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

[David Blaikie via llvm-dev]

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)
>
>
>
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