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

[Matthias Braun via llvm-dev]

> 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 <mailto: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 <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> 
>>> On Mar 8, 2017, at 3:08 PM, Hal Finkel <hfinkel at anl.gov <mailto: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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