[llvm-dev] Why we don't have eternal optimizers?

Stefanos Baziotis via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 13 09:12:55 PDT 2020

Hi everyone,

Lately I've been thinking of the optimization model of almost any optimizer
The optimizer should finish "at a reasonable time". For example,
for a 20k lines program, the optimizer should finish in a couple of minutes
and not
e.g. a couple of days.

And my question is, why? For almost all programs, the period that starts
when the program
starts being developed to the day it is released is way bigger than a
couple of minutes.
In the meantime, a couple of modules might not have been touched for months.
And an unconstrained optimizer could do its best in all this time.

Given that this eternal optimizer will respect the initial semantics, one
can compile
a module, test it, and then let the optimizer do its best. The behavior of
a module should
only be faster, no need to re-test.

In the meantime of course, we need to test the program as a whole and other
might actually change. For that we can already what happens today which is
"reasonably optimized" builds.

This is a humble opinion and I'm looking forward to hear the opinion of the
LLVM community.

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