[libunwind][include] Add SEH declarations to <unwind.h>.

[cdavis5x]

Summary

Make the _Unwind_Exception struct correct under SEH. Add a
declaration of _GCC_specific_handler(), which is used by SEH versions
of Itanium personality handlers to do common setup. Roughly corresponds
to Clang's D50380.

[mstorsjo]

Should we maybe add the same declaration of _GCC_specific_handler to clang's unwind.h? That would allow removing the forward declaration in libcxxabi from D49638.

Do you plan on implementing these SEH specific bits in libunwind, or are you just trying to sync things?

include/unwind.h

383

I think __x86_64__ is the wrong condition here. DISPATCHER_CONTEXT is defined on ARM and ARM64 as well (and probably other obscure cases like IA-64), so I would rather make this #ifndef __i386__ instead, or explicitly listing __x86_64__, __arm__ and __aarch64__.

On the other hand, we don't really ever build code with __SEH__ defined for i386 here either, so we could also just leave it out - I see that GCC's unwind.h does that.

[cdavis5x]

In D50414#1191834, @mstorsjo wrote:

Should we maybe add the same declaration of _GCC_specific_handler to clang's unwind.h? That would allow removing the forward declaration in libcxxabi from D49638.

Probably. I believe it's present in GCC's <unwind.h>, too.

Do you plan on implementing these SEH specific bits in libunwind,

Already have. Patch forthcoming.

[cdavis5x]

• Remove unneeded preprocessor condition.

[mstorsjo]

In D50414#1192267, @cdavis5x wrote:

In D50414#1191834, @mstorsjo wrote:

Do you plan on implementing these SEH specific bits in libunwind,

Already have. Patch forthcoming.

Oh, excellent - looking forward to it.

This version LGTM.

include/unwind.h

 #ifndef __UNWIND_H__
 #define __UNWIND_H__
 
 #include <__libunwind_config.h>
 
 #include <stdint.h>
 #include <stddef.h>
 
+#if defined(__SEH__) && !defined(__USING_SJLJ_EXCEPTIONS__)
+#include <windows.h>
+#endif
+
 #if defined(__APPLE__)
 #define LIBUNWIND_UNAVAIL __attribute__ (( unavailable ))
 #else
 #define LIBUNWIND_UNAVAIL
 #endif
 
 typedef enum {
   _URC_NO_REASON = 0,

 struct _Unwind_Context;   // opaque
 struct _Unwind_Exception; // forward declaration
 typedef struct _Unwind_Exception _Unwind_Exception;
 
 struct _Unwind_Exception {
   uint64_t exception_class;
   void (*exception_cleanup)(_Unwind_Reason_Code reason,
                             _Unwind_Exception *exc);
+#if defined(__SEH__) && !defined(__USING_SJLJ_EXCEPTIONS__)
+  uintptr_t private_[6];
+#else
   uintptr_t private_1; // non-zero means forced unwind
   uintptr_t private_2; // holds sp that phase1 found for phase2 to use
+#endif
 #if __SIZEOF_POINTER__ == 4
   // The implementation of _Unwind_Exception uses an attribute mode on the
   // above fields which has the side effect of causing this whole struct to
-  // round up to 32 bytes in size. To be more explicit, we add pad fields
-  // added for binary compatibility.
+  // round up to 32 bytes in size (48 with SEH). To be more explicit, we add
+  // pad fields added for binary compatibility.
   uint32_t reserved[3];
 #endif
   // The Itanium ABI requires that _Unwind_Exception objects are "double-word
   // aligned".  GCC has interpreted this to mean "use the maximum useful
   // alignment for the target"; so do we.
 } __attribute__((__aligned__));
 
 typedef _Unwind_Reason_Code (*_Unwind_Stop_Fn)
extern void __register_frame_info_table(const void *fde, void *ob)
     LIBUNWIND_UNAVAIL;
 extern void __register_frame_table(const void *fde)
     LIBUNWIND_UNAVAIL;
 extern void *__deregister_frame_info(const void *fde)
     LIBUNWIND_UNAVAIL;
 extern void *__deregister_frame_info_bases(const void *fde)
     LIBUNWIND_UNAVAIL;
 
+#if defined(__SEH__) && !defined(__USING_SJLJ_EXCEPTIONS__)
+// This is the common wrapper for GCC-style personality functions with SEH.
+extern EXCEPTION_DISPOSITION _GCC_specific_handler(PEXCEPTION_RECORD exc,
+                                                   PVOID frame,
+                                                   PCONTEXT ctx,
+                                                   PDISPATCHER_CONTEXT disp,
+                                                   _Unwind_Personality_Fn pers);
+#endif
+
 #ifdef __cplusplus
 }
 #endif
 
 #endif // __UNWIND_H__