Fix "zero divided by n" (part of this was MPR#7201)

Changes

@@ -11,6 +11,10 @@ Next version (4.05.0):
   (Stephen Dolan, review by Alain Frisch, Pierre Chambart, Mark
   Shinwell, Gabriel Scherer and Damien Doligez)
 
+- MPR#7201, GPR#954: Correct wrong optimisation of "0 / <expr>"
+  and "0 mod <expr>" in the case when <expr> was a non-constant
+  evaluating to zero (Mark Shinwell)
+
 ### Runtime system:
 
 ### Type system:

asmcomp/cmmgen.ml

@@ -339,8 +339,6 @@ let rec div_int c1 c2 is_safe dbg =
       Csequence(c1, raise_symbol dbg "caml_exn_Division_by_zero")
   | (c1, Cconst_int 1) ->
       c1
-  | (Cconst_int 0 as c1, c2) ->
-      Csequence(c2, c1)
   | (Cconst_int n1, Cconst_int n2) ->
       Cconst_int (n1 / n2)
   | (c1, Cconst_int n) when n <> min_int ->
@@ -387,8 +385,6 @@ let mod_int c1 c2 is_safe dbg =
       Csequence(c1, raise_symbol dbg "caml_exn_Division_by_zero")
   | (c1, Cconst_int (1 | (-1))) ->
       Csequence(c1, Cconst_int 0)
-  | (Cconst_int 0, c2) ->
-      Csequence(c2, Cconst_int 0)
   | (Cconst_int n1, Cconst_int n2) ->
       Cconst_int (n1 mod n2)
   | (c1, (Cconst_int n as c2)) when n <> min_int ->

testsuite/tests/basic/zero_divided_by_n.ml

@@ -0,0 +1,17 @@
+(* Mantis 7201 *)
+
+let f () = 0 [@@inline never]
+
+let () =
+  try
+    ignore ((0 / f ()) : int);
+    assert false
+  with Division_by_zero -> ()
+
+(* Not in Mantis 7201, but related: *)
+
+let () =
+  try
+    ignore ((0 mod f ()) : int);
+    assert false
+  with Division_by_zero -> ()