flambda-backend: Fix wrong shift generation in Cmm_helpers (oxcaml#347)

Author: mshinwell

asmcomp/cmm_helpers.ml

@@ -239,10 +239,10 @@ let untag_int i dbg =
   match i with
     Cconst_int (n, _) -> Cconst_int(n asr 1, dbg)
   | Cop(Cor, [Cop(Casr, [c; Cconst_int (n, _)], _); Cconst_int (1, _)], _)
-    when n > 0 && n < size_int * 8 ->
+    when n > 0 && n < size_int * 8 - 1 ->
       Cop(Casr, [c; Cconst_int (n+1, dbg)], dbg)
   | Cop(Cor, [Cop(Clsr, [c; Cconst_int (n, _)], _); Cconst_int (1, _)], _)
-    when n > 0 && n < size_int * 8 ->
+    when n > 0 && n < size_int * 8 - 1 ->
       Cop(Clsr, [c; Cconst_int (n+1, dbg)], dbg)
   | c -> asr_int c (Cconst_int (1, dbg)) dbg
 
@@ -433,6 +433,7 @@ let rec div_int c1 c2 is_safe dbg =
               res = shift-right-signed(c1 + t, l)
         *)
         Cop(Casr, [bind "dividend" c1 (fun c1 ->
+                     assert (l >= 1);
                      let t = asr_int c1 (Cconst_int (l - 1, dbg)) dbg in
                      let t =
                        lsr_int t (Cconst_int (Nativeint.size - l, dbg)) dbg
@@ -490,6 +491,7 @@ let mod_int c1 c2 is_safe dbg =
               res = c1 - t
          *)
         bind "dividend" c1 (fun c1 ->
+          assert (l >= 1);
           let t = asr_int c1 (Cconst_int (l - 1, dbg)) dbg in
           let t = lsr_int t (Cconst_int (Nativeint.size - l, dbg)) dbg in
           let t = add_int c1 t dbg in

testsuite/tests/basic/lsr_mod.ml

@@ -0,0 +1,16 @@
+(* TEST *)
+
+let [@inline never] lsr_63 (x : int) =
+  x lsr 63
+
+let [@inline never] mod_2 (x : int) =
+  x mod 2
+
+let [@inline never] lsr_63_pipe_mod_2 (x : int) =
+  (lsr_63 x) |> mod_2
+
+let [@inline never] lsr_63_mod_2 (x : int) =
+  (x lsr 63) mod 2
+
+let () =
+  assert (Int.equal (lsr_63_pipe_mod_2 0) (lsr_63_mod_2 0))