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DFG/FTL: bail out of the PerformPromiseThenOneHandler fast path when an async context is active #278

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Merged
Jarred-Sumner merged 2 commits into from
Jul 11, 2026
Merged

DFG/FTL: bail out of the PerformPromiseThenOneHandler fast path when an async context is active #278

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45bbb77
DFG/FTL: bail out of the PerformPromiseThenOneHandler fast path when …
robobun Jul 9, 2026
d95b137
DFG: allocate asyncContext GPRTemporary before any slow-path branch
robobun Jul 10, 2026
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19 changes: 17 additions & 2 deletions Source/JavaScriptCore/dfg/DFGSpeculativeJIT.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -18856,14 +18856,29 @@ void SpeculativeJIT::compilePerformPromiseThenOneHandler(Node* node)
#if USE(JSVALUE64)
GPRTemporary packed(this);
GPRReg packedGPR = packed.gpr();
#if USE(BUN_JSC_ADDITIONS)
GPRTemporary asyncContext(this);
GPRReg asyncContextGPR = asyncContext.gpr();
#endif

constexpr unsigned pointerBits = CompactPointerTuple<JSCell*, uint16_t>::maxNumberOfBitsInPointer;
constexpr uint64_t pointerMask = (1ULL << pointerBits) - 1;
constexpr uint64_t flagMask = static_cast<uint64_t>(JSPromise::stateMask | JSPromise::inlineReactionKindMask) << pointerBits;
constexpr uint64_t mask = pointerMask | flagMask;

JumpList slowCases;
load64(Address(inputPromiseGPR, JSPromise::offsetOfPacked()), packedGPR);
Jump slowPath = branchTest64(NonZero, packedGPR, TrustedImm64(mask));
slowCases.append(branchTest64(NonZero, packedGPR, TrustedImm64(mask)));

#if USE(BUN_JSC_ADDITIONS)
// Inline reactions cannot carry an async context; when one is active,
// take the slow path so performPromiseThen captures it (see JSPromise.cpp).
loadLinkableConstant(LinkableConstant::globalObject(*this, node), asyncContextGPR);
loadPtr(Address(asyncContextGPR, JSGlobalObject::offsetOfAsyncContextData()), asyncContextGPR);
Jump noAsyncContextData = branchTestPtr(Zero, asyncContextGPR);
slowCases.append(branch64(NotEqual, Address(asyncContextGPR, JSInternalFieldObjectImpl<>::offsetOfInternalField(0)), TrustedImm64(JSValue::encode(jsUndefined()))));
noAsyncContextData.link(this);
#endif

uint64_t orBits = static_cast<uint64_t>(JSPromise::isHandledFlag | (static_cast<uint16_t>(kind) << JSPromise::inlineReactionKindShift)) << pointerBits;
or64(TrustedImm64(orBits), packedGPR);
Expand All @@ -18872,7 +18887,7 @@ void SpeculativeJIT::compilePerformPromiseThenOneHandler(Node* node)
store64(handlerGPR, Address(inputPromiseGPR, JSPromise::offsetOfSlot()));
store64(packedGPR, Address(inputPromiseGPR, JSPromise::offsetOfPacked()));

addSlowPathGenerator(slowPathCall(slowPath, this, operationPerformPromiseThenOneHandler, NeedToSpill, ExceptionCheckRequirement::CheckNotNeeded, NoResult, LinkableConstant::globalObject(*this, node), inputPromiseGPR, handlerGPR, resultPromiseGPR, TrustedImm32(static_cast<int32_t>(kind))));
addSlowPathGenerator(slowPathCall(slowCases, this, operationPerformPromiseThenOneHandler, NeedToSpill, ExceptionCheckRequirement::CheckNotNeeded, NoResult, LinkableConstant::globalObject(*this, node), inputPromiseGPR, handlerGPR, resultPromiseGPR, TrustedImm32(static_cast<int32_t>(kind))));
#else
flushRegisters();
callOperationWithoutExceptionCheck(operationPerformPromiseThenOneHandler, LinkableConstant::globalObject(*this, node), inputPromiseGPR, handlerGPR, resultPromiseGPR, TrustedImm32(static_cast<int32_t>(kind)));
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20 changes: 20 additions & 0 deletions Source/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp
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Expand Up @@ -21974,11 +21974,31 @@ IGNORE_CLANG_WARNINGS_END
LBasicBlock fastPath = m_out.newBlock();
LBasicBlock slowPath = m_out.newBlock();
LBasicBlock continuation = m_out.newBlock();
#if USE(BUN_JSC_ADDITIONS)
LBasicBlock asyncContextDataCase = m_out.newBlock();
LBasicBlock asyncContextCheck = m_out.newBlock();
#endif

LValue packed = m_out.load64(inputPromise, m_heaps.JSPromise_packed);
#if USE(BUN_JSC_ADDITIONS)
// Inline reactions cannot carry an async context; when one is active,
// take the slow path so performPromiseThen captures it (see JSPromise.cpp).
m_out.branch(m_out.notZero64(m_out.bitAnd(packed, m_out.constInt64(mask))), rarely(slowPath), usually(asyncContextDataCase));

LBasicBlock lastNext = m_out.appendTo(asyncContextDataCase, asyncContextCheck);
LValue asyncContextData = m_out.loadPtr(m_out.absolute(reinterpret_cast<char*>(globalObject) + JSGlobalObject::offsetOfAsyncContextData()));
m_out.branch(m_out.isNull(asyncContextData), unsure(fastPath), unsure(asyncContextCheck));

m_out.appendTo(asyncContextCheck, fastPath);
LValue asyncContext = m_out.load64(asyncContextData, m_heaps.JSInternalFieldObjectImpl_internalFields[0]);
m_out.branch(m_out.equal(asyncContext, m_out.constInt64(JSValue::ValueUndefined)), usually(fastPath), rarely(slowPath));

m_out.appendTo(fastPath, slowPath);
#else
m_out.branch(m_out.notZero64(m_out.bitAnd(packed, m_out.constInt64(mask))), rarely(slowPath), usually(fastPath));

LBasicBlock lastNext = m_out.appendTo(fastPath, slowPath);
#endif
LValue newPacked = m_out.bitOr(m_out.bitOr(packed, m_out.constInt64(orBits)), resultPromise);
m_out.store64(handler, inputPromise, m_heaps.JSPromise_slot);
m_out.store64(newPacked, inputPromise, m_heaps.JSPromise_packed);
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1 change: 1 addition & 0 deletions Source/JavaScriptCore/runtime/JSGlobalObject.h
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Expand Up @@ -753,6 +753,7 @@ class JSGlobalObject : public JSSegmentedVariableObject {
#if USE(BUN_JSC_ADDITIONS)
bool isAsyncContextTrackingEnabled() const { return m_isAsyncContextTrackingEnabled; }
void setAsyncContextTrackingEnabled(bool isEnabled) { m_isAsyncContextTrackingEnabled = isEnabled; }
static constexpr ptrdiff_t offsetOfAsyncContextData() { return OBJECT_OFFSETOF(JSGlobalObject, m_asyncContextData); }
#endif

bool hasDebugger() const { return m_debugger; }
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