Maximum sharing: reuse sharing and detect cycles

Summary:
The previous version had a potentially exponential behavior on values with already lots of sharing.
This is fixed here at the price of a multiplicative constant factor (cost of `Hashtbl.hash`).
It also prepares for the handling of cycles.

Reviewed By: ngorogiannis

Differential Revision: D16016906

fbshipit-source-id: 611287917

infer/src/istd/MaximumSharing.ml

@@ -7,6 +7,8 @@
 
 open! IStd
 
+exception MaximumSharingCyclicValue
+
 exception MaximumSharingLazyValue
 
 module Hashing : sig
@@ -16,7 +18,8 @@ module Hashing : sig
 
   val of_int : int -> hash_value
 
-  val of_no_scan_block : 'a -> hash_value
+  val shallow : 'a -> hash_value
+  (** A deterministic hash function that visits O(1) objects, to ensure termination on cyclic values. *)
 
   val alloc_of_block : tag:int -> size:int -> state
 
@@ -30,7 +33,13 @@ end = struct
 
   let of_int = Fn.id
 
-  let of_no_scan_block = Caml.Hashtbl.hash
+  let shallow =
+    (*
+      [hash x] is defined as [seeded_hash_param 10 100 0 x].
+      Here we don't care about the specific numbers as long as the function is deterministic.
+    *)
+    Caml.Hashtbl.hash
+
 
   let alloc_of_block ~tag ~size =
     let state = Hash.alloc () in
@@ -51,77 +60,134 @@ module Sharer : sig
 
   val normalize_value : t -> 'a -> 'a
 end = struct
-  module HashedNormalizedObj = struct
-    type t = Hashing.hash_value * Obj.t
+  let hashed_obj eq =
+    ( module struct
+      type t = Hashing.hash_value * Obj.t
+
+      let hash ((h, _) : t) = (h :> int)
+
+      let equal ((h1, o1) : t) ((h2, o2) : t) = Int.equal (h1 :> int) (h2 :> int) && eq o1 o2
+    end
+    : Caml.Hashtbl.HashedType
+      with type t = Hashing.hash_value * Obj.t )
+
+
+  module HashedNoscanBlock = (val hashed_obj Polymorphic_compare.equal)
 
-    let equal ((h1, o1) : t) ((h2, o2) : t) =
-      Int.equal (h1 :> int) (h2 :> int)
-      &&
-      if Obj.tag o1 >= Obj.no_scan_tag then Polymorphic_compare.equal o1 o2
-      else PhysEqual.shallow_equal o1 o2
+  module PhysEqualedHashedScannableBlock = (val hashed_obj phys_equal)
 
+  module HashedNormalizedScannableBlock = (val hashed_obj PhysEqual.shallow_equal)
 
-    let hash ((h, _) : t) = (h :> int)
-  end
+  module HNoscan = Caml.Hashtbl.Make (HashedNoscanBlock)
+  module HPhysEq = Caml.Hashtbl.Make (PhysEqualedHashedScannableBlock)
+  module HNorm = Caml.Hashtbl.Make (HashedNormalizedScannableBlock)
 
-  module H = Caml.Hashtbl.Make (HashedNormalizedObj)
+  type visited = Visiting | Normalized of HashedNormalizedScannableBlock.t
 
   type t =
     { inplace: bool
           (** Uses [Obj.set_field] on possibly immutable values, hence should be avoided when used with flambda *)
-    ; hash_normalized: HashedNormalizedObj.t H.t
+    ; noscan_blocks: HashedNoscanBlock.t HNoscan.t
+    ; visited_blocks: visited HPhysEq.t
+    ; hash_normalized: HashedNormalizedScannableBlock.t HNorm.t
     ; fail_on_forward: bool
     ; fail_on_nonstring: bool
     ; fail_on_objects: bool }
 
   let create () =
     { inplace= false
-    ; hash_normalized= H.create 1
+    ; noscan_blocks= HNoscan.create 1
+    ; visited_blocks= HPhysEq.create 1
+    ; hash_normalized= HNorm.create 1
     ; (* these are just for safety because the code hasn't been tested on them, it should work fine though *)
       fail_on_forward= true
     ; fail_on_nonstring= true
     ; fail_on_objects= true }
 
 
-  let normalize_block sharer hash block =
-    let hash_block = (hash, block) in
-    match H.find_opt sharer.hash_normalized hash_block with
-    | Some hash_normalized ->
-        hash_normalized
-    | None ->
-        H.add sharer.hash_normalized hash_block hash_block ;
-        hash_block
-
-
   let hash_and_normalize_int o = (Hashing.of_int (Obj.obj o : int), o)
 
