Runtime: record backtraces should be explicit

CHANGES.md

@@ -3,8 +3,10 @@
 ## Features/Changes
 * Misc: update testsuite to OCmaml 5.2
 * Misc: CI uses opam.2.2 and no longer use sunset repo
-* Runtime: change Sys.os_type on windows (Cygwin -> Win32)
 * Compiler: speedup global_flow/global_deadcode pass on large bytecode
+* Runtime: change Sys.os_type on windows (Cygwin -> Win32)
+* Runtime: backtraces are really expensive, they need to be be explicitly
+  requested at compile time (--enable with-js-error) or at startup (OCAMLRUNPARAM=b=1)
 
 ## Bug fixes
 * Runtime: fix parsing of unsigned integers (0u2147483648)

runtime/backtrace.js

@@ -16,36 +16,40 @@
 // Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 
-//Provides: caml_record_backtrace_flag
+//Provides: caml_record_backtrace_env_flag
 //Requires: jsoo_sys_getenv
-var caml_record_backtrace_flag = FLAG("with-js-error");
+var caml_record_backtrace_env_flag = FLAG("with-js-error");
 
 (function () {
   var r = jsoo_sys_getenv("OCAMLRUNPARAM")
   if(r !== undefined){
     var l = r.split(",");
     for(var i = 0; i < l.length; i++){
-      if(l[i] == "b") { caml_record_backtrace_flag = 1; break }
+      if(l[i] == "b") { caml_record_backtrace_env_flag = 1; break }
       else if (l[i].startsWith("b=")) {
-        caml_record_backtrace_flag = +(l[i].slice(2))}
+        caml_record_backtrace_env_flag = +(l[i].slice(2)) }
       else continue;
     }
   }
 }) ()
 
+//Provides: caml_record_backtrace_runtime_flag
+//Requires: caml_record_backtrace_env_flag
+var caml_record_backtrace_runtime_flag = caml_record_backtrace_env_flag;
+
 
 //Provides: caml_ml_debug_info_status const
 function caml_ml_debug_info_status () { return 0; }
 //Provides: caml_backtrace_status
-//Requires: caml_record_backtrace_flag
-function caml_backtrace_status (_unit) { return caml_record_backtrace_flag ? 1 : 0; }
+//Requires: caml_record_backtrace_runtime_flag
+function caml_backtrace_status (_unit) { return caml_record_backtrace_runtime_flag ? 1 : 0; }
 //Provides: caml_get_exception_backtrace const
 function caml_get_exception_backtrace () { return 0; }
 //Provides: caml_get_exception_raw_backtrace const
 function caml_get_exception_raw_backtrace () { return [0]; }
 //Provides: caml_record_backtrace
-//Requires: caml_record_backtrace_flag
-function caml_record_backtrace (b) { caml_record_backtrace_flag = b; return 0; }
+//Requires: caml_record_backtrace_runtime_flag
+function caml_record_backtrace (b) { caml_record_backtrace_runtime_flag = b; return 0; }
 //Provides: caml_convert_raw_backtrace const
 function caml_convert_raw_backtrace () { return [0]; }
 //Provides: caml_raw_backtrace_length

runtime/jslib.js

@@ -161,9 +161,14 @@ function caml_wrap_exception(e) {
 
 //Provides: caml_maybe_attach_backtrace
 //Requires: caml_exn_with_js_backtrace
-//Requires: caml_record_backtrace_flag
+//Requires: caml_record_backtrace_env_flag
+//Requires: caml_record_backtrace_runtime_flag
 function caml_maybe_attach_backtrace(exn, force) {
-  if(caml_record_backtrace_flag)
+  // Backtraces are very expensive, we only enable them when explicitly requested
+  // at compile-time (--enable with-js-error) or at startup with OCAMLRUNPARAM=b=1.
+  // Libraries such as Base unconditionally enable backtraces (programmatically) but
+  // it's way to slow. Here, we force the end-user to opt-in to backtraces.
+  if(caml_record_backtrace_env_flag && caml_record_backtrace_runtime_flag)
     return caml_exn_with_js_backtrace(exn, force);
   else return exn
 }