Re: [PATCH] Disable non-ABI-compliant optimisations for live patching
From: Pavel Machek
Date: Mon Jun 27 2016 - 04:21:28 EST
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
--
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