3 ms·

C++ vtables need 2 levels of indirection. See the asm or decompile it with ghidra. First the vtable field, and then the method field. Of course you have to wor

by rurban 3y ago

C++ vtables need 2 levels of indirection. See the asm or decompile it with ghidra. First the vtable field, and then the method field. Of course you have to worry about pointer chasing, when you can easily avoid it. Either via a switch to a single indirection (by passing method pointers around) or inlining with final. Or other compile-time specialization.

simonask 3y ago
Show me the receipts. :-) In general it takes a significant amount of nondeterministic pointer chasing to fool modern branch predictors. Decades of research have been put into optimizing the hardware for languages like C++ and Java, both of which exhibit a lot of pointer chasing.
phire 3y ago
Though the branch predictor can chew though both layers of indirection. It can actually start fetching code from the function (and even executing it) before it even reads the function pointer from the vtable. Though, that assumes a correct prediction. But modern branch predictors are really good, they can track and correctly predict hundreds (if not thousands) of indirect calls, taking into account the history of the last few branches (so it can even get an idea of what class is currently being executed, and make branch predictions based on that). Modern branch predictors do a really good job at chewing up indirect branches in hot sequences of code. Virtual functions are probably the most harmful for warm code. We are talking about code that's executed too often to be considered cold code, but not often enough to stick around in the branch predictors' cache, executed only a few hundred times a second. It's a death by a thousand cuts type thing. And that's where devirtualisation will help the most... As long as you don't go too far with the inlining and start causeing icache misses with code bloat. In an ideal would the compiler would inline enough to devirtualise the class, but not necessarily inline the actual function (unless they are small, or only called from one place)

by rurban 3y ago

simonask 3y ago

Show me the receipts. :-) In general it takes a significant amount of nondeterministic pointer chasing to fool modern branch predictors. Decades of research have been put into optimizing the hardware for languages like C++ and Java, both of which exhibit a lot of pointer chasing.

phire 3y ago

Though the branch predictor can chew though both layers of indirection. It can actually start fetching code from the function (and even executing it) before it even reads the function pointer from the vtable. Though, that assumes a correct prediction. But modern branch predictors are really good, they can track and correctly predict hundreds (if not thousands) of indirect calls, taking into account the history of the last few branches (so it can even get an idea of what class is currently being executed, and make branch predictions based on that). Modern branch predictors do a really good job at chewing up indirect branches in hot sequences of code. Virtual functions are probably the most harmful for warm code. We are talking about code that's executed too often to be considered cold code, but not often enough to stick around in the branch predictors' cache, executed only a few hundred times a second. It's a death by a thousand cuts type thing. And that's where devirtualisation will help the most... As long as you don't go too far with the inlining and start causeing icache misses with code bloat. In an ideal would the compiler would inline enough to devirtualise the class, but not necessarily inline the actual function (unless they are small, or only called from one place)