flambda-backend: Keep Sys.opaque_identity in Cmm and Mach (port upstream PR 9412) (#238)

Author: stedolan

testsuite/tests/lib-sys/opaque.ml

@@ -0,0 +1,38 @@
+(* TEST *)
+
+let[@inline never] float_unboxing s f =
+  let x = Sys.opaque_identity (s +. 1.) in
+  let mw1 = Gc.minor_words () in
+  let mw2 = Gc.minor_words () in
+  f x;
+  let mw3 = Gc.minor_words () in
+  Printf.printf "unbox: %.0f\n" ((mw3 -. mw2) -. (mw2 -. mw1))
+
+let[@inline never] lifetimes () =
+  let final = ref false in
+  let go () =
+    let r = ref 42 in
+    Gc.finalise (fun _ -> final := true) r;
+    let f1 = !final in
+    Gc.full_major ();
+    let f2 = !final in
+    ignore (Sys.opaque_identity r);
+    (f1, f2) in
+  let (f1, f2) = go () in
+  Gc.full_major ();
+  let f3 = !final in
+  Printf.printf "lifetime: %b %b %b\n" f1 f2 f3
+
+let[@inline never] dead_alloc a =
+  let mw1 = Gc.minor_words () in
+  let mw2 = Gc.minor_words () in
+  ignore (Sys.opaque_identity (a, a));
+  let mw3 = Gc.minor_words () in
+  Printf.printf "dead: %.0f\n" ((mw3 -. mw2) -. (mw2 -. mw1))
+  
+
+let () =
+  float_unboxing 50. (fun _ -> ());
+  lifetimes ();
+  dead_alloc 10
+          

testsuite/tests/lib-sys/opaque.reference

@@ -0,0 +1,3 @@
+unbox: 0
+lifetime: false false true
+dead: 3