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Disable postMessage optimization when string is < 256 chars (#22006)

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### What does this PR do?

Disable postMessage optimization when string is < 256 chars

If you're going to potentially use these strings as a property or
identifier, which is much more likely for short strings than long
strings, we shouldn't ban atomizing them and the cost of cloning isn't
so much in that case

### How did you verify your code works?
This commit is contained in:
Jarred Sumner
2025-08-20 16:05:43 -07:00
committed by GitHub
parent ede4ba567b
commit 87a5fac697
1 changed files with 12 additions and 4 deletions
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16
src/bun.js/bindings/BunString.cpp
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@@ -280,12 +280,17 @@ BunString toStringView(StringView view)
};
}
// We don't want to ban atomiziation for tiny strings that are potentially going
// to appear as properties/identifiers in JS. So we should only do this for long
// strings that are unlikely to ever be atomized.
static constexpr unsigned int kMinCrossThreadShareableLength = 256;
bool isCrossThreadShareable(const WTF::String& string)
{
if (!string.impl())
if (string.length() < kMinCrossThreadShareableLength)
return false;
auto* impl = string.impl();
const auto* impl = string.impl();
// 1) Never share AtomStringImpl/symbols - they have special thread-unsafe behavior
if (impl->isAtom() || impl->isSymbol())
@@ -306,6 +311,9 @@ Ref<WTF::StringImpl> toCrossThreadShareable(Ref<WTF::StringImpl> impl)
if (impl->bufferOwnership() == StringImpl::BufferSubstring)
return impl->isolatedCopy();
if (impl->length() < kMinCrossThreadShareableLength)
return impl->isolatedCopy();
// 3) Ensure we won't lazily touch hash/flags on the consumer thread
// Force hash computation on this thread before sharing
impl->hash();
@@ -316,8 +324,8 @@ Ref<WTF::StringImpl> toCrossThreadShareable(Ref<WTF::StringImpl> impl)
WTF::String toCrossThreadShareable(const WTF::String& string)
{
if (!string.impl())
return string;
if (string.length() < kMinCrossThreadShareableLength)
return string.isolatedCopy();
auto* impl = string.impl();