[llvm-dev] Use of host/target compiler when building compiler-rt

[Sterling Augustine via llvm-dev]

It would be really good to update https://compiler-rt.llvm.org/ with more
detail on how to build this in a supported way.

On Wed, Mar 8, 2017 at 4:48 PM, Matthias Braun <mbraun at apple.com> wrote:

>
> On Mar 8, 2017, at 4:44 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Wed, Mar 8, 2017 at 3:52 PM Matthias Braun via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Mar 8, 2017, at 3:14 PM, Chris Bieneman via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>
>> On Mar 8, 2017, at 3:08 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>>
>> On 03/08/2017 04:55 PM, Chris Bieneman via llvm-dev wrote:
>>
>> David,
>>
>> This is an area that has had a lot of development over the last two years.
>>
>> There are two supported ways in the LLVM build system to build
>> compiler-rt with the just-built compiler.
>>
>> 1) The legacy way is for if compiler-rt is under LLVM/projects. You can
>> specify -DLLVM_BUILD_EXTERNAL_COMPILER_RT=On, which will configure
>> compiler-rt using the just-built clang after clang is built.
>>
>>
>> Why is this not the default?
>>
>>
>> Two reasons. (1) It is buggy, and nobody fixed the issues because (2) the
>> long-term plan is to not support building compiler-rt (or any other runtime
>> library) under llvm/projects. The goal is to migrate entirely to
>> llvm/runtimes where the support is more complete.
>>
>>
>> From a compiler developers point of view I wanted to stress that we at
>> least should keep the option not to do it, as in my experience development
>> workflow heavily slows down if you have to wait around for compiler-rt to
>> rebuild after every small change you make to the compiler.
>>
>
> If that's a valid option, I'd be open to it - it does really slow down the
> dev cycle & it's one of the reasons I often end up moving compiler-rt
> out/not building it anymore.
>
> I'm not sure if that's an option at the moment - if it is, I'd love to
> know how to do that & it might at least unblock my use case anyway, even if
> there's still a need for a broader fix.
>
> The legacy way (I wasn't aware it is legacy) of "-DLLVM_BUILD_EXTERNAL_COMPILER_RT=On/Off"
> has worked fine for me so far. I haven't tried the runtimes directory style
> yet, but it looks like a nicer more explicit way than specifying cmake
> flags and is just suffering from a bit probably?
>
> - Matthias
>
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