llvm dev - Mar 2017 - Use of host/target compiler when building compiler-rt

> 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
<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.
Of course I agree that we should switch the default to something that uses the
just-built compiler so people unaware of these issues get the safe and correct
thing.

- Matthias

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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.

> Of course I agree that we should switch the default to something that uses
> the just-built compiler so people unaware of these issues get the safe and
> correct thing.
>
> - Matthias
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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> 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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