[flang-dev] CMake standalone and compile flags

[Isuru Fernando via flang-dev]

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

>
>
> On Mon, May 4, 2020 at 10:41 AM Isuru Fernando <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> wrote:
>>
>>>
>>>
>>> On Mon, May 4, 2020 at 10:17 AM David Truby <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> on behalf of
>>>> Mehdi AMINI via flang-dev <flang-dev at lists.llvm.org>
>>>> *Sent:* 04 May 2020 18:12
>>>> *To:* Flang Development List <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
>>>>
>>>> _______________________________________________
>>> flang-dev mailing list
>>> flang-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/flang-dev
>>>
>>
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