[Issue 12440] Implement whole-program analysis

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12440


bearophile_hugs at eml.cc changed:

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--- Comment #1 from bearophile_hugs at eml.cc 2014-03-22 13:20:36 PDT ---
(In reply to comment #0)
> The idea is to examine the whole program's class hierarchies to determine which
> classes can be made final. Final classes can then benefit from non-virtual
> dispatch of its virtual functions, and even inline them.

Good. (But this can't solve the other problem listed by Manu and C# designers
of virtual calls. It's not just a matter of performance).

Beside finding what class/method can be final, whole program's class analysis
can find what call points of _virtual functions_ always call the same method,
so it can turn those virtual calls into non-virtual calls.

Whole program's call analysis can also find what points call two different
virtual methods. In such points the compiler can use a faster tiny (2-slots
wide) inline cache and avoid the virtual call.

Monomorphic and 2-polimorphic virtual call points cover a large percentage of
all virtual call points (3-polimorphic and megamorphic call points are
comparatively). So doing this you remove a significant percentage of all
virtual calls.



> Hence, to work, this would have to add a flag to the compiler, such as
> -wholeprogram, that causes the compiler to make the assumption that all classes
> defined in files supplied to the compiler on the command line (not imports) are
> never derived from from other object files or DLLs.

I suggest to avoid the "one trick pony" syndrome: "whole program" analysis is
useful for some other purposes too (like executable libraries:
http://rosettacode.org/wiki/Executable_library a pattern currently not
supported in D. Or to support Link-time-optimization, etc), so perhaps it's a
good idea to give a little stronger meaning to "-wholeprogram", so it's useful
for more than just OO de-virtualization.


> This could be implemented as a pass inserted before the inliner runs.
> 
> Note that if the user throws this flag, and the assumption is not true, the
> resulting program will behave badly in unpredictable ways.

Is it possible to enforce/verify the condition and avoid such undefined
situations?

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