[libunwind] Enable the SEH unwinding codepaths if building in MSVC mode

[mstorsjo]

Summary

The __SEH__ built-in define is only defined in mingw mode; in MSVC mode it's implied.

This makes a libunwind built with clang-cl in MSVC mode actually do unwinding using the SEH information instead of DWARF.

After D86041, this should make the now successfully built libunwind actually do the right thing as well.

[compnerd]

I don't think that this is entirely true. I believe it is possible to disable SEH with cl, so I think that it is better to conditionalize it as if defined(__SEH__) || defined(_SEH) since IIRC, _SEH is the define that cl uses.

[mstorsjo]

In D86612#2242965, @compnerd wrote:

I don't think that this is entirely true. I believe it is possible to disable SEH with cl, so I think that it is better to conditionalize it as if defined(__SEH__) || defined(_SEH) since IIRC, _SEH is the define that cl uses.

Hmm, I don't see it being defined with cl.exe as far as I can see, and in any case, clang-cl doesn't define it either.

[mstorsjo]

@compnerd - Any followup on this one?

[compnerd]

It is definitely possible to disable SEH via /EHs- though

[mstorsjo]

In D86612#2262441, @compnerd wrote:

It is definitely possible to disable SEH via /EHs- though

Sure, but that's not visible via preprocessor macros as far as I know. (It's not mentioned in https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros, MSVC doesn't support -E -dM, and this option doesn't change clang-cl's predefined macros.)

And regardless - if building libunwind for an MSVC environment, the SEH backend (as opposed to the dwarf or SJLJ ones) is the one that makes most sense to enable by default, as far as I can see. I guess you'd theoretically want to be able to enable the dwarf backend as well, but that should probably require more explicit opt-in.

[compnerd]

I was thinking more the SjLj backend. I am okay with an additional option, but this currently would force enable the SEH backend.

[mstorsjo]

Abandoning this before the transition to PRs - will reopen as a PR later if I want to pursue this further then.

libunwind/include/__libunwind_config.h

 #  define _LIBUNWIND_TARGET_I386
 #  define _LIBUNWIND_CONTEXT_SIZE 8
 #  define _LIBUNWIND_CURSOR_SIZE 15
 #  define _LIBUNWIND_HIGHEST_DWARF_REGISTER _LIBUNWIND_HIGHEST_DWARF_REGISTER_X86
 # elif defined(__x86_64__)
 #  define _LIBUNWIND_TARGET_X86_64 1
 #  if defined(_WIN64)
 #    define _LIBUNWIND_CONTEXT_SIZE 54
-#    ifdef __SEH__
+#    if defined(__SEH__) || defined(_MSC_VER)
 #      define _LIBUNWIND_CURSOR_SIZE 204
 #    else
 #      define _LIBUNWIND_CURSOR_SIZE 66
 #    endif
 #  else
 #    define _LIBUNWIND_CONTEXT_SIZE 21
 #    define _LIBUNWIND_CURSOR_SIZE 33
 #  endif
 #  define _LIBUNWIND_HIGHEST_DWARF_REGISTER _LIBUNWIND_HIGHEST_DWARF_REGISTER_X86_64
 # elif defined(__powerpc64__)
 #  define _LIBUNWIND_TARGET_PPC64 1
 #  define _LIBUNWIND_CONTEXT_SIZE 167
 #  define _LIBUNWIND_CURSOR_SIZE 179
 #  define _LIBUNWIND_HIGHEST_DWARF_REGISTER _LIBUNWIND_HIGHEST_DWARF_REGISTER_PPC64
 # elif defined(__ppc__)
 #  define _LIBUNWIND_TARGET_PPC 1
 #  define _LIBUNWIND_CONTEXT_SIZE 117
 #  define _LIBUNWIND_CURSOR_SIZE 124
 #  define _LIBUNWIND_HIGHEST_DWARF_REGISTER _LIBUNWIND_HIGHEST_DWARF_REGISTER_PPC
 # elif defined(__aarch64__)
 #  define _LIBUNWIND_TARGET_AARCH64 1
 #  define _LIBUNWIND_CONTEXT_SIZE 66
-#  if defined(__SEH__)
+#  if defined(__SEH__) || defined(_MSC_VER)
 #    define _LIBUNWIND_CURSOR_SIZE 164
 #  else
 #    define _LIBUNWIND_CURSOR_SIZE 78
 #  endif
 #  define _LIBUNWIND_HIGHEST_DWARF_REGISTER _LIBUNWIND_HIGHEST_DWARF_REGISTER_ARM64
 # elif defined(__arm__)
 #  define _LIBUNWIND_TARGET_ARM 1
-#  if defined(__SEH__)
+#  if defined(__SEH__) || defined(_MSC_VER)
 #    define _LIBUNWIND_CONTEXT_SIZE 42
 #    define _LIBUNWIND_CURSOR_SIZE 80
 #  elif defined(__ARM_WMMX)
 #    define _LIBUNWIND_CONTEXT_SIZE 61
 #    define _LIBUNWIND_CURSOR_SIZE 68
 #  else
 #    define _LIBUNWIND_CONTEXT_SIZE 42
 #    define _LIBUNWIND_CURSOR_SIZE 49

libunwind/include/libunwind.h

#else
     #else
       #define LIBUNWIND_AVAIL __attribute__((unavailable))
     #endif
   #endif
 #else
   #define LIBUNWIND_AVAIL
 #endif
 
