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, 54 insertions, 1 deletions

diff --git a/rts/StgMiscClosures.cmm b/rts/StgMiscClosures.cmm
index 9fd5fb8b88..c307293cce 100644
--- a/rts/StgMiscClosures.cmm
+++ b/rts/StgMiscClosures.cmm
@@ -518,7 +518,60 @@ CLOSURE(stg_END_STM_CHUNK_LIST_closure,stg_END_STM_CHUNK_LIST);
 CLOSURE(stg_NO_TREC_closure,stg_NO_TREC);
 
 /* ----------------------------------------------------------------------------
-   Messages
+   SRTs
+
+   See Note [SRTs] in compiler/cmm/CmmBuildInfoTable.hs
+   ------------------------------------------------------------------------- */
+
+INFO_TABLE_CONSTR(stg_SRT_1, 1, 0, 0, CONSTR, "SRT_1", "SRT_1")
+{ foreign "C" barf("SRT_1 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_2, 2, 0, 0, CONSTR, "SRT_2", "SRT_2")
+{ foreign "C" barf("SRT_2 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_3, 3, 0, 0, CONSTR, "SRT_3", "SRT_3")
+{ foreign "C" barf("SRT_3 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_4, 4, 0, 0, CONSTR, "SRT_4", "SRT_4")
+{ foreign "C" barf("SRT_4 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_5, 5, 0, 0, CONSTR, "SRT_5", "SRT_5")
+{ foreign "C" barf("SRT_5 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_6, 6, 0, 0, CONSTR, "SRT_6", "SRT_6")
+{ foreign "C" barf("SRT_6 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_7, 7, 0, 0, CONSTR, "SRT_7", "SRT_7")
+{ foreign "C" barf("SRT_7 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_8, 8, 0, 0, CONSTR, "SRT_8", "SRT_8")
+{ foreign "C" barf("SRT_8 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_9, 9, 0, 0, CONSTR, "SRT_9", "SRT_9")
+{ foreign "C" barf("SRT_9 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_10, 10, 0, 0, CONSTR, "SRT_10", "SRT_10")
+{ foreign "C" barf("SRT_10 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_11, 11, 0, 0, CONSTR, "SRT_11", "SRT_11")
+{ foreign "C" barf("SRT_11 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_12, 12, 0, 0, CONSTR, "SRT_12", "SRT_12")
+{ foreign "C" barf("SRT_12 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_13, 13, 0, 0, CONSTR, "SRT_13", "SRT_13")
+{ foreign "C" barf("SRT_13 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_14, 14, 0, 0, CONSTR, "SRT_14", "SRT_14")
+{ foreign "C" barf("SRT_14 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_15, 15, 0, 0, CONSTR, "SRT_15", "SRT_15")
+{ foreign "C" barf("SRT_15 object entered!") never returns; }
+
+INFO_TABLE_CONSTR(stg_SRT_16, 16, 0, 0, CONSTR, "SRT_16", "SRT_16")
+{ foreign "C" barf("SRT_16 object entered!") never returns; }
+
+/* ---------------------------------------------------------------------------   Messages
    ------------------------------------------------------------------------- */
 
 // PRIM rather than CONSTR, because PRIM objects cannot be duplicated by the GC.