Reduce non-determinism in ABI hashes with RULES and instance decls

Summary:
Before this change the `RULES` would be attached to one for the names from
the module that appear on the left hand side. The choice depended on the
`uniq` that was generated, which are known to be non-deterministic (a
separate, bigger problem). Now we use `OccName`s which should be stable.

Analogously for instance declarations, but they are attached to one of
the types involved.

Test Plan:
contbuild
it made `Data.Text.Internal.Fusion.Common` interface stable, previously
stream fusion rule would be attached either to `streamList` or
`unstreamList` depending on if the module was compiled after `cabal
clean` or after `find | grep '\.o$' | xargs rm`.

Reviewers: simonpj, austin, bgamari, simonmar

Subscribers: puffnfresh, thomie

Differential Revision: https://phabricator.haskell.org/D1073

GHC Trac Issues: #4012

2 files changed, 19 insertions, 6 deletions

diff --git a/compiler/specialise/Rules.hs b/compiler/specialise/Rules.hs
index cb71e3a3f3..7cf6e56bb7 100644
--- a/compiler/specialise/Rules.hs
+++ b/compiler/specialise/Rules.hs
@@ -180,9 +180,15 @@ mkRule this_mod is_auto is_local name act fn bndrs args rhs
     lhs_names = nameSetElems (extendNameSet (exprsOrphNames args) fn)
         -- TODO: copied from ruleLhsOrphNames
 
-    orph = case filter (nameIsLocalOrFrom this_mod) lhs_names of
-                        (n : _) -> NotOrphan (nameOccName n)
-                        []      -> IsOrphan
+        -- Since rules get eventually attached to one of the free names
+        -- from the definition when compiling the ABI hash, we should make
+        -- it deterministic. This chooses the one with minimal OccName
+        -- as opposed to uniq value.
+    local_lhs_names = filter (nameIsLocalOrFrom this_mod) lhs_names
+    anchor = minimum $ map nameOccName local_lhs_names
+    orph = case local_lhs_names of
+             (_ : _) -> NotOrphan anchor
+             []      -> IsOrphan
 
 --------------
 roughTopNames :: [CoreExpr] -> [Maybe Name]
diff --git a/compiler/types/InstEnv.hs b/compiler/types/InstEnv.hs
index f810850d4b..db8f531a58 100644
--- a/compiler/types/InstEnv.hs
+++ b/compiler/types/InstEnv.hs
@@ -236,10 +236,17 @@ mkLocalInstance dfun oflag tvs cls tys
     do_one (_ltvs, rtvs) = choose_one [ns | (tv,ns) <- cls_tvs `zip` arg_names
                                           , not (tv `elem` rtvs)]
 
+    -- Since instance declarations get eventually attached to one of the types
+    -- from the definition when compiling the ABI hash, we should make
+    -- it deterministic. This chooses the one with minimal OccName
+    -- as opposed to uniq value.
     choose_one :: [NameSet] -> IsOrphan
-    choose_one nss = case nameSetElems (unionNameSets nss) of
-                        []      -> IsOrphan
-                        (n : _) -> NotOrphan (nameOccName n)
+    choose_one nss = case local_names of
+                       []      -> IsOrphan
+                       (_ : _) -> NotOrphan anchor
+      where
+      local_names = nameSetElems (unionNameSets nss)
+      anchor = minimum $ map nameOccName local_names
 
 mkImportedInstance :: Name
                    -> [Maybe Name]