search for: getmyobjectthathappenstobet

...validly formed > to an object of dynamic type T will always refer to an object of that > dynamic type. So does that mean that we're allowed to assume that, given an "old" pointer to that memory, the vtbl-ptr slot hasn't changed? Which allows us to devirtualize: Base* pT = GetMyObjectThatHappensToBeT(); // ... // stuff that we can't tell what it does to our memory // ... pT->A(); // more mysterious stuff pT->B(); // etc. as long as we can tell that a given pointer is actually pT and not another pointer copied from it or aliased to it?

...of dynamic type T will always refer to an object of that >> dynamic type. > > So does that mean that we're allowed to assume that, given an "old" > pointer to that memory, the vtbl-ptr slot hasn't changed?  Which > allows us to devirtualize: > > Base* pT = GetMyObjectThatHappensToBeT(); > // ... > // stuff that we can't tell what it does to our memory > // ... > pT->A(); > // more mysterious stuff > pT->B(); > // etc. > > as long as we can tell that a given pointer is actually pT and not > another pointer copied from it or aliased to it?...

...7:00 AM, Kenneth Uildriks <kennethuil at gmail.com> wrote: >> So does that mean that we're allowed to assume that, given an "old" >> pointer to that memory, the vtbl-ptr slot hasn't changed? Which >> allows us to devirtualize: >> >> Base* pT = GetMyObjectThatHappensToBeT(); >> // ... >> // stuff that we can't tell what it does to our memory >> // ... >> pT->A(); >> // more mysterious stuff >> pT->B(); >> // etc. >> >> as long as we can tell that a given pointer is actually pT and not >> another...

...e vptr in advance, so invariant doesn't work here. Making it non-local just means that we would need to know the static call graph, which we don't because we haven't devirtualized yet so all the calls are indirect. Nick > Which > allows us to devirtualize: > > Base* pT = GetMyObjectThatHappensToBeT(); > // ... > // stuff that we can't tell what it does to our memory > // ... > pT->A(); > // more mysterious stuff > pT->B(); > // etc. > > as long as we can tell that a given pointer is actually pT and not > another pointer copied from it or aliased to it?...

On Oct 11, 2010, at 2:01 PM, Kenneth Uildriks wrote: > A better way for a front-end to declare that vtbl-ptr-hacking is not > expected and not supported is for it to emit llvm.invariant.start and > llvm.invariant.end calls for it. Some of us were talking about this apropos your earlier post. @llvm.invariant.start/end aren't appropriate, because the memory *isn't* invariant; the

On Mon, Oct 11, 2010 at 3:55 PM, John McCall <rjmccall at apple.com> wrote: > > On Oct 11, 2010, at 10:43 AM, Kenneth Uildriks wrote: > > On Mon, Oct 11, 2010 at 12:30 PM, John McCall <rjmccall at apple.com> wrote: > > On Oct 11, 2010, at 9:12 AM, Kenneth Uildriks wrote: > > 3. The front-end, recognizing that scribbling on an instance's vtbl > >