Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

[Jamie Lokier]

Linus Torvalds wrote:
> > This is about storage allocation, not aliases.  Storage allocation only
> > depends on lifetime.
> 
> Well, the thing is, code motion does extend life-times, and if you think 
> you can move stores across each other (even when you can see that they 
> alias statically) due to type-based alias decisions, that does essentially 
> end up making what _used_ to be disjoint lifetimes now be potentially 
> overlapping.

Sometimes code motion makes code faster and/or smaller but use more
stack space.  If you want to keep the stack use down, it blocks some
other optimisations.

Register allocation is similar: code motion optimisations may use more
registers due to overlapping lifetimes, which causes more register
spills and changes the code.  The two interact; it's not trivial to
optimise fully.

-- Jamie
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