RISC-V LLVM sync-up call 3rd October 2019

For background on these calls, see

Reminder: the purpose is to co-ordinate between active contributors.
If you have support questions etc then it's best to post to llvm-dev.

We have a call each Thursday at 4pm BST, via

Issues to discuss include the following. I think it's mostly pretty
brief / heads-up type items, as I haven't had many topic submissions
this week:

* Machine outliner and related patches
  * It would be good to further discuss
⚙ D68290 [RISCV] Added isCompressibleInst() to estimate size in getInstSizeInBytes() and the possibility of using the new
helper function for getInstSizeInBytes
  * It looks like both the machine outliner and save/restore libcall
patches are waiting for miscompiles to be addressed
  * Let's decouple shrink-wrapping from this, as we should be able to
get that merged pretty rapidly if the dependency on the libcall
save/restore patch is broken

* Issues from the Zig frontend
  * f16 expansion
  * getRegisterByName issue

* Target-specific scheduling model - any updates from those working on this?

* Splitting SP adjustment ⚙ D68011 [RISCV] Split SP adjustment to reduce the offset of callee saved register spill and restore We think
this is good to merge. Thanks Shiva for the patch and Luis for the
careful review!

* R_RISCV_RELAX_NORVC proposed relocation type - see the psABI
discussion at Add relocation `R_RISCV_RELAX_NORVC` for linker relaxation. by Nelson1225 · Pull Request #116 · riscv-non-isa/riscv-elf-psabi-doc · GitHub -
the original submitter has updated the proposal based on feedback

* libunwind RV64 support - patch posted yesterday
⚙ D68362 [libunwind][RISCV] Add 64-bit RISC-V support - haven't had a chance to start to
review, but more eyes very welcome

* Making RA callee-saved ⚙ D67698 [RISCV] Remove RA from reserved register to use as callee saved register - we
discussed this before. I'm going to do another survey of what other
backends do, but given GCC treats it as callee saved I'm leaning
towards going with this. Speak up if you have any concerns

* Anything else (as usual, always helpful to drop me an email ahead of
time to add to the agenda - but feel free to list things here)