[cfe-dev] [llvm-dev] [Github] RFC: linear history vs merge commits

Jeremy Lakeman via cfe-dev cfe-dev at lists.llvm.org
Thu Jan 31 16:34:23 PST 2019

I realise that llvm trunk can move fairly quickly.
So my original, but brief, suggestion was to merge the current set of
approved patches together rather than attempting them one at a time.
Build on a set of fast smoke test bots. If something breaks, it should be
possible to bisect it to reject a PR and make forward progress.
Occasionally bisecting a large set of PR's should still be less bot time
than attempting to build each of them individually.
Blocking the PR's due to target specific and or slow bot failures would be
a different question.
You could probably do this with a linear history, so long as the final tree
is the same as the merge commit, it should still build.
I'm just fond of the idea of trying to prevent bad commits from ever being
merged. Since they sometimes waste everyone's time.

On Fri, 1 Feb 2019 at 04:02, David Greene <dag at cray.com> wrote:

> <paul.robinson at sony.com> writes:
> > Systems that I've seen will funnel all submitted PRs into a single queue
> > which *does* guarantee that the trial builds are against HEAD and there
> > are no "later commits" that can screw it up. If the trial build passes,
> > the PR goes in and becomes the new HEAD.
> The downside of a system like this is that when changes are going in
> rapidly, the queue grows very large and it takes forever to get your
> change merged.  PR builds are serialized and if a "build" means "make
> sure it builds on all the Buildbots" then it takes a very long time
> indeed.
> There are ways to parallelize builds by speculating that PRs will build
> cleanly but it gets pretty complicated quickly.
> > But this would be a radical redesign of the LLVM bot system, and would
> > have to wait until we're done with the GitHub migration and can spend
> > a couple of years debating the use of PRs. :-)
> Heh.  Fully guaranteeing buildability of a branch is not a trivial task
> and will take a LOT of thought and discussion.
>                               -David
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