unwind: Export _Unwind_{Get,Set}{GR,IP}() for ARM EHABI.

[logan]

Summary

I have removed the exported symbols of _Unwind_{Get,Set}{GR,IP}()
from libunwind.so in D11190. However, it is debatable whether we should
export those symbols. Thus, I am uploading this patch for discussion.

Arguments for this patch:

• The programs that do not include <unwind.h> and declares their own function declarations can still be linked with our libunwind.so.

• _Unwind_{Get,Set}{GR,IP}() are listed in C++ Itanium Level 1 unwinding API.

Arguments against this patch:

• libgcc does not provide these symbols at all. We should not extend the ABI without deliberate consideration.

• Programs that didn't include <unwind.h> deserves not being compiled. If the application developers have tried to port their code to ARM, they should already found the problem.

• ARM EHABI only specified _Unwind_VRS_{Get,Set}() for such purpose.

I personally against this patch, but I would like to see if there are other
comments/concerns.

Depends on D11190

[compnerd]

include/unwind.h

217

The double negative hurts. Would you be opposed to switching this around so that we can have:

__LIBUWNIND_INLINE_LEVEL1

[logan]

include/unwind.h

217

How about changing this to:

#if defined(__LIBUNWIND_UNWIND_LEVEL1_EXTERNAL_LINKAGE
#define __LIBUNWIND_EXPORT_UNWIND_LEVEL1 extern
#else
#define __LIBUNWIND_EXPORT_UNWIND_LEVEL1 static __inline__
#endif

My intuition is to make sure that these functions are inlined by default, i.e. other files can get the inlined version without any extra #defines.

[compnerd]

Sure that would be fine too.

[logan]

Stashing the change with D11190.

include/unwind.h

 extern uintptr_t _Unwind_GetGR(struct _Unwind_Context *context, int index);
 extern void _Unwind_SetGR(struct _Unwind_Context *context, int index,
                           uintptr_t new_value);
 extern uintptr_t _Unwind_GetIP(struct _Unwind_Context *context);
 extern void _Unwind_SetIP(struct _Unwind_Context *, uintptr_t new_value);
 
 #else  // LIBCXXABI_ARM_EHABI
 
+#if !defined(__LIBUNWIND_DONT_INLINE_UNWIND_LEVEL1)
+#define __LIBUNWIND_EXPORT_UNWIND_LEVEL1 static __inline__
+#else
+#define __LIBUNWIND_EXPORT_UNWIND_LEVEL1 extern
+#endif
+
 // These are de facto helper functions for ARM, which delegate the function
 // calls to _Unwind_VRS_Get/Set().  These are not a part of ARM EHABI
 // specification, thus these function MUST be inlined.  Please don't replace
 // these with the "extern" functin declaration; otherwise, the program
 // including this <unwind.h> header won't be ABI compatible and will result in
 // link error when we are linking the program with libgcc.
 
-static __inline__ uintptr_t _Unwind_GetGR(struct _Unwind_Context *context,
-                                          int index) {
+__LIBUNWIND_EXPORT_UNWIND_LEVEL1
+uintptr_t _Unwind_GetGR(struct _Unwind_Context *context, int index) {
   uintptr_t value = 0;
   _Unwind_VRS_Get(context, _UVRSC_CORE, (uint32_t)index, _UVRSD_UINT32, &value);
   return value;
 }
 
-static __inline__ void _Unwind_SetGR(struct _Unwind_Context *context,
-                                     int index, uintptr_t value) {
+__LIBUNWIND_EXPORT_UNWIND_LEVEL1
+void _Unwind_SetGR(struct _Unwind_Context *context, int index,
+                   uintptr_t value) {
   _Unwind_VRS_Set(context, _UVRSC_CORE, (uint32_t)index, _UVRSD_UINT32, &value);
 }
 
-static __inline__ uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
+__LIBUNWIND_EXPORT_UNWIND_LEVEL1
+uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
   // remove the thumb-bit before returning
   return _Unwind_GetGR(context, 15) & (~(uintptr_t)0x1);
 }
 
-static __inline__ void _Unwind_SetIP(struct _Unwind_Context *context,
-                                     uintptr_t value) {
+__LIBUNWIND_EXPORT_UNWIND_LEVEL1
+void _Unwind_SetIP(struct _Unwind_Context *context, uintptr_t value) {
   uintptr_t thumb_bit = _Unwind_GetGR(context, 15) & ((uintptr_t)0x1);
   _Unwind_SetGR(context, 15, value | thumb_bit);
 }
 #endif  // LIBCXXABI_ARM_EHABI
 
 extern uintptr_t _Unwind_GetRegionStart(struct _Unwind_Context *context);
 extern uintptr_t
     _Unwind_GetLanguageSpecificData(struct _Unwind_Context *context);

src/UnwindLevel1.c

 //===------------------------- UnwindLevel1.c -----------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //
 // Implements C++ ABI Exception Handling Level 1 as documented at:
 //      http://mentorembedded.github.io/cxx-abi/abi-eh.html
 // using libunwind
 //
 //===----------------------------------------------------------------------===//
 
+// ARM EHABI does not specify _Unwind_{Get,Set}{GR,IP}().  Thus, we are
+// defining inline functions to delegate the function calls to
+// _Unwind_VRS_{Get,Set}().  However, some applications might declare the
+// function protetype directly (instead of including <unwind.h>), thus we need
+// to export these functions in libunwind.so as well.
+#define __LIBUNWIND_DONT_INLINE_UNWIND_LEVEL1 1
+
 #include <inttypes.h>
 #include <stdint.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 
 #include "libunwind.h"