Avoid int literals that overflow a 31-bit target

src/int63_emul.ml

@@ -99,7 +99,11 @@ let to_unsigned_int x =
   then to_int x
   else invalid_arg "Int63.to_unsigned_int: %Lx can not fit into a 31 bits unsigned integer" x
 
-let without_bit_sign (x:int) = if x >= 0 then x else x land (lnot 0x40000000)
+(* [0x40000000] does not fit a 31-bit [int], where it is the sign bit; build it
+   from an [int64] so that nothing is truncated at compile time. *)
+let sign_bit = Int64.to_int 0x40000000L
+
+let without_bit_sign (x:int) = if x >= 0 then x else x land (lnot sign_bit)
 
 let of_unsigned_int x =
   if x < 0

src/optint_emul.ml

@@ -10,7 +10,11 @@ let of_int64 = Int64.to_int32
 
 let pp ppf (x:t) = Format.fprintf ppf "%ld" x
 
-let without_bit_sign (x:int) = if x >= 0 then x else x land (lnot 0x40000000)
+(* [0x40000000] does not fit a 31-bit [int], where it is the sign bit; build it
+   from an [int32] so that nothing is truncated at compile time. *)
+let sign_bit = Int32.to_int 0x40000000l
+
+let without_bit_sign (x:int) = if x >= 0 then x else x land (lnot sign_bit)
 
 let invalid_arg fmt = Format.kasprintf invalid_arg fmt
 

src/optint_native.ml

@@ -53,11 +53,15 @@ let equal : int -> int -> bool = fun a b -> a = b
 
 let invalid_arg fmt = Format.kasprintf invalid_arg fmt
 
+(* Reached only when [Sys.word_size = 64], so these masks assume an [int] wider
+   than 32 bits: [int32_sign_mask] and the [lsl 32] in [of_int32] are already
+   unspecified on a narrower one. They are computed rather than written as
+   literals so that a 31-bit backend has nothing to truncate. *)
 let uint32_max = (0xffff lsl 16) lor 0xffff
 let int32_sign_maskl = 0x80000000l
 let int32_sign_mask = 1 lsl 31
 let int32_maxl = 0x7fffffffl
-let int32_max = 0x7fffffff
+let int32_max = uint32_max lsr 1
 
 let to_int32 x =
   let truncated = x land uint32_max in