OccurAnal: Ensure SourceNotes don't interfere with join-point analysis

In general ticks are problematic for join point analysis as described
in #14242.  However, source notes are intended to be a best-effort
annotation which shouldn't interfere with optimization. Special-case
these to ensure that tail-call information is still correct, even in the
presence of source note
ticks.

Test Plan: Validate

Reviewers: simonpj, austin

Reviewed By: simonpj

Subscribers: rwbarton, thomie

GHC Trac Issues: #14242

Differential Revision: https://phabricator.haskell.org/D3978

1 files changed, 7 insertions, 0 deletions

diff --git a/compiler/simplCore/OccurAnal.hs b/compiler/simplCore/OccurAnal.hs
index dace6a04c2..0c237cabe0 100644
--- a/compiler/simplCore/OccurAnal.hs
+++ b/compiler/simplCore/OccurAnal.hs
@@ -1723,6 +1723,12 @@ we can sort them into the right place when doing dependency analysis.
 -}
 
 occAnal env (Tick tickish body)
+  | SourceNote{} <- tickish
+  = (usage, Tick tickish body')
+                  -- SourceNotes are best-effort; so we just proceed as usual.
+                  -- If we drop a tick due to the issues described below it's
+                  -- not the end of the world.
+
   | tickish `tickishScopesLike` SoftScope
   = (markAllNonTailCalled usage, Tick tickish body')
 
@@ -1742,6 +1748,7 @@ occAnal env (Tick tickish body)
                   -- Making j a join point may cause the simplifier to drop t
                   -- (if the tick is put into the continuation). So we don't
                   -- count j 1 as a tail call.
+                  -- See #14242.
 
 occAnal env (Cast expr co)
   = case occAnal env expr of { (usage, expr') ->