feat(string): implement RopeString with thin vtable and lazy flattening by akash-R-A-J · Pull Request #5006 · boa-dev/boa

akash-R-A-J · 2026-03-11T23:58:35Z

This Pull Request fixes/closes #5005 .

This PR introduces Rope strings to boa_string and integrates them into the engine’s concatenation pipeline to eliminate the pathological O(N²) behavior of repeated string concatenation.

Key Changes

Rope Strings

Added RopeString representation (core/string/src/vtable/rope.rs)
Lazy flattening: ropes are only flattened when a contiguous buffer is required
Balanced tree construction for batch concatenations
Fibonacci-based rope rebalancing to maintain bounded depth
Leaf sinking optimization to reduce rope node depth during concatenation
Depth limit (32) and small-string threshold to avoid pathological trees
Iterative traversal to prevent recursion overflow
Devirtualized hot paths (clone, drop, as_str, code_unit_at)

Engine Integration

JsValue::add now creates ropes for large concatenations (len(lhs) + len(rhs) > 1024)
ConcatToString updated to use balanced batch concatenation
String.prototype.concat refactored to use the new API

Thin VTable & Performance Recovery

Moved method pointers to static vtables, reducing string header size
Devirtualized hot paths (clone, drop, as_str, code_unit_at)
Added cached u64 hash to speed up property lookups
Added ptr_eq + hash checks to optimize equality comparisons

Benchmarks

Concatenation Stress Test

main : 2.876 s ±  0.025 s
rope : 132.4 ms ±   0.3 ms

≈ 21.7× faster

V8 Combined Suite

main : 181.562 s ±  8.539 s
rope : 181.642 s ±  9.381 s

Summary

This change removes quadratic concatenation behavior while maintaining baseline performance for general workloads. The thin vtable and devirtualization ensure that ropes introduce minimal overhead when not used.

codecov · 2026-03-12T00:31:13Z

Codecov Report

❌ Patch coverage is 41.98047% with 416 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.33%. Comparing base (6ddc2b4) to head (f157f37).
⚠️ Report is 928 commits behind head on main.

Files with missing lines	Patch %	Lines
core/string/src/iter.rs	23.07%	120 Missing ⚠️
core/string/src/vtable/rope.rs	36.58%	104 Missing ⚠️
core/string/src/lib.rs	60.00%	72 Missing ⚠️
core/string/src/str.rs	39.04%	64 Missing ⚠️
core/runtime/src/text/encodings.rs	0.00%	10 Missing ⚠️
cli/src/debug/string.rs	0.00%	8 Missing ⚠️
core/string/src/vtable/slice.rs	63.63%	8 Missing ⚠️
core/engine/src/builtins/regexp/mod.rs	36.36%	7 Missing ⚠️
core/engine/src/builtins/intl/segmenter/mod.rs	0.00%	6 Missing ⚠️
core/engine/src/builtins/date/utils.rs	28.57%	5 Missing ⚠️
... and 6 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5006       +/-   ##
===========================================
+ Coverage   47.24%   59.33%   +12.09%     
===========================================
  Files         476      591      +115     
  Lines       46892    63973    +17081     
===========================================
+ Hits        22154    37959    +15805     
- Misses      24738    26014     +1276

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

jedel1043 · 2026-03-12T02:33:36Z

-        // SAFETY: Caller must ensure the type matches.
-        unsafe { self.ptr.cast::<T>().as_ref() }
+    #[must_use]
+    pub fn as_str(&self) -> JsStr<'static> {


This is unsound. It's returning a 'static reference to a temporary JsString, so you could deallocate the original string and the compiler will happily let you access the JsStr<'static>:

let s = JsString::from_str("undefined behaviour!"); let temp = s.as_str(); drop(s) println!("{}", temp.display_lossy()); // UB!

Thanks for pointing this out! you're absolutely right, that was a leftover from the initial pointer-based POC while focusing on the memory layout. I've now refactored the vtable to use HRTBs for as_str which replaces that temporary shim with a lifetime binding. It was my fault.

zhuzhu81998

ci shows segmentaion fault (potential cause below)

zhuzhu81998 · 2026-03-12T09:03:48Z

+    let specifier = specifier.cow_replace('/', "\\");

-    let short_path = Path::new(&specifier);
+    let short_path = Path::new(&*specifier);


what does this have to do with rope strings?

zhuzhu81998 · 2026-03-12T09:13:32Z

+pub(crate) mod rope;
+pub(crate) use rope::RopeString;
+
+/// Header for all `JsString` allocations.