   (* 
-  TODO: currently the function explores the graph without sharing.  It is possible to build values
-  on which this will behave exponentially.  This could be avoided by keeping a hashtbl of visited
-  values.  But it would be much more efficient to write it in C to be able to use the GC flags to
-  mark visited values.
+    TODO: be much more efficient and write it in C to be able to use the GC flags to
+    mark visited values.
    *)
   let rec hash_and_normalize_obj sharer o =
     if Obj.is_int o then hash_and_normalize_int o else hash_and_normalize_block sharer o
 
 
   and hash_and_normalize_block sharer block =
+    let shallow_hash = Hashing.shallow block in
+    let shallow_hash_block = (shallow_hash, block) in
     let tag = Obj.tag block in
     if tag >= Obj.no_scan_tag then (
+      (*
+        No-scan blocks (strings, int64, closures, weird stuff) are treated separately.
+        They are hashed and compared using the Stdlib polymorphic functions.
+      *)
       assert ((not sharer.fail_on_nonstring) || Int.equal tag Obj.string_tag) ;
-      hash_and_normalize_no_scan_block sharer block )
-    else if Int.equal tag Obj.forward_tag then (
-      assert (not sharer.fail_on_forward) ;
-      hash_and_normalize_obj sharer (Obj.field block 0) )
-    else if Int.equal tag Obj.lazy_tag then raise MaximumSharingLazyValue
-    else (
-      assert ((not sharer.fail_on_objects) || not (Int.equal tag Obj.object_tag)) ;
-      let size = Obj.size block in
-      hash_and_normalize_block_fields sharer block block size 0 (Hashing.alloc_of_block ~tag ~size) )
+      match HNoscan.find_opt sharer.noscan_blocks shallow_hash_block with
+      | Some hash_normalized ->
+          hash_normalized
+      | None ->
+          HNoscan.add sharer.noscan_blocks shallow_hash_block shallow_hash_block ;
+          shallow_hash_block )
+    else if Int.equal tag Obj.lazy_tag then
+      (*
+        For now MaximumSharing is used before marshalling.
+        It makes little sense to marshal lazy values. 
+        Because lazy blocks are normal scannable blocks, this special case could be safely removed.
+      *)
+      raise MaximumSharingLazyValue
+    else
+      (*
+        This is where we could win by mutating the value directly.
+        Instead we need to use a hashtbl using a shallow hash (for termination), which adds a
+        multiplicative factor to the running time.
+      *)
+      match HPhysEq.find_opt sharer.visited_blocks shallow_hash_block with
+      | Some (Normalized hash_normalized) ->
+          (* The block has already been visited, we can reuse the result. *)
+          hash_normalized
+      | Some Visiting ->
+          (*
+            The block is being visited, which means we have a cycle.
+          *)
+          raise MaximumSharingCyclicValue
+      | None ->
+          HPhysEq.add sharer.visited_blocks shallow_hash_block Visiting ;
+          let hash_normalized =
+            if Int.equal tag Obj.forward_tag then (
+              assert (not sharer.fail_on_forward) ;
+              (*
+                Forward_tag is an intermediate block resulting from the evaluating of a lazy.
+                As an optimization, let's replace it directly with the normalization of the result
+                as if this intermediate block didn't exist.
+
+                This remains untested for now (hence the assertion above).
+                Not obvious to test as optimizations or the GC can already do the substitution.
+              *)
+              hash_and_normalize_obj sharer (Obj.field block 0) )
+            else (
+              (* For regular blocks, normalize each field then use a shallow comparison. *)
+              assert ((not sharer.fail_on_objects) || not (Int.equal tag Obj.object_tag)) ;
+              let hash_shallow_normalized =
+                let size = Obj.size block in
+                hash_and_normalize_block_fields sharer block block size 0
+                  (Hashing.alloc_of_block ~tag ~size)
+              in
+              match HNorm.find_opt sharer.hash_normalized hash_shallow_normalized with
+              | Some hash_normalized ->
+                  hash_normalized
+              | None ->
+                  HNorm.add sharer.hash_normalized hash_shallow_normalized hash_shallow_normalized ;
+                  hash_shallow_normalized )
+          in
+          HPhysEq.replace sharer.visited_blocks shallow_hash_block (Normalized hash_normalized) ;
+          hash_normalized
 
 
   and hash_and_normalize_block_fields sharer original_block new_block size field_i hash_state =
-    if field_i >= size then normalize_block sharer (Hashing.get_hash_value hash_state) new_block
+    if field_i >= size then (Hashing.get_hash_value hash_state, new_block)
     else
       let field_v = Obj.field original_block field_i in
       let field_hash, field_v' = hash_and_normalize_obj sharer field_v in
@@ -142,10 +208,6 @@ end = struct
         (field_i + 1) hash_state
 
