[libunwind] Export the unw_* symbols as weak symbols

libunwind defines the _Unwind_* ABI used by libc++abi. This ABI is a
stable quasi-standard common between multiple implementations such as
LLVM and GNU. The _U* symbol name space is also safely within the symbol
name space that standard C & C++ reserve for the implementation.

Futhermore, libunwind also defines several unw_* symbols, and references
these from the _Unwind_* entry points so the standard/reserved part of
the ABI is dependent on the unw_* part of the ABI. This is not OK for a
C or C++ implementation. The unw_* symbols are reserved for C and extern
"C" used by application code.

This change renames each unw_* function to __unw* and adds a weak alias
unw_* to keep the public <libunwind.h> ABI unchanged for backwards
compatibility. Every reference to unw_* in the implementation has been
changed to use __unw* so that if other unw_* definitions are in force
because nothing uses <libunwind.h> in a particular program, no _Unwind*
code path depends on any unw_* symbol. Furthemore, __unw_* symbols are
hidden, which saves PLT overhead in the shared library case.

In the future, we should cconsider untangling the unw_* API/ABI from the
_Unwind_* API/ABI. The internal API backing the _Unwind_* ABI
implementation should not rely on any nonstandard symbols not in the
implementation-reserved name space. This would then allow separating the
_Unwind_* API/ABI from unw_* entirely, but that's a more substantial
change that's going to require more significant refactoring.

Differential Revision: https://reviews.llvm.org/D59921

llvm-svn: 357640

1 files changed, 9 insertions, 7 deletions

diff --git a/libunwind/src/Unwind-sjlj.c b/libunwind/src/Unwind-sjlj.c
index 22041618631..b8bb7c83bdf 100644
--- a/libunwind/src/Unwind-sjlj.c
+++ b/libunwind/src/Unwind-sjlj.c
@@ -177,9 +177,10 @@ unwind_phase2(struct _Unwind_Exception *exception_object) {
 
     // check for no more frames
     if (c == NULL) {
-      _LIBUNWIND_TRACE_UNWINDING("unwind_phase2(ex_ojb=%p): unw_step() reached "
-                                 "bottom => _URC_END_OF_STACK",
-                                 (void *)exception_object);
+      _LIBUNWIND_TRACE_UNWINDING(
+          "unwind_phase2(ex_ojb=%p): __unw_step() reached "
+          "bottom => _URC_END_OF_STACK",
+          (void *)exception_object);
       return _URC_END_OF_STACK;
     }
 
@@ -215,7 +216,7 @@ unwind_phase2(struct _Unwind_Exception *exception_object) {
         // we may get control back if landing pad calls _Unwind_Resume()
         __Unwind_SjLj_SetTopOfFunctionStack(c);
         __builtin_longjmp(c->jbuf, 1);
-        // unw_resume() only returns if there was an error
+        // __unw_resume() only returns if there was an error
         return _URC_FATAL_PHASE2_ERROR;
       default:
         // something went wrong
@@ -242,9 +243,10 @@ unwind_phase2_forced(struct _Unwind_Exception *exception_object,
 
     // get next frame (skip over first which is _Unwind_RaiseException)
     if (c == NULL) {
-      _LIBUNWIND_TRACE_UNWINDING("unwind_phase2(ex_ojb=%p): unw_step() reached "
-                                 "bottom => _URC_END_OF_STACK",
-                                 (void *)exception_object);
+      _LIBUNWIND_TRACE_UNWINDING(
+          "unwind_phase2(ex_ojb=%p): __unw_step() reached "
+          "bottom => _URC_END_OF_STACK",
+          (void *)exception_object);
       return _URC_END_OF_STACK;
     }