Then call this JsStringHeader instead of RawJsString perhaps?

sounds good, will replace RawJsString with the suggested JsStringHeader

zhuzhu81998 · 2026-03-12T09:16:20Z

+unsafe impl Sync for RawJsString {}
+// SAFETY: RawJsString contains only thread-safe data.
+unsafe impl Send for RawJsString {}


do you need this somewhere?

zhuzhu81998 · 2026-03-12T09:16:45Z

+// SAFETY: We only mutate refcount and hash via atomic-casts when kind != Static.
+unsafe impl Sync for JsStringVTable {}
+// SAFETY: JsStringVTable contains only thread-safe data.
+unsafe impl Send for JsStringVTable {}


same question: where is the need?

zhuzhu81998 · 2026-03-12T09:37:35Z

+/// A rope string that is a tree of other strings.
+#[repr(C)]
+pub(crate) struct RopeString {
+    /// Standardized header for all strings.
+    pub(crate) header: RawJsString,
+    pub(crate) left: JsString,
+    pub(crate) right: JsString,
+    flattened: OnceCell<JsString>,
+    pub(crate) depth: u8,
+}


hmmm so it is my understanding that JsString here will most likely GC-tracked. And there is no way for GC to know that RopeString is holdig reference to the left and right here?
are yo doing something to prevent this?

To clarify: JSStrings are not GC-tracked, they're independently ref-counted, so it's fine to not trace through them. I'm fairly certain the UB comes from the modifications made to as_str that removed the lifetime inheritance for JsStr<'static>.

indeed its not gc. disabling the gc does not resolve the segmentation fault XD.

github-actions · 2026-03-14T00:57:11Z

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,073	50,063	-10
Ignored	2,072	2,072	0
Failed	818	828	+10
Panics	0	0	0
Conformance	94.54%	94.52%	-0.02%

Broken tests (10):

test/built-ins/parseFloat/S15.1.2.3_A2_T1.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T7.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T8.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T6.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T4.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T3.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T10.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T5.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T2.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T9.js (previously Passed)

Tested main commit: 197b736bb87523a14b41834c078adbec3059d237
Tested PR commit: af923c75d12138009f1d7dc46a8106c310998475
Compare commits: 197b736...af923c7

akash-R-A-J · 2026-03-14T02:36:58Z

@jedel1043 I believe all requested changes have now been addressed. CI is green across all platforms, and test262 results match main.

Please let me know if anything else needs adjustment.

akash-R-A-J · 2026-03-17T15:07:54Z

Hey @jedel1043, whenever you have some time, could you please take another look at this PR?

All previously mentioned issues should now be addressed, including the rope rebalancing changes, safety fixes, and flattening adjustments discussed in the earlier review.

Here's the latest benchmark results:

github-actions · 2026-03-22T07:03:19Z

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	50,827	50,817	-10
Ignored	1,482	1,482	0
Failed	816	826	+10
Panics	0	0	0
Conformance	95.67%	95.66%	-0.02%

Broken tests (10):

test/built-ins/parseFloat/S15.1.2.3_A2_T7.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T4.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T5.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T10.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T9.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T1.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T8.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T2.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T6.js (previously Passed)
test/built-ins/parseFloat/S15.1.2.3_A2_T3.js (previously Passed)

Tested main commit: 8a3ca7d0978ffadf165989ad0a9a93a6cafd61b4
Tested PR commit: f157f374746e85b51ba29c340da753d24376405c
Compare commits: 8a3ca7d...f157f37

… issues

…ration

zhuzhu81998 · 2026-03-29T16:30:28Z

the failed tests are probably related.