 
-  and hash_and_normalize_no_scan_block sharer block =
-    normalize_block sharer (Hashing.of_no_scan_block block) block
-
-
   (**
     Returns a value structurally equal but with potentially more sharing.
     Potentially unsafe if used on mutable values that are modified later.

infer/src/istd/MaximumSharing.mli

@@ -8,8 +8,8 @@
 open! IStd
 
 (**
-  Current implementation will stack overflow on deep (TODO: a tailrec version)
-  or circular values (much harder to detect sharing, also not needed for now).
+  Current implementation will stack overflow on deep values (TODO: a tailrec version)
+  and throw on circular values.
 *)
 
 module Sharer : sig

infer/src/unit/MaximumSharingTests.ml

@@ -11,11 +11,13 @@ open OUnit2
 let inputs =
   let a = `A 'a' in
   let b = Array.create ~len:10_000 a in
-  let c = Array.create ~len:100 b in
-  [ ("unit", Obj.repr ())
-  ; ("same representation", Obj.repr ([42], [|42; 0|]))
-  ; ("10K times the same element", Obj.repr b)
-  ; ("100 times 10K times the same element", Obj.repr c) ]
+  let c = Array.create ~len:1_000 b in
+  let d = Array.create ~len:1_000 c in
+  [ ("unit", Obj.repr (), `PhysEqual)
+  ; ("same representation", Obj.repr ([42], [|42; 0|]), `Marshal_MustBeBetter)
+  ; ("10K times the same element", Obj.repr b, `PhysEqual)
+  ; ("1K times 10K times the same element", Obj.repr c, `PhysEqual)
+  ; ("1K times 1K times 10K times the same element", Obj.repr d, `PhysEqual) ]
 
 
 let tests =
@@ -23,10 +25,17 @@ let tests =
     let sharer = MaximumSharing.Sharer.create () in
     MaximumSharing.Sharer.normalize_value sharer input
   in
-  let test_one input _ =
+  let test_one input checks _ =
     (* Save this now, in case `MaximumSharing` mutates the [input], even though it shouldn't *)
     let serialized_input_with_sharing = Marshal.to_string input [] in
-    let serialized_input_no_sharing = Marshal.to_string input [Marshal.No_sharing] in
+    let serialized_input_no_sharing =
+      match checks with
+      | `PhysEqual ->
+          "UNUSED"
+      | `Marshal_MustBeBetter ->
+          (* OOMs for big or cyclic values *)
+          Marshal.to_string input [Marshal.No_sharing]
+    in
     let reachable_words_input = Obj.reachable_words input in
     let normalized = normalize input in
     (*
@@ -46,15 +55,25 @@ let tests =
     assert_bool "less reachable words" (reachable_words_normalized <= reachable_words_input) ;
     let eq =
       (* Cannot use [assert_equal] because it doesn't shortcut physical equalities *)
-      Polymorphic_compare.equal input normalized
+      match checks with
+      | `PhysEqual ->
+          phys_equal input normalized
+      | `Marshal_MustBeBetter ->
+          Polymorphic_compare.equal input normalized
     in
     assert_bool "equal" eq ;
-    (*
+    match checks with
+    | `PhysEqual ->
+        ()
+    | `Marshal_MustBeBetter ->
+        assert_bool "strictly less reachable words"
+          (reachable_words_normalized < reachable_words_input) ;
+        (*
       In case structural equality and marshalling have slightly different semantics,
       let's also make sure the serialized versions are indistinguishable
     *)
-    let serialized_normalized_no_sharing = Marshal.to_string normalized [Marshal.No_sharing] in
-    assert_equal serialized_input_no_sharing serialized_normalized_no_sharing
+        let serialized_normalized_no_sharing = Marshal.to_string normalized [Marshal.No_sharing] in
+        assert_equal serialized_input_no_sharing serialized_normalized_no_sharing
   in
-  let tests_ = List.map inputs ~f:(fun (name, input) -> name >:: test_one input) in
+  let tests_ = List.map inputs ~f:(fun (name, input, checks) -> name >:: test_one input checks) in
   "MaximumSharing_tests" >::: tests_