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

Wed Mar 8 15:08:46 PST 2017

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?

Thanks again,
Hal

>
> 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 <mailto: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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>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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