unwind: Allow the building of libunwind for MIPS.

[vkalintiris]

Summary

Currently, libunwind doesn't support MIPS. However, with this patch
we do allow the library to build, and we warn the user about the lack of
support for MIPS. Also, the dummy unw_getcontext() implementation for MIPS just
traps on function entry in order to avoid any confusion with silent/weird
failures at runtime.

This allows us to test an LLVM-based toolchain without the dependency on a
GCC toolchain. Of course, C++ exception handling and other things that depend
on stack unwinding will not work until we add a proper implementation of the
stub functions.

[vkalintiris]

@compnerd: Thanks for the review. I was wondering, are there any public buildbots testing libunwind commits? I can't see anything at the llvm.org buildbots.

libunwind/trunk/src/UnwindRegistersSave.S

DEFINE_LIBUNWIND_FUNCTION(unw_getcontext)
   movq  %rsi,128(%rdi) # store return address as rip
   # skip rflags
   # skip cs
   # skip fs
   # skip gs
   xorl  %eax, %eax    # return UNW_ESUCCESS
   ret
 
+# elif defined(__mips__)
+
+#
+# extern int unw_getcontext(unw_context_t* thread_state)
+#
+# Just trap for the time being.
+DEFINE_LIBUNWIND_FUNCTION(unw_getcontext)
+  teq $0, $0
+
 #elif defined(__ppc__)
 
 ;
 ; extern int unw_getcontext(unw_context_t* thread_state)
 ;
 ; On entry:
 ;  thread_state pointer is in r3
 ;

libunwind/trunk/src/config.h

#endif
     void __assert_rtn(const char *, const char *, int, const char *)
                                                       __attribute__((noreturn));
   #ifdef __cplusplus
     }
   #endif
 
   #define _LIBUNWIND_BUILD_ZERO_COST_APIS (defined(__i386__) || \
                                            defined(__x86_64__) || \
-                                           defined(__arm64__))
+                                           defined(__arm64__) || \
+                                           defined(__mips__))
   #define _LIBUNWIND_BUILD_SJLJ_APIS      defined(__arm__)
   #define _LIBUNWIND_SUPPORT_FRAME_APIS   (defined(__i386__) || \
                                            defined(__x86_64__))
   #define _LIBUNWIND_EXPORT               __attribute__((visibility("default")))
   #define _LIBUNWIND_HIDDEN               __attribute__((visibility("hidden")))
   #define _LIBUNWIND_LOG(msg, ...) fprintf(stderr, "libuwind: " msg, __VA_ARGS__)
   #define _LIBUNWIND_ABORT(msg) __assert_rtn(__func__, __FILE__, __LINE__, msg)
 

libunwind/trunk/src/libunwind.cpp

#elif defined(__arm64__) || defined(__aarch64__)
   new ((void *)cursor) UnwindCursor<LocalAddressSpace, Registers_arm64>(
                                  context, LocalAddressSpace::sThisAddressSpace);
 #elif _LIBUNWIND_ARM_EHABI
   new ((void *)cursor) UnwindCursor<LocalAddressSpace, Registers_arm>(
                                  context, LocalAddressSpace::sThisAddressSpace);
 #elif defined(__or1k__)
   new ((void *)cursor) UnwindCursor<LocalAddressSpace, Registers_or1k>(
                                  context, LocalAddressSpace::sThisAddressSpace);
+#elif defined(__mips__)
+#warning The MIPS architecture is not supported.
 #else
 #error Architecture not supported
 #endif
   AbstractUnwindCursor *co = (AbstractUnwindCursor *)cursor;
   co->setInfoBasedOnIPRegister();
 
   return UNW_ESUCCESS;
 }