On Fri, Sep 04, 2020 at 12:18:12PM -0500, Qing Zhao wrote: > > I call this very expensive, already, > > Yes, I think that 17.56% on average is quite expensive. That’s the data for > -fzero-call-used-regs=all, the worst case i.e, clearing all the call-used > registers at the return. > > However, if we only clear USED registers, the worst case is 1.72% on average. > This overhead is very reasonable.
No, that is the number I meant. 2% overhead is extremely much, unless this is magically super effective, and actually protects many things from exploitation (that aren't already protected some other way, SSP for example). > > and it is benchmarked on a target > > where this should be very cheap (it has few registers) :-/ > > It’s a tradeoff to improve the software security with some runtime overhead. Yes. Which is why I asked for numbers of both sides of the equation: how much it costs, vs. how much value it brings. > For compiler, we should provide such option to the users to satisfy their > security need even though the runtime overhead. Of course, during compiler > implementation, we will do our best to minimize the runtime overhead. There also is a real cost to the compiler *developers*. Which is my prime worry here. If this gives users at most marginal value, then it is real cost to us, but nothing to hold up to that. Segher
