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

[David Truby via flang-dev]

Hi all,

I agree with Michael here: having every possible build configuration fixed in every commit is an impossible task. I submitted the patch in question because earlier builds broke my build configuration (and any other builds with multi-config generators, e.g. msbuild), and I believe submitting such a patch to fix these configurations was reasonable, just as submitting patches to fix out-of-tree builds is reasonable.

I do not believe that this patch should have been reverted, as such a revert seems to suggest that one (non-standard?) build configuration (out of tree builds) is more important than another (non-standard?) build configuration (multi-config builds). The patch in question didn't fail any build bots and doesn't (I believe) fall short of the LLVM Developer Guidelines guidance on quality of patches. I believe a better course of action here would have been a patch from someone that requires out-of-tree builds to work, and I submitted some suggestions on places to look to develop an easy fix.

In response to out of tree builds being a critical feature that no patch should break: I think in general it is unreasonable to state that _any_ feature that is not defended by buildbots is a critical feature that no patch should break. This puts a really unreasonable expectation on the developer of every patch to manually test with every possible configuration that some people might care about, rather than using the automated structure we already have to detect regressions.

Thanks
David Truby
________________________________
From: flang-dev <flang-dev-bounces at lists.llvm.org> on behalf of Isuru Fernando via flang-dev <flang-dev at lists.llvm.org>
Sent: 31 July 2020 20:52
To: Steve Scalpone <sscalpone at nvidia.com>
Cc: Peter Steinfeld via flang-dev <flang-dev at lists.llvm.org>
Subject: Re: [flang-dev] Out-of-tree flang builds

> Support for out-of-tree builds, like BUILD_SHARED_LIBS or gn builds, are helpful for developers, but not relevant for release builds.

I disagree. Out-of-tree builds are also very useful for downstream distributors when packaging individual components of the LLVM project.

Isuru

On Fri, Jul 31, 2020 at 2:45 PM Steve Scalpone via flang-dev <flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>> wrote:
+1 on Pete's and Tim's reply. I am often building multiple branches at the same time.
________________________________
From: flang-dev <flang-dev-bounces at lists.llvm.org<mailto:flang-dev-bounces at lists.llvm.org>> on behalf of Peter Steinfeld via flang-dev <flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>>
Sent: Friday, July 31, 2020 12:23 PM
To: Michael Kruse <llvm at meinersbur.de<mailto:llvm at meinersbur.de>>
Cc: flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org> <flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>>
Subject: Re: [flang-dev] Out-of-tree flang builds

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Iterative builds are very useful and work well.



But when I’m reviewing a change from someone else or starting on a new change for myself, I build all of flang.  In the last six hours, I’ve done four non-iterative builds, for example.



Pete



From: Michael Kruse <llvm at meinersbur.de<mailto:llvm at meinersbur.de>>
Sent: Friday, July 31, 2020 12:01 PM
To: Peter Steinfeld <psteinfeld at nvidia.com<mailto:psteinfeld at nvidia.com>>
Cc: flang-dev at lists.llvm.org<mailto: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.


Michael







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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