perf: convert strings across the V8/Obj-C bridge without the UTF-8 round trip - #442
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ToV8String(Isolate*, NSString*) built a UTF-8 buffer that neither side wanted. -UTF8String encodes the whole string and mallocs an autoreleased buffer, -lengthOfBytesUsingEncoding: encodes it a second time just to count the bytes, and V8 then decodes UTF-8 back into its own one-byte or two-byte form. Three passes and an allocation to move between two representations that already match. Hand CFString's existing buffer to V8 at its native width instead: an ASCII interior pointer feeds NewFromOneByte, a UTF-16 interior pointer feeds NewFromTwoByte, and the shapes that expose neither (tagged pointers, some bridged strings) are copied out once into a stack buffer. Foundation-side cost, measured at -O2 on arm64: ASCII, 15 chars 47.8 ns -> 8.2 ns ASCII, 4000 chars 1040 ns -> 8.0 ns non-ASCII, 17 chars 148 ns -> 7.1 ns CJK, 400 chars 1893 ns -> 7.3 ns tagged pointer 54.2 ns -> 6.2 ns The copy V8 performs is unchanged, so end-to-end this is largest for short strings and settles around 3-8x for multi-KB ones. It also drops one malloc per conversion: -UTF8String allocated on every call for everything but constant literals (10061 mallocs per 10k calls, ~880 KB held until the pool drained). This fixes silent data loss as a side effect. -UTF8String returns nil for a string containing a lone surrogate and -lengthOfBytesUsingEncoding: returns 0, which V8 turns into "" via its length == 0 branch, so a lone surrogate did not survive JS -> native -> JS. The reverse direction was already fixed for exactly this; the bridge is now symmetric.
tns::ToString ran every conversion through v8::String::Utf8Value, which heap-allocates a buffer and UTF-8-encodes into it, and then copied that buffer a second time into the returned std::string. Two allocations and two passes for a value that is almost always an ASCII identifier. Read the string's native buffer with ValueView instead. A one-byte V8 string that is pure ASCII already is its own UTF-8 encoding, so it is copied straight into the result; everything else encodes once with WriteUtf8V2 writing directly into the string's storage. Since libc++ keeps strings up to 22 characters inline, property names, selectors and class names now convert without touching the heap at all. The unwrap step moves into ToStringLocal so the same handling is shared with the other text conversions. It keeps Utf8Value's TryCatch: a throwing toString() stayed swallowed rather than becoming a pending exception in the 100-odd call sites that never expected one. ConcurrentMap, ArgConverter::GetMeta and InlineFunctions::IsGlobalFunction took their keys by value, so the global property interceptor copied the name twice more on every miss. They take const references now.
ToV8String had overloads for std::string, (const char*, int) and NSString*, but none for a bare const char*. Every string literal, every c_str(), every jsName() therefore picked the std::string overload and paid a strlen, a malloc, a copy and a free purely to select it. Adding the missing overload removes that from roughly 270 literal and 37 pointer call sites without touching any of them. Selectors no longer detour through Foundation. The argument path built a std::string, wrapped it in an NSString and handed it to NSSelectorFromString, which just calls sel_registerName on the UTF-8 bytes; it now calls sel_registerName directly. The return path asked NSStringFromSelector for an NSString only to read -UTF8String back out, where sel_getName already returns the bytes. The unichar argument path went V8 -> UTF-8 -> std::u16string -> std::vector to read a single code unit, by way of the deprecated std::codecvt_utf8_utf16. It reads that code unit off ValueView now, which retires tns::ToVector and the last use of <codecvt>. The unichar return path was also wrong. It packed the code unit into a char[2] with no room for a terminator and passed it to std::string, which read past the array, and for values above 127 it wrote the high byte first, so a unichar of 0xE9 produced a leading NUL and the call returned "" instead of "é". A unichar is one UTF-16 code unit and is now handed to V8 as one. The existing coverage only passed ASCII 'i', so this went unnoticed; the new case covers 'é' and '✓'.
Every Obj-C instance method call from JS allocated a std::string for the receiver's class name purely to probe Caches::ClassPrototypes, and class names routinely exceed the inline buffer a std::string can hold (UITableViewController is 21 characters). Giving that map a transparent hash and equality lets object_getClassName's const char* probe it directly. MethodCallback copied item->className_ on every invocation even though only the class-side branch replaces it, and InvokeMethod then took the name by value and immediately called c_str() on it. The common path now reads the cached name in place.
Several sites handed [x UTF8String] to ToV8String. -UTF8String returns nil for any string holding a lone surrogate, so those calls strlen a null pointer rather than producing a string. Passing the NSString straight to the overload that reads CFString's own buffer is nil-safe, keeps lone surrogates intact and skips the encode entirely. Covers the NSException name and reason, NSError's localizedDescription and domain, NSString elements yielded by fast enumeration, and the description shown for a native object. The sites that build a std::string from -UTF8String (Interop.mm:1677 and 1718, ModuleInternal, DevFlags) have the same hazard but need a separate NSString-to-std::string helper; they are left as they are.
