RFR: 8273563: Improve performance of implicit exceptions with -XX:-OmitStackTraceInFastThrow [v6]

[Martin Doerr]

On Thu, 11 Nov 2021 09:40:44 GMT, Volker Simonis <simonis at openjdk.org> wrote:

>> src/hotspot/share/prims/whitebox.cpp line 987:
>> 
>>> 985:     bool overflow = false;
>>> 986:     for (uint reason = 0; reason < mdo->trap_reason_limit(); reason++) {
>>> 987:       if (reason_str != NULL && !strcmp(reason_str, Deoptimization::trap_reason_name(reason))) {
>> 
>> Maybe the code would be better readable when checking `reason_str != NULL` first and then use 2 loops? Just a minor suggestion. Should only be done if readability is better.
>
> I've tried it but the resulting version is slightly longer and in my opinion not really more readable:
> 
> WB_ENTRY(jint, WB_GetMethodTrapCount(JNIEnv* env, jobject o, jobject method, jstring reason_obj))
>   jmethodID jmid = reflected_method_to_jmid(thread, env, method);
>   CHECK_JNI_EXCEPTION_(env, 0);
>   methodHandle mh(THREAD, Method::checked_resolve_jmethod_id(jmid));
>   uint cnt = 0;
>   MethodData* mdo = mh->method_data();
>   if (mdo != NULL) {
>     ResourceMark rm(THREAD);
>     if (reason_obj != NULL) {
>       char* reason_str = java_lang_String::as_utf8_string(JNIHandles::resolve_non_null(reason_obj));
>       for (uint reason = 0; reason < mdo->trap_reason_limit(); reason++) {
>         if (!strcmp(reason_str, Deoptimization::trap_reason_name(reason))) {
>           cnt = mdo->trap_count(reason);
>           // Count in the overflow trap count on overflow
>           if (cnt == (uint)-1) {
>             cnt = mdo->trap_count_limit() + mdo->overflow_trap_count();
>           }
>           break;
>         }
>       }
>     } else {
>       bool overflow = false;
>       for (uint reason = 0; reason < mdo->trap_reason_limit(); reason++) {
>         uint c = mdo->trap_count(reason);
>         if (c == (uint)-1) {
>           c = mdo->trap_count_limit();
>           if (!overflow) {
>             // Count overflow trap count just once
>             overflow = true;
>             c += mdo->overflow_trap_count();
>           }
>         }
>         cnt += c;
>       }
>     }
>   }
>   return cnt;
> WB_END
> 
> 
> But for me it's actually no difference. Please just let me know if you'd still prefer the alternative version.
> 
> PS: I've updated the documentation of the method which was inaccurate for `reason==NULL`.

Your two loop version looks a bit easier to read for me, but that may be a matter of taste. I leave you free to decide.

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PR: https://git.openjdk.java.net/jdk/pull/5488