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...s like Java or Scala can safely use it. This is because virtual pointer in Java is set only once for every dynamic type, and you can’t do things like placement new, which means 2 aliasing pointers will always have the same dynamic type (also because every pointer still points to a valid object).The problemThe old model miscompiles on the following example:A *a = new A;a->foo(); A *b = new(a) B;if (a == b) b->foo(); // This call could be devirtualized to A::foo()The problem is that GVN and other pass replaces the SSA value of b with a based on the a==b comparison, which is a totally legal optimiza...

...can safely use it. This is > because virtual pointer in Java is set only once for every dynamic type, > and you can’t do things like placement new, which means 2 aliasing pointers > will always have the same dynamic type (also because every pointer still > points to a valid object).The problemThe old model miscompiles on the > following example:A *a = new A;a->foo(); A *b = new(a) B;if (a == b) > b->foo(); // This call could be devirtualized to A::foo()The problem is > that GVN and other pass replaces the SSA value of b with a based on the > a==b comparison, which is a to...

> On Mar 19, 2018, at 7:27 PM, Piotr Padlewski via cfe-dev <cfe-dev at lists.llvm.org> wrote: > > Hi folks, > > here is a link to the proposal that we've been working on lately: > https://docs.google.com/document/d/16GVtCpzK8sIHNc2qZz6RN8amICNBtvjWUod2SujZVEo/edit?usp=sharing