertions, 9 deletions

diff --git a/compiler/stgSyn/CoreToStg.hs b/compiler/stgSyn/CoreToStg.hs
index 3ee3ba5cc4..e2ed3953ae 100644
--- a/compiler/stgSyn/CoreToStg.hs
+++ b/compiler/stgSyn/CoreToStg.hs
@@ -131,15 +131,6 @@ import Control.Monad (liftM, ap)
 --
 -- The CafInfo has already been calculated during the CoreTidy pass.
 --
--- During CoreToStg, we then pin onto each binding and case expression, a
--- list of Ids which represents the "live" CAFs at that point.  The meaning
--- of "live" here is the same as for live variables, see above (which is
--- why it's convenient to collect CAF information here rather than elsewhere).
---
--- The later SRT pass takes these lists of Ids and uses them to construct
--- the actual nested SRTs, and replaces the lists of Ids with (offset,length)
--- pairs.
-
 -- Note [What is a non-escaping let]
 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 --