unwind: Fix libc++abi and libgcc build.

[logan]

Summary

To build libc++abi without libunwind, we should make sure that all
function calls to _Unwind_{Get,Set}{GR,IP}() are inlined as function
calls to _Unwind_VRS_{Get,Set}(). Otherwise, libc++abi.so will fail to
link since libgcc does not provide these symbol at all.

The commit reverts the changes in r228903, which breaks libc++abi/libgcc
build since then.

[asl]

include/unwind.h

224

Why can't we simply make them static functions of UnwindLevel1.c for EHABI case?

[logan]

include/unwind.h

224

They can't be static functions of UnwindLevel1.c because they will be called from the other source code (e.g. libc++abi calls these functions.) If they are declared as static functions in UnwindLevel1.c, then the other source code can't access these functions.

We can't declare these functions as extern and define these functions in UnwindLevel1.c either. If we do so, the source code including this header and using these functions will be compiled to object files with external symbols (undefined references) to these functions. This means that these symbols should be available to the linkers. However, libgcc does not provide these symbols. If we are building libc++abi with libgcc without libunwinder, then we will encounter the following link error:

libcxxabi/src/cxa_personality.cpp:(.text+0x3c): undefined reference to `_Unwind_SetGR'
(... and etc ...)

This is the reason why these functions must be inlined. AFAICT, both the unwind.h from gcc or clang (lib/Header/unwind.h) are following the same practice.

[asl]

include/unwind.h

224

I see. My biggest concern is the use of LIBCXXABI_ARM_EHABI define. We need to resolve this beforehand, because currently we're having a cyclic dependency between libunwind and libcxxabi via cxxabi_config.h. I'd simply fold it inside libwunwind (or introduce libunwind_config.h)

[logan]

include/unwind.h

224

OK. I think I will simply copy the ifdefs from __cxxabi_config.h since they are simple. Will update the patch soon.

My long term plan is to merge this unwind.h with the one in the clang repository. And then, update libc++abi so that we can use the unwind.h from the compiler (both clang and gcc) directly.

[compnerd]

Im not sure that this is the best way to handle this. Currently, use of the savannah unwind would provide these interfaces on ARM. I realize that the alternate implementation of unwind is not EHABI compliant, but that does make this slightly challenging. Perhaps we could just provide a macro-like function which duplicates the interfaces, but bridges to _Unwind_VRS_{G,S}et. (And, since Im sure you will ask, yes, I do have code which relies on these interfaces).

include/unwind.h

224

The long term plan sounds great. I think that is definitely the right direction.

[logan]

Address asl's comment.

[logan]

@compnerd:

For your use case, please have a look at the upcoming D11191. I believe it should work for you. Although I am really hesitating, I have just confirmed that Savannah libunwind[1] is working on ARM. So I am not strongly oppose to the idea now.

[1] http://savannah.nongnu.org/projects/libunwind/

[compnerd]

Yeah, I think that approach would suffice.

[logan]

Stash the changes from D11191.

[logan]

Hi,

Please have a look on the revised version. This should address all of the comments at the moment. I really wish to commit this change before 3.7 release. Thanks.

Logan

[compnerd]

I think that this allows us to be compatible with both libgcc and the Savannah libunwind. Thanks for all the revisions related to this.

[logan]

Thanks for reviewing. Committed as rL243073.

include/unwind.h

_Unwind_VRS_Set(_Unwind_Context *context, _Unwind_VRS_RegClass regclass,
                 void *valuep);
 
 extern _Unwind_VRS_Result
 _Unwind_VRS_Pop(_Unwind_Context *context, _Unwind_VRS_RegClass regclass,
                 uint32_t discriminator,
                 _Unwind_VRS_DataRepresentation representation);
 #endif
 
+#if !_LIBUNWIND_ARM_EHABI
+
 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  // _LIBUNWIND_ARM_EHABI
+
+#if defined(_LIBUNWIND_UNWIND_LEVEL1_EXTERNAL_LINKAGE)
+#define _LIBUNWIND_EXPORT_UNWIND_LEVEL1 extern
+#else
+#define _LIBUNWIND_EXPORT_UNWIND_LEVEL1 static __inline__
+#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" function 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.
+
+_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;
+}
+
+_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);
+}
+
+_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);
+}
+
+_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  // _LIBUNWIND_ARM_EHABI
+
 extern uintptr_t _Unwind_GetRegionStart(struct _Unwind_Context *context);
 extern uintptr_t
     _Unwind_GetLanguageSpecificData(struct _Unwind_Context *context);
 #ifdef __USING_SJLJ_EXCEPTIONS__
 extern _Unwind_Reason_Code
     _Unwind_SjLj_ForcedUnwind(_Unwind_Exception *exception_object,
                               _Unwind_Stop_Fn stop, void *stop_parameter);
 #else

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 from libunwind.so as well.
+#define _LIBUNWIND_UNWIND_LEVEL1_EXTERNAL_LINKAGE 1
+
 #include <inttypes.h>
 #include <stdint.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 
 #include "libunwind.h"

 _LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *context,
                                      uintptr_t value) {
   _LIBUNWIND_TRACE_API("_Unwind_SetIP(context=%p, value=0x%0" PRIx64 ")\n",
                        (void *)context, (uint64_t)value);
   unw_cursor_t *cursor = (unw_cursor_t *)context;
   unw_set_reg(cursor, UNW_REG_IP, value);
 }
 
-#else
-
-_LIBUNWIND_EXPORT 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);
-  _LIBUNWIND_TRACE_API("_Unwind_GetGR(context=%p, reg=%d) => 0x%" PRIx64 "\n",
-                       (void *)context, index, (uint64_t)value);
-  return value;
-}
-
-_LIBUNWIND_EXPORT void _Unwind_SetGR(struct _Unwind_Context *context, int index,
-                                     uintptr_t value) {
-  _LIBUNWIND_TRACE_API("_Unwind_SetGR(context=%p, reg=%d, value=0x%0"PRIx64")\n",
-                       (void *)context, index, (uint64_t)value);
-  _Unwind_VRS_Set(context, _UVRSC_CORE, (uint32_t)index, _UVRSD_UINT32, &value);
-}
-
-_LIBUNWIND_EXPORT uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
-  // remove the thumb-bit before returning
-  uintptr_t value = _Unwind_GetGR(context, 15) & (~(uintptr_t)0x1);
-  _LIBUNWIND_TRACE_API("_Unwind_GetIP(context=%p) => 0x%" PRIx64 "\n",
-                       (void *)context, (uint64_t)value);
-  return value;
-}
-
-_LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *context,
-                                     uintptr_t value) {
-  _LIBUNWIND_TRACE_API("_Unwind_SetIP(context=%p, value=0x%0" PRIx64 ")\n",
-                       (void *)context, (uint64_t)value);
-  uintptr_t thumb_bit = _Unwind_GetGR(context, 15) & ((uintptr_t)0x1);
-  _Unwind_SetGR(context, 15, value | thumb_bit);
-}
-
 #endif // !_LIBUNWIND_ARM_EHABI
-