flambda-backend: Better results from meet (#406)

Co-authored-by: Pierre Chambart <pierre.chambart@ocamlpro.com>
Co-authored-by: Guillaume Bury <guillaume.bury@gmail.com>

utils/misc.ml

@@ -233,6 +233,15 @@ module Stdlib = struct
         else loop (succ i) in
       loop 0
 
+    let fold_left2 f x a1 a2 =
+      if Array.length a1 <> Array.length a2
+      then invalid_arg "Misc.Stdlib.Array.fold_left2";
+      let r = ref x in
+      for i = 0 to Array.length a1 - 1 do
+        r := f !r (Array.unsafe_get a1 i) (Array.unsafe_get a2 i)
+      done;
+      !r
+
     let for_alli p a =
       let n = Array.length a in
       let rec loop i =
@@ -260,6 +269,17 @@ module Stdlib = struct
           false
       in
       loop 0
+
+    let map_sharing f a =
+      let same = ref true in
+      let f' x =
+        let x' = f x in
+        if x != x' then
+          same := false;
+        x'
+      in
+      let a' = (Array.map [@inlined hint]) f' a in
+      if !same then a else a'
   end
 
   module String = struct

utils/misc.mli

@@ -192,6 +192,14 @@ module Stdlib : sig
     val exists2 : ('a -> 'b -> bool) -> 'a array -> 'b array -> bool
     (** Same as [Array.exists2] from the standard library. *)
 
+    val fold_left2 :
+      ('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a array -> 'b array -> 'acc
+    (** [fold_left2 f init [|a1; ...; an|] [|b1; ...; bn|]] is
+        [f (... (f (f init a1 b1) a2 b2) ...) an bn].
+        @raise Invalid_argument if the two arrays are determined
+        to have different lengths.
+    *)
+
     val for_alli : (int -> 'a -> bool) -> 'a array -> bool
     (** Same as [Array.for_all] from the standard library, but the
         function is applied with the index of the element as first argument,
@@ -202,6 +210,10 @@ module Stdlib : sig
     val equal : ('a -> 'a -> bool) -> 'a array -> 'a array -> bool
     (** Compare two arrays for equality, using the supplied predicate for
         element equality *)
+
+    val map_sharing : ('a -> 'a) -> 'a array -> 'a array
+    (** [map_sharing f a] is [map f a]. If for all elements of the array
+        [f e == e] then [map_sharing f a == a] *)
   end
 
 (** {2 Extensions to the String module} *)