Source notes (Core support)

This patch introduces "SourceNote" tickishs that link Core to the
source code that generated it. The idea is to retain these source code
links throughout code transformations so we can eventually relate
object code all the way back to the original source (which we can,
say, encode as DWARF information to allow debugging).  We generate
these SourceNotes like other tickshs in the desugaring phase. The
activating command line flag is "-g", consistent with the flag other
compilers use to decide DWARF generation.

Keeping ticks from getting into the way of Core transformations is
tricky, but doable. The changes in this patch produce identical Core
in all cases I tested -- which at this point is GHC, all libraries and
nofib. Also note that this pass creates *lots* of tick nodes, which we
reduce somewhat by removing duplicated and overlapping source
ticks. This will still cause significant Tick "clumps" - a possible
future optimization could be to make Tick carry a list of Tickishs
instead of one at a time.

(From Phabricator D169)

1 files changed, 2 insertions, 0 deletions

diff --git a/compiler/simplCore/SetLevels.hs b/compiler/simplCore/SetLevels.hs
index e7000409e7..c3ee112de4 100644
--- a/compiler/simplCore/SetLevels.hs
+++ b/compiler/simplCore/SetLevels.hs
@@ -592,6 +592,7 @@ notWorthFloating e abs_vars
     go (_, AnnVar {}) n    = n >= 0
     go (_, AnnLit lit) n   = ASSERT( n==0 )
                              litIsTrivial lit   -- Note [Floating literals]
+    go (_, AnnTick t e) n  = not (tickishIsCode t) && go e n
     go (_, AnnCast e _)  n = go e n
     go (_, AnnApp e arg) n
        | (_, AnnType {}) <- arg = go e n
@@ -606,6 +607,7 @@ notWorthFloating e abs_vars
     is_triv (_, AnnCast e _)              = is_triv e
     is_triv (_, AnnApp e (_, AnnType {})) = is_triv e
     is_triv (_, AnnApp e (_, AnnCoercion {})) = is_triv e
+    is_triv (_, AnnTick t e)              = not (tickishIsCode t) && is_triv e
     is_triv _                             = False
 
 {-