Changes

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/jslib.js

@@ -164,6 +164,10 @@ function caml_wrap_exception(e) {
 //Requires: caml_record_backtrace_env_flag
 //Requires: caml_record_backtrace_runtime_flag
 function caml_maybe_attach_backtrace(exn, force) {
+  // 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