[Bug rtl-optimization/64164] [4.9/5 Regression] one more stack slot used due to one less inlining level

[amacleod at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64164

Andrew Macleod <amacleod at redhat dot com> changed:

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                 CC|                            |amacleod at redhat dot com

--- Comment #8 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Alexandre Oliva from comment #6)

> 
> Coalescing SSA names even when neither has a root removes the optimization
> differences for this testcase.

I would worry about side effects... Coalescing is always a balancing act.   If
you allow things to coalesce when neither has a root, a few of those coalesced
together may prevent a coalesce with a much more desirable root in other
programs.  

And of course more coalesces may mean longer live ranges...  so any changes
like this need a lot of studying of side effects on a much larger set of
problems. 

It was a long time ago, so I don't remember the reasons, but most of these
kinds of restrictions were rooted in some kind of rationale... usually from
examining an issue of some sort and introducing the restriction to avoid it. 
The original conditions may no longer exist, but we'd have to run tests to make
sure nothing shows up.  Perhaps tweaking the costing model could help too.