Fix popcnt CPU support detection

src/discover/discover.ml

@@ -15,6 +15,33 @@ let () =
     ~name:"discover"
     ~args:[ "-o", Set_string output, "FILENAME output file" ]
     (fun c ->
-      let has_popcnt = c_test c ~c_flags:[ "-mpopcnt" ] program in
-      Flags.write_sexp !output (if has_popcnt then [ "-mpopcnt" ] else []))
+       let arch = ocaml_config_var c "architecture" in
+       let cc = ocaml_config_var_exn c "native_c_compiler" in
+       let is_gcc =
+         (Process.run c cc [ "--version" ] ~env:[]).stdout
+         |> String.lowercase_ascii
+         |> String.starts_with ~prefix:"gcc"
+       in
+       let try_flag c flag = if c_test c ~c_flags:[ flag ] program then flag else "" in
+       let flags =
+         match arch with
+         | Some s when String.length s >= 3 ->
+           (match String.sub s 0 3 with
+            | "ppc" | "pow" ->
+              let popcntd = try_flag c "-mpopcntd" in
+              if popcntd <> ""
+              then popcntd
+              (* needed to prevent crashing, only gcc has this option *)
+              else if is_gcc
+              then try_flag c "-mpopcntb"
+              else ""
+            | "spa" -> try_flag c "-mpopc"
+            | "x86" | "amd" -> try_flag c "-mpopcnt"
+            | _ -> "")
+           (* armv8/arm64/aarch64 support popcnt via vcnt by default and don't require any flags *)
+         | _ -> ""
+       in
+       let flags = if flags <> "" then [ flags ] else [] in
+       Flags.write_sexp !output flags)
 ;;
+