[flang-dev] CMake standalone and compile flags

[David Truby via flang-dev]

I believe out of tree builds are also used by distributors; For example most linux distributions have separate packages for llvm, clang, libcxx etc and I believe make this work by building out of tree. I'm not a distributor though so I'm not fully sure this is the case.

David
________________________________
From: Isuru Fernando <isuruf at gmail.com>
Sent: 04 May 2020 19:13
To: Mehdi AMINI <joker.eph at gmail.com>
Cc: David Truby <David.Truby at arm.com>; Flang Development List <flang-dev at lists.llvm.org>
Subject: Re: [flang-dev] CMake standalone and compile flags



On Mon, May 4, 2020 at 12:58 PM Mehdi AMINI <joker.eph at gmail.com<mailto:joker.eph at gmail.com>> wrote:


On Mon, May 4, 2020 at 10:41 AM Isuru Fernando <isuruf at gmail.com<mailto:isuruf at gmail.com>> wrote:


On Mon, May 4, 2020 at 12:30 PM Mehdi AMINI via flang-dev <flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>> wrote:


On Mon, May 4, 2020 at 10:17 AM David Truby <David.Truby at arm.com<mailto:David.Truby at arm.com>> wrote:
Hi Mehdi,

That branch enables out of tree builds; I believe there are people that still rely on those which is why we added that branch in when rewriting the CMake infrastructure.

I assume that folks were relying on this before the merge in the monorepo, but are there still reason to maintain this from now on? Can we put an end-date on it?
I'm trying to figure out if this is just a transition thing or if this is really something useful on the long run (and why).

Most LLVM subprojects have out-of-tree builds. For eg: clang, lld, lldb, libcxx, openmp, clang-rt  and it's a useful option to have.

But I'd claim that most of them only have this because that was the norm until 6 months ago when everything merged in the monorepo :)
(builtin/runtime libraries are a different case though, they often must built against a just-built compiler)

Tim had a good point about the usefulness of it though: scaling the builds of multiple flang variants against the same LLVM. I wonder how fragile / useful is this though (I never done so)? You likely have to disable LLVM_ABI_BREAKING_CHECKS if you're trying to build a Debug flang against a release LLVM (or vice versa). If your LLVM is not a Debug one you also don't have debug info for the LLVM data structure / libraries, this may be limiting when debugging flang.

It's not only the build type. Some subprojects have cmake options that developers want to check. For eg: clang has lots of options that are specific to clang. https://github.com/llvm/llvm-project/blob/master/clang/CMakeLists.txt#L266-L275. This is useful for users too who may want a subproject with a different option from their package manager version or if the subproject is not available from their package manager.

Isuru


mlir is an exception.

That's because it is the only project that started *after* the monorepo was a thing and always assumed it.

--
Mehdi



Isuru


There's an open review to fix a similar issue as is reported in the discussion you linked but it seems to have stalled on a discussion about whether to continue to force Werror on, perhaps you could take a look at that and comment there if it fixes this issue or not?

The review is at https://reviews.llvm.org/D78306

Thanks, that was roughly what I was about to start doing!
In general I think we should avoid doing directly `set(CMAKE_CXX_FLAGS ...` and instead use the `add_flag_if_supported` macros provided by LLVM, e.g.: https://github.com/llvm/llvm-project/blob/master/mlir/lib/CMakeLists.txt#L2

Werror is always difficult to maintain because different compilers (and compiler versions) have different handling (possibly even opposite!) warnings. It is a good thing to have for developers, but it may be a bit hostile to users to have it on by default.

--
Mehdi




Thanks
David Truby
________________________________
From: flang-dev <flang-dev-bounces at lists.llvm.org<mailto:flang-dev-bounces at lists.llvm.org>> on behalf of Mehdi AMINI via flang-dev <flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>>
Sent: 04 May 2020 18:12
To: Flang Development List <flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>>
Subject: [flang-dev] CMake standalone and compile flags

Hi,

I was looking into fixing this error that someone has when building flang: https://llvm.discourse.group/t/error-when-building-with-cmake/935

Then I noticed that flang/CMakeLists.txt still has this branch: https://github.com/llvm/llvm-project/blob/master/flang/CMakeLists.txt#L42 ; is there anyone that relies on it or can we simplify this file and the way the options are setup?

Thanks,

--
Mehdi

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