[flang-dev] Out-of-tree flang builds

Timothy Keith via flang-dev flang-dev at lists.llvm.org
Fri Jul 31 12:22:00 PDT 2020

Out-of-tree builds are useful when you're working on more than one thing at once (including trying out other people's changes). You don't need multiple builds of llvm and mlir, just of flang.
From: flang-dev <flang-dev-bounces at lists.llvm.org> on behalf of Michael Kruse via flang-dev <flang-dev at lists.llvm.org>
Sent: Friday, July 31, 2020 12:01 PM
To: Peter Steinfeld <psteinfeld at nvidia.com>
Cc: flang-dev at lists.llvm.org <flang-dev at lists.llvm.org>
Subject: Re: [flang-dev] Out-of-tree flang builds

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Support for out-of-tree builds, like BUILD_SHARED_LIBS or gn builds, are helpful for developers, but not relevant for release builds. Requiring to maintain all combinations of build configurations is not feasible before every commit, this is why they are only best effort. When you notice a out-of-tree build breaking, you are free to fix the problem (and assuming the fix is straightforward, low-risk, without review).

However, I am wondering, why isn't an iterative build sufficient? Assuming you are not touching any non-flang files, make/ninja should not try to rebuild them.


Am Fr., 31. Juli 2020 um 13:41 Uhr schrieb Peter Steinfeld via flang-dev <flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>>:

Out-of-tree builds are a way to build only the flang code using a pre-existing full build of llvm.  Out-of-tree flang builds are four times faster than full builds.  Thus, for those of us actively developing code and reviewing other people's changes, out-of-tree builds significantly improve our productivity.

Unfortunately, out-of-tree builds are not currently supported by the llvm buildbots.  Thus, people changing build files must test this feature themselves.  The process of doing an out-of-tree build is described in the flang overview document -- https://github.com/llvm/llvm-project/tree/master/flang.  Alternatively, people changing build files could ask someone familiar with out-of-tree builds to review their changes.

A recent update from David Truby broke out-of-tree builds -- https://reviews.llvm.org/D84022.  In the conversation in the Phabricator review, David states that he understands that "out-of-tree builds are considered a "best effort" feature that isn't guaranteed to work".  But since out-of-tree builds are critical for my development, I don't consider them optional.

I'm writing this note to bring this issue to the broader flang community.  I consider out-of-tree builds to be a critical feature that no change should break.  What do you think?

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