Fix marking of if condition as inconstant in flambda Fix #10603 (#10611)

The if condition was marked as systematically inconstant. This was not
often problematic since most of the time this means that the if is
eliminated by simplify. But this is not the case in let rec's where
the approximation of the value is not known by simplify.

But the problem is noticeable in this example

type t = A | B of (int -> int)

let p = 1 + 1

let rec b = B g
and g n =
  let b' = b in
  match b' with
  | A -> n + p
  | B f -> f n

The inconstant_ident pass correctly identifies b and g as constants, but
marks b' as inconstant due to this bug (because match on this kind of
type is compiled as an if). When lifting the definition of g into a
let_rec_symbol, the alias_analysis does not keep the alias of b' to b
because b' is considered as inconstant. Hence no substitution is applied
to it. This leads to building a closed function with free variables,
which is obviously wrong.

Changes

@@ -162,6 +162,9 @@ Working version
 - #10590: Some typechecker optimisations
   (Stephen Dolan, review by Gabriel Scherer and Leo White)
 
+- #10603, #10611: Fix if condition marked as inconstant in flambda
+  (Vincent Laviron and Pierre Chambart, report by Marcello Seri)
+
 OCaml 4.13.0
 -------------
 

middle_end/flambda/inconstant_idents.ml

@@ -269,9 +269,9 @@ module Inconstants (P:Param) (Backend:Backend_intf.S) = struct
       mark_curr curr;
       mark_loop ~toplevel [] f1;
       mark_loop ~toplevel:false [] body
-    | If_then_else (f1,f2,f3) ->
+    | If_then_else (cond,f2,f3) ->
       mark_curr curr;
-      mark_curr [Var f1];
+      mark_var cond curr;
       mark_loop ~toplevel [] f2;
       mark_loop ~toplevel [] f3
     | Static_raise (_,l) ->

testsuite/tests/regression/pr10611/pr10611.ml

@@ -0,0 +1,12 @@
+(* TEST *)
+
+type t = A | B of (int -> int)
+
+let p = 1 + 1
+
+let rec b = B g
+and g n =
+  let b' = b in
+  match b' with
+  | A -> n + p
+  | B f -> f n