Add missing _US_ACTION_MASK constant to unwind.h

[dim]

Summary

During building of recent compiler-rt sources on FreeBSD for arm, I
noticed that our unwind.h (which originates in libunwind) was missing
the _US_ACTION_MASK constant:

compiler-rt/lib/builtins/gcc_personality_v0.c:187:18: error: use of undeclared identifier '_US_ACTION_MASK'
    if ((state & _US_ACTION_MASK) != _US_UNWIND_FRAME_STARTING)
                 ^

It appears that both clang's internal unwind.h, and libgcc's unwind.h
define this constant as 3, so let's add this to libunwind's version too.

[davide]

lg

[joerg]

Please restrict this to ARM EHABI.

[dim]

In D24222#533867, @joerg wrote:

Please restrict this to ARM EHABI.

Huh? This whole is under #if _LIBUNWIND_ARM_EHABI already? :)

[joerg]

Missed line 54, ignore that remark.

[dim]

One additional note: it seems libgcc and other unwinders use _US_ACTION_MASK to mask out the _US_UNWIND_FRAME_STARTING and _US_UNWIND_FRAME_RESUME values, as they handle _US_FORCE_UNWIND (and _US_END_OF_STACK which we also don't have) separately. It doesn't look like libunwind uses this method?

include/unwind.h

 typedef struct _Unwind_Context _Unwind_Context;   // opaque
 
 #if _LIBUNWIND_ARM_EHABI
 typedef uint32_t _Unwind_State;
 
 static const _Unwind_State _US_VIRTUAL_UNWIND_FRAME   = 0;
 static const _Unwind_State _US_UNWIND_FRAME_STARTING  = 1;
 static const _Unwind_State _US_UNWIND_FRAME_RESUME    = 2;
+static const _Unwind_State _US_ACTION_MASK            = 3;
 /* Undocumented flag for force unwinding. */
 static const _Unwind_State _US_FORCE_UNWIND           = 8;
 
 typedef uint32_t _Unwind_EHT_Header;
 
 struct _Unwind_Control_Block;
 typedef struct _Unwind_Control_Block _Unwind_Control_Block;
 typedef struct _Unwind_Control_Block _Unwind_Exception; /* Alias */