[llvm-dev] RFC: Large, especially super-linear, compile time regressions are bugs.

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Thu Mar 31 13:28:33 PDT 2016

LLVM has a wonderful policy regarding broken commits: we revert to green.
We ask that a test case be available within a reasonable time frame
(preferably before, but some exceptions can be made), but otherwise we
revert the offending patch, even if it contains nice features that people
want, and keep the tree green. This is an awesome policy.

I would like to suggest we adopt and follow the same policy for compile
time regressions that are large, and especially for ones that are
super-linear. As an example from the previous thread:

On Mon, Mar 14, 2016 at 12:14 PM Bruno Cardoso Lopes via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> > There is a possibility that r259673 could play a role here.
> >
> > For the buildSchedGraph() method, there is the -dag-maps-huge-region that
> > has the default value of 1000. When I commited the patch, I was expecting
> > people to lower this value as needed and also suggested this, but this
> has
> > not happened. 1000 is very high, basically "unlimited".
> >
> > It would be interesting to see what results you get with e.g. -mllvm
> > -dag-maps-huge-region=50. Of course, since this is a trade-off between
> > compile time and scheduler freedom, some care should be taken before
> > lowering this in trunk.
> Indeed we hit this internally, filed a PR:
> https://llvm.org/bugs/show_bug.cgi?id=26940

I think we should have rolled back r259673 as soon as the test case was

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