On Mon 2016-06-27 10:13:28, Jiri Kosina wrote: > On Mon, 27 Jun 2016, Pavel Machek wrote: > > > > Live patching, as we use it, deliberately disrupts the fabric of > > > compile units; thus all assumptions a compiler can make about the > > > control flow may be invalid. As an example, it could analyse that a > > > callee does not touch a caller-saved register at all, so why waste > > > memory bandwidth saving it? The register allocations for the live > > > patch replacement function may however be quite different. > > > > > > Starting with this example, disable all compiler optimisations that > > > do not strictly comply with the established calling conventions. > > > > I thought that in such case, person creating the live patch should > > notice and adjust patch appropriately, at assembly level if > > neccessary..? > > Yes, that still holds; a lot of things could be automated though, and > creating the automation tools is one of the big TODO items. So the patch is not a bugfix, it is just something that slows down kernel to make stuff easier for the person doing the live patching...? > > If this is not true, and we want gcc to help us, what other > > optimalizations do we need to disable? Even changes inside one compiler > > unit can be "interesting"... > > What would actually be helpful is gcc providing us with a list of > functions where it performed this ABI-violating optimization (similarly, > we're already obtaining list of "what got inlined where"). Unfortunately, > -fdump-ipa-ra is currently missing; I'm talking to gcc guys now to have it > implemented. What you actually want is "whenever source of function A influenced code in function B, I want to be notified", right? If gcc can eliminate an if() brach in function B, because it can tell reading function A it can not happen, you need to know. Maybe that's limited to ABI today, but... Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
