[PATCH] D20119: [libunwind] Improve unwinder stack usage

[Asiri Rathnayake via cfe-commits]

rmaprath created this revision.
rmaprath added reviewers: jroelofs, bcraig.
rmaprath added a subscriber: cfe-commits.
Herald added a subscriber: aemerson.

This patch is generalization of D19920.

A new native-only libunwind variant (selectable through the `-DLIBUNWIND_ENABLE_CROSS_UNWINDING=OFF` cmake option) is introduced.

The sizes of `Registers_xxx` and the associated `UnwindCursor<A, R>` types are strictly matched against the sizes of `unw_context_t` and `unw_cursor_t` types defined in `libunwind.h`. This improves the stack usage of the unwinder as only just enough memory is reserved on the stack to hold the virtual register set on each architecture.

I've locally tested ARM, AArch64, x86 (compile-only) and x86_64 builds of the new variant. Could not perform any testing of PPC or OpenRISK as I couldn't get the cross compiling to work properly. Ideally, we'll have to switch the buildbots to use this new build option so that these strict limits will be validated (and will continue to be validated). This should be fine I think - if the library works for these restricted limits, the general case (which supports cross-unwinding) should follow.

If the patch is accepted, I will start a discussion on cfe-dev about switching the buildbot configurations (I'm assuming we don't have any cross-unwinding buildbot setups). 
 

http://reviews.llvm.org/D20119

Files:
  CMakeLists.txt
  include/__libunwind_config.h
  include/libunwind.h
  src/CompactUnwinder.hpp
  src/Registers.hpp
  src/UnwindCursor.hpp
  src/config.h
  src/libunwind.cpp

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