-#if defined(_WIN32) && defined(__SEH__)
+#if defined(_WIN32) && (defined(__SEH__) || defined(_MSC_VER))
   #define LIBUNWIND_CURSOR_ALIGNMENT_ATTR __attribute__((__aligned__(16)))
 #else
   #define LIBUNWIND_CURSOR_ALIGNMENT_ATTR
 #endif
 
 /* error codes */
 enum {
   UNW_ESUCCESS      = 0,     /* no error */
   UNW_EUNSPEC       = -6540, /* unspecified (general) error */

libunwind/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__) && defined(_WIN32)
+#if (defined(__SEH__) || defined(_MSC_VER)) && !defined(__USING_SJLJ_EXCEPTIONS__) && defined(_WIN32)
 #include <windows.h>
 #include <ntverp.h>
 #endif
 
 #if defined(__APPLE__)
 #define LIBUNWIND_UNAVAIL __attribute__ (( unavailable ))
 #else
 #define LIBUNWIND_UNAVAIL

 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__)
+#if (defined(__SEH__) || defined(_MSC_VER)) && !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
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__)
+#if (defined(__SEH__) || defined(_MSC_VER)) && !defined(__USING_SJLJ_EXCEPTIONS__)
 #ifndef _WIN32
 typedef struct _EXCEPTION_RECORD EXCEPTION_RECORD;
 typedef struct _CONTEXT CONTEXT;
 typedef struct _DISPATCHER_CONTEXT DISPATCHER_CONTEXT;
 #elif !defined(__MINGW32__) && VER_PRODUCTBUILD < 8000
 typedef struct _DISPATCHER_CONTEXT DISPATCHER_CONTEXT;
 #endif
 // This is the common wrapper for GCC-style personality functions with SEH.

libunwind/src/Unwind-seh.cpp

case _URC_CONTINUE_UNWIND:
     // at the target frame, we're in real trouble.
     if (action & _UA_HANDLER_FRAME)
       _LIBUNWIND_ABORT("Personality continued unwind at the target frame!");
     return ExceptionContinueSearch;
   case _URC_HANDLER_FOUND:
     // If we were called by __libunwind_seh_personality(), indicate that
     // a handler was found; otherwise, initiate phase 2 by unwinding.
     if (ours && ms_exc->NumberParameters > 1)
-      return 4 /* ExecptionExecuteHandler in mingw */;
+      return (EXCEPTION_DISPOSITION) 4 /* ExecptionExecuteHandler in mingw */;
     // This should never happen in phase 2.
     if (IS_UNWINDING(ms_exc->ExceptionFlags))
       _LIBUNWIND_ABORT("Personality indicated exception handler in phase 2!");
     exc->private_[1] = (ULONG_PTR)frame;
     if (ours) {
       ms_exc->NumberParameters = 4;
       ms_exc->ExceptionInformation[1] = (ULONG_PTR)frame;
     }
     // FIXME: Indicate target frame in foreign case!
     // phase 2: the clean up phase
     RtlUnwindEx(frame, (PVOID)disp->ControlPc, ms_exc, exc, ms_ctx, disp->HistoryTable);
     _LIBUNWIND_ABORT("RtlUnwindEx() failed");
   case _URC_INSTALL_CONTEXT: {
     // If we were called by __libunwind_seh_personality(), indicate that
     // a handler was found; otherwise, it's time to initiate a collided
     // unwind to the target.
     if (ours && !IS_UNWINDING(ms_exc->ExceptionFlags) && ms_exc->NumberParameters > 1)
-      return 4 /* ExecptionExecuteHandler in mingw */;
+      return (EXCEPTION_DISPOSITION) 4 /* ExecptionExecuteHandler in mingw */;
     // This should never happen in phase 1.
     if (!IS_UNWINDING(ms_exc->ExceptionFlags))
       _LIBUNWIND_ABORT("Personality installed context during phase 1!");
 #ifdef __x86_64__
     exc->private_[2] = disp->TargetIp;
     __unw_get_reg(&cursor, UNW_X86_64_RAX, &retval);
     __unw_get_reg(&cursor, UNW_X86_64_RDX, &exc->private_[3]);
 #elif defined(__arm__)

libunwind/src/config.h

 #ifdef __APPLE__
   #if defined(FOR_DYLD)
     #define _LIBUNWIND_SUPPORT_COMPACT_UNWIND
   #else
     #define _LIBUNWIND_SUPPORT_COMPACT_UNWIND
     #define _LIBUNWIND_SUPPORT_DWARF_UNWIND   1
   #endif
 #elif defined(_WIN32)
-  #ifdef __SEH__
+  #if defined(__SEH__) || defined(_MSC_VER)
     #define _LIBUNWIND_SUPPORT_SEH_UNWIND 1
   #else
     #define _LIBUNWIND_SUPPORT_DWARF_UNWIND 1
   #endif
 #else
   #if defined(__ARM_DWARF_EH__) || !defined(__arm__)
     #define _LIBUNWIND_SUPPORT_DWARF_UNWIND 1
     #define _LIBUNWIND_SUPPORT_DWARF_INDEX 1