…uiltins

…into feat/rope-strings

HalidOdat · 2026-04-02T16:03:06Z

+    /// Reference to the static vtable for this string kind.
+    pub vtable: &'static JsStringVTable,


I like the thin vtable here, we could make this even more space efficient if we store the kind only and have a table of vtables (u8 that we could potentially even encode in the len or refcount), the reference to a vtable makes more sense for object that we can't determine at compile time and are not limited (but JsString types are limited 😄).

const VTABLES = [JsStringVTable; {num vtables}] = [ LATIN1_VABLE, ROPE_VTABLE, // ... ]; // ... // usage: let dealloc = VTABLES[header.kind as usize].dealloc;

Keeping it by value instead of reference in the table may also help with cache locality for all JsString internal implementations.

HalidOdat · 2026-04-02T16:11:07Z

+
+/// Fibonacci numbers for rope balancing thresholds.
+/// `F(n) = Fib(n + 2)`. A rope of depth `n` is balanced if its length >= `F(n)`.
+static FIBONACCI_THRESHOLDS: [usize; 46] = [


Suggestion: We can shrink this lookup table by half here (at least on 64bit systems) 😄

Suggested change

static FIBONACCI_THRESHOLDS: [usize; 46] = [

static FIBONACCI_THRESHOLDS: [u32; 46] = [

HalidOdat · 2026-04-02T16:49:57Z

    };
    ( $x:expr, $y:expr ) => {
-        $crate::string::JsString::concat($crate::string::JsStr::from($x), $crate::string::JsStr::from($y))
+        $crate::string::JsString::concat(&$crate::string::JsString::from($x), &$crate::string::JsString::from($y))


Suggestion: Recursively calling js_string here, has the benefit of creating a static string instead of two dynamic strings. (i.e. js_string!(s, ";"))

Suggested change

$crate::string::JsString::concat(&$crate::string::JsString::from($x), &$crate::string::JsString::from($y))

$crate::string::JsString::concat(&crate::js_string!($x), &crate::js_string!($y))

HalidOdat · 2026-04-02T16:51:00Z

    };
    ( $( $s:expr ),+ ) => {
-        $crate::string::JsString::concat_array(&[ $( $crate::string::JsStr::from($s) ),+ ])
+        $crate::string::JsString::concat_array_strings(&[ $( $crate::string::JsString::from($s) ),+ ])


Suggested change

$crate::string::JsString::concat_array_strings(&[ $( $crate::string::JsString::from($s) ),+ ])

$crate::string::JsString::concat_array_strings(&[ $( $crate::js_string!($s) ),+ ])

akash-R-A-J requested a review from a team as a code owner March 11, 2026 23:58

jedel1043 reviewed Mar 12, 2026

View reviewed changes

Comment thread core/string/src/vtable/mod.rs Outdated

jedel1043 reviewed Mar 12, 2026

View reviewed changes

Comment thread core/string/src/vtable/rope.rs Outdated

jedel1043 reviewed Mar 12, 2026

View reviewed changes

zhuzhu81998 suggested changes Mar 12, 2026

View reviewed changes

akash-R-A-J force-pushed the feat/rope-strings branch from 6587b0c to 951985f Compare March 14, 2026 00:46

jedel1043 reviewed Mar 14, 2026

View reviewed changes

Comment thread core/string/src/vtable/rope.rs Outdated

akash-R-A-J requested a review from jedel1043 March 14, 2026 02:23

akash-R-A-J force-pushed the feat/rope-strings branch from c21f4cb to b6134ad Compare March 15, 2026 02:06

github-actions bot added the Waiting On Review Waiting on reviews from the maintainers label Mar 15, 2026

jedel1043 reviewed Mar 15, 2026

View reviewed changes

Comment thread core/string/src/vtable/rope.rs

akash-R-A-J requested a review from jedel1043 March 15, 2026 03:23

akash-R-A-J requested a review from nekevss as a code owner March 15, 2026 14:59

github-actions bot added C-CLI Issues and PRs related to the Boa command line interface. C-Intl Changes related to the `Intl` implementation C-Runtime Issues and PRs related to Boa's runtime features labels Mar 15, 2026

jedel1043 added the A-Performance Performance related changes and issues label Mar 17, 2026

akash-R-A-J force-pushed the feat/rope-strings branch from f21309a to af923c7 Compare March 17, 2026 12:06

jedel1043 removed the C-Builtins PRs and Issues related to builtins/intrinsics label Mar 20, 2026