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📝 WalkthroughWalkthroughThe runtime now converts Objective-C and V8 strings through native buffers, preserves UTF-16 values, supports string-view cache lookups, and reduces string copies. Tests cover Unicode, surrogate, storage-width, unichar, and large-string cases. ChangesRuntime string conversion
Estimated code review effort: 4 (Complex) | ~45 minutes Mergeability Score: ⚪ Minimal · up to The PR replaces unnecessary UTF-8 conversions with width-preserving string handling and fixes related Unicode and safety issues, improving bridge performance and correctness. No actionable merge-blocking risk remains after normal checks and review. Sequence Diagram(s)sequenceDiagram
participant JavaScript
participant Interop
participant Helpers
participant ObjectiveC
JavaScript->>Interop: pass string or unichar value
Interop->>Helpers: request direct V8 conversion
Helpers->>ObjectiveC: read native NSString buffer when needed
Helpers-->>Interop: return V8 string
Interop->>ObjectiveC: invoke Objective-C method
ObjectiveC-->>Interop: return converted value
Interop-->>JavaScript: return JavaScript string
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In `@NativeScript/runtime/Interop.mm`:
- Around line 1682-1687: Remove all UTF8String-based conversions in this error
path, including the initial error text and the fallbacks near the name/message
assignments and later fallback. Create the initial V8 string with the direct
tns::ToV8String conversion, derive the NativeScriptException message via
tns::ToString from that V8 string, and use the same direct conversion for both
fallback cases.
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NativeScript/runtime/ArgConverter.hNativeScript/runtime/ArgConverter.mmNativeScript/runtime/Caches.hNativeScript/runtime/ConcurrentMap.hNativeScript/runtime/Helpers.hNativeScript/runtime/Helpers.mmNativeScript/runtime/InlineFunctions.cppNativeScript/runtime/InlineFunctions.hNativeScript/runtime/Interop.mmNativeScript/runtime/MetadataBuilder.hNativeScript/runtime/MetadataBuilder.mmNativeScript/runtime/SymbolIterator.mmTestRunner/app/tests/Marshalling/NSStringTests.jsTestRunner/app/tests/Marshalling/Primitives/Instance.js
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- NativeScript/runtime/Helpers.mm
The NSException path constructed its message from [nsReason UTF8String] before reaching the conversions that handle nil, so an exception whose reason held a lone surrogate built a std::string from a null pointer and crashed while reporting another failure. The NSException and NSError fallbacks did the same with -description and -localizedDescription. Add a ToString overload that encodes through V8, which substitutes U+FFFD for the unpaired half rather than giving back nil. An unpaired surrogate has no UTF-8 spelling, so a replacement character is the only answer a std::string can carry. Interop.mm no longer calls -UTF8String anywhere.
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Fixed in 9c2628b. Confirmed the finding against the source: Rather than open-code the V8 round-trip at three sites, added a Suite green: 1200 tests, 0 failures. |
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Current behavior
Both V8 and CFString/NSString store text as either 8-bit or UTF-16, never UTF-8 — V8 as
SeqOneByteString(Latin-1) orSeqTwoByteString, CFString as an 8-bit backing store, a UTF-16 backing store, or a tagged pointer. The runtime nevertheless routed several conversions through UTF-8, which costs an encode on one side and a decode on the other, plus an allocation in between, to move data between two representations that already match.ToNSString(V8 → NSString) was already width-preserving. The other direction was not:That is three passes —
-UTF8Stringencodes and mallocs an autoreleased buffer,-lengthOfBytesUsingEncoding:encodes the whole string a second time purely to count bytes, and V8 then decodes UTF-8 back into its own representation. A commented-out sketch of the UTF-16 fast path had been sitting above it with// TODO: profile if this is faster.New behavior
Conversions hand over the buffer at its existing width. Five commits, each independently reviewable:
1efc9436ToV8String(NSString*): ASCII interior pointer →NewFromOneByte, UTF-16 interior pointer →NewFromTwoByte, otherwise one copy into a stack bufferdd6c6758tns::ToString:ValueViewwith a pure-ASCII fast path instead ofString::Utf8Value+ a second copy1ed65716const char*overload; selectors viasel_registerName/sel_getName;unicharviaValueView;ToVectorand<codecvt>removed0e935971ClassPrototypes; no per-call class-name copies on the invoke path1922d1a9-UTF8StringMeasured
Foundation-side cost,
-O2on arm64, 300k iterations. V8's own copy is unchanged and excluded, so end-to-end this is largest for short strings and settles around 3–8× for multi-KB ones:It also removes one malloc per conversion.
-UTF8Stringallocated on every call for everything except constant literals — 10,061 mallocs per 10k calls, ~880 KB held until the autorelease pool drained.Bugs fixed along the way
Lone surrogates were silently dropped.
-UTF8Stringreturnsnilfor a string containing an unpaired surrogate and-lengthOfBytesUsingEncoding:returns 0, which V8'slength == 0branch turns into"". A lone surrogate therefore did not survive JS → native → JS. The reverse direction had already been fixed for exactly this; the bridge is now symmetric.The
unicharreturn path read out of bounds. It packed the code unit into achar[2]with no room for a terminator and passed it tostd::string, which read past the array. For values above 127 it also wrote the high byte first, so aunicharof0xE9produced a leading NUL and the call returned""rather than"é". Existing coverage only passed ASCII'i'.nilfrom-UTF8Stringwas strlen'd at the NSException, NSError, fast-enumeration and object-description sites.Testing
./run_tests.shgreen at every commit: 1196 tests on main → 1200 here, 0 failures. Four new specs — NSString round trips for a lone surrogate, for both storage widths (ASCII / Latin-1 / CJK / non-BMP), for strings longer than the conversion stack buffer, and a non-ASCIIunicharround trip.Independently reviewed for
ValueViewlifetime (no V8 allocation, throw, or pointer escape inside a no-GC scope), fallback buffer arithmetic, the blast radius of the newconst char*overload across every affected call site, transparent-hash consistency, and pointer lifetime inMethodCallback.Two caveats worth stating plainly:
-UTF8StringreturnsNULL(measured),-lengthOfBytesUsingEncoding:returns 0 (measured), andNEW_STRINGmapslength == 0toString::Empty(v8/src/api/api.cc:7568).Summary by CodeRabbit
Bug Fixes
Performance
Tests