On Fri, Nov 03, 2017 at 11:07:04AM -0700, Nick Desaulniers wrote: > On Fri, Nov 3, 2017 at 10:51 AM, Mark Rutland <[email protected]> wrote: > > I had to create an aarch64-linux-gnu-clang wrapper, too. I'm not sure if > > there's build system help to avoid needing that? > > Gah! So a BIG difference with Clang vs GCC for cross compiling is > that Clang by default ships with all backends enabled, and uses a > `-target <triple>` CFLAG to determine the arch to cross compile for, > while GCC is configured at compile time to support one back end, IIUC.
Yup. I initally tried passing CC=clang\ --target=aarch64-linux-gnu ... but since that conflicts with CROSS_COMPILE, you then have to override AS, LD, OBJCOPY, etc, separately too. This is a little less than ideal. :/ > Clang _can_ be built with all other back ends disabled but one, but I > have a problem with this approach: distributions of Clang are rarely > configured this way (all backends enabled by default), and it seems we > just found this past week that Clang if configured with just one > backend, it will silently ignore -target flags for other backends and > generate code for the configured backend (this led to: > https://lkml.org/lkml/2017/11/2/892). I consider this a bug in Clang, > so I just filed: https://bugs.llvm.org/show_bug.cgi?id=35196 Ouch. Thanks for attacking that. > > CC arch/arm64/crypto/aes-ce-cipher.o > > fatal error: error in backend: Do not know how to split the result of this > > operator! > > Yep, we've seen this. It was the FINAL bug in Clang for compiling the > kernel for arm64. It was recently fixed upstream in llvm by > [email protected], but so recent that you'll need to either compiler > Clang from source from ToT or work around it like we have in Android > with: > https://android.googlesource.com/kernel/msm/+/9093342a0186dad05095b70f1806938310ace6e7 Sure. I can live with patch 7 for now, assuming there's nothing else lurking in the shadows. Thanks, Mark.
