Determining the size of C++ vtables (original) (raw)

Ioi Lam ioi.lam at oracle.com
Fri Feb 24 03:47:03 UTC 2017

Hi,

I am working on https://bugs.openjdk.java.net/browse/JDK-8005165 (Remove CPU-dependent code in self-patching vtables), I need a way find out the size of a C++ vtable. I ended up doing this:

// Objects of the Metadata types (such as Klass and ConstantPool) have C++ vtables. // (In GCC this is the field ::_vptr, i.e., first word in the object.) // // Addresses of the vtables and the methods may be different across JVM runs, // if libjvm.so is dynamically loaded at a different base address. // // To ensure that the Metadata objects in the CDS archive always have the correct vtable: // // + at dump time: we redirect the _vptr to point to our own vtables inside // the CDS image // + at run time: we clone the actual contents of the vtables from libjvm.so // into our own tables. // // To determine the size of the vtable for each type, we use the following // trick by declaring 2 subclasses: // // class CppVtabTesterA: public InstanceKlass { // virtual int last_virtual_method() {return 1;} // }; // class CppVtabTesterB: public InstanceKlass { // virtual void* last_virtual_method() {return NULL}; // }; // // CppVtabTesterA and CppVtabTesterB's vtables have the following properties: // - Their size (N+1) is exactly one more than the size of InstanceKlass's vtable (N) // - The first N entries have are exactly the same as in InstanceKlass's vtable. // - Their last entry is different. // // So to determine the value of N, we just walk CppVtabTesterA and CppVtabTesterB's tables // and find the first entry that's different

Could anyone comment if this is acceptable? I know it's not 100% portable (C++ doesn't specify where to find the vtable, or what's inside), but my assumptions is the same as the existing code. I.e., _vptr is a pointer located at offset 0 of the object, and it points to a one-dimensional array.

So at least it's not any worse than before?

Thanks

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