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, 7 insertions, 13 deletions

diff --git a/compiler/main/HscMain.hs b/compiler/main/HscMain.hs
index 223886a1fc..9012025ece 100644
--- a/compiler/main/HscMain.hs
+++ b/compiler/main/HscMain.hs
@@ -1397,15 +1397,13 @@ hscCompileCmmFile hsc_env filename output_filename = runHsc hsc_env $ do
     let dflags = hsc_dflags hsc_env
     cmm <- ioMsgMaybe $ parseCmmFile dflags filename
     liftIO $ do
-        us <- mkSplitUniqSupply 'S'
-        let initTopSRT = initUs_ us emptySRT
         dumpIfSet_dyn dflags Opt_D_dump_cmm_verbose "Parsed Cmm" (ppr cmm)
-        (_, cmmgroup) <- cmmPipeline hsc_env initTopSRT cmm
-        rawCmms <- cmmToRawCmm dflags (Stream.yield cmmgroup)
         let -- Make up a module name to give the NCG. We can't pass bottom here
             -- lest we reproduce #11784.
             mod_name = mkModuleName $ "Cmm$" ++ FilePath.takeFileName filename
             cmm_mod = mkModule (thisPackage dflags) mod_name
+        (_, cmmgroup) <- cmmPipeline hsc_env (emptySRT cmm_mod) cmm
+        rawCmms <- cmmToRawCmm dflags (Stream.yield cmmgroup)
         _ <- codeOutput dflags cmm_mod output_filename no_loc NoStubs [] []
              rawCmms
         return ()
@@ -1456,21 +1454,17 @@ doCodeGen hsc_env this_mod data_tycons
         osSubsectionsViaSymbols (platformOS (targetPlatform dflags))
         = {-# SCC "cmmPipeline" #-}
           let run_pipeline us cmmgroup = do
-                let (topSRT', us') = initUs us emptySRT
-                (topSRT, cmmgroup) <- cmmPipeline hsc_env topSRT' cmmgroup
-                let srt | isEmptySRT topSRT = []
-                        | otherwise         = srtToData topSRT
-                return (us', srt ++ cmmgroup)
+                (_topSRT, cmmgroup) <-
+                  cmmPipeline hsc_env (emptySRT this_mod) cmmgroup
+                return (us, cmmgroup)
 
           in do _ <- Stream.mapAccumL run_pipeline us ppr_stream1
                 return ()
 
       | otherwise
         = {-# SCC "cmmPipeline" #-}
-          let initTopSRT = initUs_ us emptySRT
-              run_pipeline = cmmPipeline hsc_env
-          in do topSRT <- Stream.mapAccumL run_pipeline initTopSRT ppr_stream1
-                Stream.yield (srtToData topSRT)
+          let run_pipeline = cmmPipeline hsc_env
+          in void $ Stream.mapAccumL run_pipeline (emptySRT this_mod) ppr_stream1
 
     let
         dump2 a = do dumpIfSet_dyn dflags Opt_D_dump_cmm