Re: [PATCH][RFA/RFC] Stack clash mitigation patch 07/08

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Jul 12, 2017 at 12:20:32AM +0000, Wilco Dijkstra wrote:
> No - like I mentioned before that's not correct nor acceptable as it would imply
> that ~70% of functions need a probe at entry. I did a quick run across SPEC and
> found the outgoing argument size is > 1024 in just 9 functions out of 147000!
> Those 9 were odd special cases due to auto generated code to interface between
> C and Fortran. This is extremely unlikely to occur anywhere else. So even assuming
> an unchecked caller, large outgoing arguments are simply not a realistic threat.
> 
> Therefore even when using a tiny 4K probe size we can safely adjust SP by 3KB
> before needing an explicit probe - now only 0.6% of functions need a probe.
> If we choose a proper minimum probe distance, say 64KB, explicit probes are
> basically non-existent (just 35 functions, or ~0.02% of all functions are > 64KB).
> Clearly inserting probes can be the default as the impact on code quality is negligible.

For non-leaf functions you need at least one probe no matter how small the
frame size is (if it is bigger than 0), explicit or implicit, unless you
perform IPA analysis on the callgraph and determine when that isn't needed,
because you can have deep call stacks that would through functions that
don't touch anything skip stack pages.  Of course, such probes can be stores
of call used registers, it can be any store to the stack.

	Jakub