[DWARF] Get RA from RA register even if it appears unused

[whitequark]

Summary

If prolog info lists the RA register as unused, it means that
the current stack frame corresponds to a leaf function which never
needs to save the RA register. The RA register is of course used
to return from this leaf function.

Such a frame may be in the middle of the stack if another frame was
pushed on top of it by a signal or CPU exception. In this case,
unless we extract RA from the RA register, _Unwind_Backtrace would
stop at the frame right above the signal or exception frame.

[whitequark]

Reuploaded diff with context.

[whitequark]

@compnerd ping

[compnerd]

It would be nice if we had a test case added for this, but, seems correct to me.

src/DwarfInstructions.hpp

195

I think that we should stick to LLVM style (coddled braces) and use C++ style casts.

[whitequark]

It would be nice if we had a test case added for this

This is inherently architecture and platform specific, and libunwind currently doesn't have any provisions for this... In principle, would a x86-only Linux-only test suffice? I'm not sure if I can spend time making it more portable.

src/DwarfInstructions.hpp

195

Ack re braces.

I was copying that cast from ten lines above. Should I change that too? This should be a static_cast, right?

[compnerd]

x86-Linux-only test is good. LG with the discussed changes.

src/DwarfInstructions.hpp

195

Yeah, it should be a static_cast. And, yes, it should be fine to adjust the style there too (just do it as a separate commit).

src/DwarfInstructions.hpp

if (CFI_Parser<A>::parseFDEInstructions(addressSpace, fdeInfo, cieInfo, pc,
                                              prolog.savedRegisters[i]);
           else if (registers.validRegister(i))
             newRegisters.setRegister(
                 i, getSavedRegister(addressSpace, registers, cfa,
                                     prolog.savedRegisters[i]));
           else
             return UNW_EBADREG;
         }
+        else if (i == (int)cieInfo.returnAddressRegister)
+          returnAddress = registers.getRegister(i);
       }
 
       // By definition, the CFA is the stack pointer at the call site, so
       // restoring SP means setting it to CFA.
       newRegisters.setSP(cfa);
 
       // Return address is address after call site instruction, so setting IP to
       // that does simualates a return.