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

[Evgenii Stepanov via llvm-dev]

I think the long term plan is to build compiler-rt with just build
clang. Nothing else makes sense, really. AFAIK that's what
LLVM_BUILD_EXTERNAL_COMPILER_RT is for.

On Wed, Mar 8, 2017 at 2:35 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
> On Wed, Mar 8, 2017 at 2:03 PM Sterling Augustine <saugustine at google.com>
> wrote:
>>
>> Yes, this is a aspect of the larger problem that clang bootstrap doesn't
>> work for a cross-compiler. The build (mostly?) assumes that host==target
>> during the build of clang itself, and then if you want another architecture
>> also, you run a second build of the target libraries, and manually merge the
>> trees.
>
>
> I kind of roughly follow that, but not too well.
>
>>
>> If you think about compiler-rt as being compiled for the target rather
>> than the host, the problem you describe here is exactly the same one, and we
>> have been getting lucky.
>
>
> Sure - if a PPC clang is being built from an x86 host, how would compiler-rt
> be built (OK, it could be built with the just-built clang, which it isn't at
> the moment) and tested (can't really be tested because the host can't run
> PPC binaries).
>
>>
>> At the moment, the blaze builds of clang do exactly the procedure
>> described above, so this hasn't been a terrible problem for Google, but I do
>> think it is something that should be fixed (I'm working on another aspect of
>> compiler-rt bringup at the moment, so won't solve this in the immediate
>> future.)
>
>
> Rightio
>
>>
>>
>> gnu systems have a make variable, "CC_FOR_TARGET" that addresses this
>> problem. I imagine llvm should adopt a similar mechanism inside cmake.
>
>
> Not sure I follow on the need/use of CC_FOR_TARGET compared to using the
> just-built clang as the CC_FOR_TARGET (which it seems we have some plumbing
> for already - the just-built clang is used for building the compiler-rt
> tests, but not for building the library. I /think/ it should be used for
> both)
>
> - Dave
>
>>
>>
>> On Wed, Mar 8, 2017 at 1:54 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>> I stumbled across what seems to be a bug (to me) in the compiler-rt
>>> build:
>>>
>>> The compiler-rt libraries themselves are built with the host compiler
>>> while the tests are built and then linked with the just-built clang.
>>>
>>> It was my understanding that the goal/intent/need was to have the
>>> compiler-rt library build with the just-built clang? Did I misunderstand
>>> that?*
>>>
>>> Sterling: Chandler seemed to think you might be interested in this issue
>>> & possibly addressing it given you're working on compiler-rt bring-up? It'd
>>> probably be useful to have compiler-rt built with the just-built clang for
>>> performance reasons.
>>>
>>> Evgeniy - not sure if you're interested in this or have much context?
>>> Know if this is right/wrong/neutral, etc?
>>>
>>> * reasons include performance, ABI compatibility, etc (I thought this was
>>> necessary for correctness in some way) - also, otherwise it seems excessive
>>> to hold up the whole build on waiting for just-built clang to finish, then
>>> use that to compile some tests. (well, I realize some of the tests are
>>> end-to-end, so they do need the just-built compiler)
>>
>>
>