Re: [PATCH] Disable non-ABI-compliant optimisations for live patching

[Pavel Machek <pavel@ucw.cz>]

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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