[ammarahm-ed]

Strings cross the JNI bridge as UTF-16 in both directions, in the Node-API and the direct runtime.

JS to Java. ConvertToJavaString read the string as UTF-8 into a new char[] that was never freed, then passed it to NewStringUTF. That leaked a buffer on every string argument (the Node-API heap below), and NewStringUTF takes modified UTF-8, so non-BMP characters came out wrong. It now copies the UTF-16 code units into a stack buffer (heap only past 256 units) and calls NewString.

Java to JS. jstringToJsString now reads GetStringChars instead of re-encoding through GetStringUTFChars. A char return, field read and array element no longer build a java.lang.String first.

Engine layer. The direct runtime gets Value::createStringFromUtf16 and Value::copyUtf16 on V8 (NewFromTwoByte / WriteV2), JSC (JSStringCreateWithCharacters / JSStringGetCharactersPtr), QuickJS-NG (JS_NewStringUTF16 / JS_ToCStringLenUTF16) and Hermes (jsi::String::createFromUtf16 / getStringData), which replaces the hand-written UTF-16 to UTF-8 transcoder in ArgConverter.cpp. Unpaired surrogates survive the round trip.

JSC. JSString::CopyTo in vendor/jsc/jsc-api.cpp copied size bytes instead of size * sizeof(JSChar), so napi_get_value_string_utf16 returned half a string.

Results

V8 release build on a Sony SO-01M (Android 11, arm64), AOT-compiled with cmd package compile -m speed. One warm-up launch is discarded, then 5 launches; values are the median ns per call.

Case	Node-API before	after	Δ	Direct before	after	Δ
Pass and return string	2,340	1,979	-15%	2,186	1,975	-10%
Pass a String	1,168	1,094	-6%	1,295	1,186	-8%
Return a String	976	936	-4%	918	922	+0%
Pass and return non-ASCII string	2,457	2,007	-18%	2,444	1,959	-20%
Pass and return 1KB string	7,395	5,816	-21%	6,667	5,991	-10%
Return a Char	958	409	-57%	481	414	-14%
Native heap after run (MB)	485 MB	256 MB	-47%	180 MB	178 MB	-1%

Rows this change does not touch moved by up to ±15% between runs (array and object returns by up to ±26%), which is this device's noise floor; only changes well beyond that are attributable.

All cases

Case	Node-API before	after	Δ	Direct before	after	Δ
Void Method on instance	764	717	-6%	688	680	-1%
Field on instance	915	910	-1%	932	882	-5%
Int Field on instance	487	425	-13%	440	387	-12%
Get Static Field	1,053	898	-15%	915	928	+1%
Void Static Method	410	362	-12%	364	377	+4%
multiply(a, b)	774	776	+0%	824	878	+6%
Pass and return string	2,340	1,979	-15%	2,186	1,975	-10%
Return a String	976	936	-4%	918	922	+0%
Return an Int	389	367	-6%	367	376	+2%
Pass a String	1,168	1,094	-6%	1,295	1,186	-8%
Return a Boolean	374	363	-3%	377	377	-0%
Return a Double	518	464	-10%	465	458	-1%
Pass an Int	585	626	+7%	647	693	+7%
Pass a Double	594	624	+5%	655	702	+7%
Pass a Boolean	715	701	-2%	666	670	+1%
Return an Int Array	27,612	26,453	-4%	17,913	17,506	-2%
Pass an Int Array	3,231	3,302	+2%	4,026	4,162	+3%
Return a String Array	26,595	27,122	+2%	19,409	19,109	-2%
Pass a String Array	5,808	5,691	-2%	7,564	7,092	-6%
Return a Double Array	36,162	26,921	-26%	19,724	20,357	+3%
Pass a Double Array	3,699	3,144	-15%	3,886	3,987	+3%
Return a Boolean Array	33,383	24,781	-26%	18,409	18,246	-1%
Pass a Boolean Array	3,439	3,014	-12%	3,907	3,842	-2%
Int Array[0] read	442	379	-14%	254	248	-2%
String Array[0] read	910	849	-7%	675	698	+3%
Double Array[0] read	497	492	-1%	348	348	-0%
Boolean Array[0] read	378	369	-2%	248	247	-1%
Int Array[0] write	542	546	+1%	386	389	+1%
String Array[0] write	1,417	1,289	-9%	1,292	1,141	-12%
Double Array[0] write	524	551	+5%	391	381	-3%
Boolean Array[0] write	521	552	+6%	400	394	-2%
Return a Date Object	28,412	29,666	+4%	22,592	23,960	+6%
Pass a Date Object	4,410	3,774	-14%	3,658	3,727	+2%
Invoke void callback	9,364	7,637	-18%	6,997	6,848	-2%
Instance add(a, b)	1,186	1,135	-4%	1,135	1,193	+5%
Pass and return non-ASCII string	2,457	2,007	-18%	2,444	1,959	-20%
Pass and return 1KB string	7,395	5,816	-21%	6,667	5,991	-10%
Return a Char	958	409	-57%	481	414	-14%
Return this	1,616	1,607	-1%	1,630	1,622	-0%
Return a new object	33,642	29,868	-11%	23,862	23,988	+1%
Call a method on a new object	42,810	38,563	-10%	34,684	33,581	-3%
Pass a 16 Int Array	4,744	4,512	-5%	5,371	5,325	-1%
Pass a 16 Double Array	5,316	5,351	+1%	5,785	6,043	+4%
Invoke int callback	13,217	10,851	-18%	10,889	11,006	+1%
Invoke string callback	10,233	8,868	-13%	8,518	8,511	-0%
Native heap after run (MB)	485 MB	256 MB	-47%	180 MB	178 MB	-1%

Only V8 was built and run for this table. The JSC, QuickJS-NG and Hermes backends in this change have not been built or run yet.

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