github-actions bot added the C-Builtins PRs and Issues related to builtins/intrinsics label Mar 22, 2026

akash-R-A-J added 15 commits March 22, 2026 12:47

feat(string): implement RopeString with thin vtable and lazy flattening

b1e7e58

fix: cross-platform path resolution and clippy warnings

10e9fce

docs: fix broken intra-doc links in boa_string

e1c8c53

feat(string): implement robust rope rebalancing and fix memory safety…

8c117b4

… issues

fix(string): resolve seg fault, memory leak, and recursion limits

5bafd8e

refactor(string): refine rope implementation and cleanup vtable integ…

388c014

…ration

fix(string): escape brackets in rope doc comments

6145e31

perf(string): switch OnceLock to OnceCell

c12a38e

fix(string): revert OnceCell to OnceLock due to reentrancy panic

6024453

fix(string): resolve rope SIGSEGV via raw-buffer cache

3b0d198

fix(string): ensure 32-bit compatibility for rope thresholds

0d31e83

refactor(string): unify JsStr as enum and support unflattened ropes

b8cdbd0

perf(string): optimize rope performance with leaf sinking

76acd1e

docs: fix rustdoc intra-doc links for private items

951b4da

fix(string): stabilize rope implementation and address edge cases

fe779dc

akash-R-A-J force-pushed the feat/rope-strings branch from 3482050 to fe779dc Compare March 22, 2026 08:27

Merge branch 'main' into feat/rope-strings

a0b8538

jedel1043 reviewed Mar 29, 2026

View reviewed changes

Comment thread core/engine/src/builtins/date/utils.rs Outdated

Comment thread core/engine/src/builtins/number/globals.rs Outdated

Comment thread core/engine/src/builtins/regexp/mod.rs Outdated

akash-R-A-J added 2 commits April 2, 2026 05:00

refactor(string): maintain rope laziness in date, number and regexp b…

0024baf

…uiltins

Merge branch 'feat/rope-strings' of https://github.com/akash-R-A-J/boa …

f157f37

…into feat/rope-strings

akash-R-A-J requested a review from jedel1043 April 2, 2026 00:01

HalidOdat reviewed Apr 2, 2026

View reviewed changes

		/// Reference to the static vtable for this string kind.
		pub vtable: &'static JsStringVTable,

	static FIBONACCI_THRESHOLDS: [usize; 46] = [
	static FIBONACCI_THRESHOLDS: [u32; 46] = [

	$crate::string::JsString::concat(&$crate::string::JsString::from($x), &$crate::string::JsString::from($y))
	$crate::string::JsString::concat(&crate::js_string!($x), &crate::js_string!($y))

	$crate::string::JsString::concat_array_strings(&[ $( $crate::string::JsString::from($s) ),+ ])
	$crate::string::JsString::concat_array_strings(&[ $( $crate::js_string!($s) ),+ ])

Uh oh!

Conversation

akash-R-A-J commented Mar 11, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Key Changes

Rope Strings

Engine Integration

Thin VTable & Performance Recovery

Benchmarks

Concatenation Stress Test

V8 Combined Suite

Summary

Uh oh!

codecov bot commented Mar 12, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

Uh oh!

Uh oh!

jedel1043 Mar 12, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

akash-R-A-J Mar 12, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

zhuzhu81998 left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

github-actions bot commented Mar 14, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Test262 conformance changes

Uh oh!

Uh oh!

akash-R-A-J commented Mar 14, 2026

Uh oh!

Uh oh!

akash-R-A-J commented Mar 17, 2026

Uh oh!

github-actions bot commented Mar 22, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Test262 conformance changes

Uh oh!

zhuzhu81998 commented Mar 29, 2026

Uh oh!

Uh oh!

Uh oh!

Uh oh!

HalidOdat Apr 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

akash-R-A-J commented Mar 11, 2026 •

edited

Loading

codecov bot commented Mar 12, 2026 •

edited

Loading

jedel1043 Mar 12, 2026 •

edited

Loading

akash-R-A-J Mar 12, 2026 •

edited

Loading

github-actions bot commented Mar 14, 2026 •

edited

Loading

github-actions bot commented Mar 22, 2026 •

edited

Loading

HalidOdat Apr 2, 2026 •

edited

Loading