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

Wed Mar 8 14:55:17 PST 2017

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.

2) The new way, is to place compiler-rt under LLVM/runtimes. In this path the build system will automatically build with the just-built compiler. This path also splits compiler-rt into two separate build steps, one that configures and builds the builtins with the just-built compiler, and a second that configures and builds the sanitizer libraries.

The second path also works for many (but not all) of our other runtime library projects. I know it works for libcxx, libcxxabi, and libunwind. Petr Hosek (CC'd) has also been working on support for multi-arch builtin and runtime library builds so that you can generate full cross-compilers from a single cmake invocation.

-Chris

> On Mar 8, 2017, at 2:35 PM, David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
> On Wed, Mar 8, 2017 at 2:03 PM Sterling Augustine <saugustine at google.com <mailto: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 <mailto: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)
> 
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