[libunwind][AIX] Implement _Unwind_FindEnclosingFunction() using traceback table on AIX

[xingxue]

Summary

The implementation of _Unwind_FindEnclosingFunction(void *ip) takes the context of itself and then uses the context to get the info of the function enclosing ip. This approach does not work for AIX because on AIX, the TOC base in GPR2 is used as the base for calculating relative addresses. Since _Unwind_FindEnclosingFunction() may be in a different shared lib than the function containing ip, their TOC bases can be different. Therefore, using the value of GPR2 in the context from _Unwind_FindEnclosingFunction() as the base results in incorrect addresses. On the other hand, the start address of a function is available in the traceback table following the instructions of each function on AIX. To get to the traceback table, search a word of 0 starting from ip and the traceback table is located after the word 0. This patch implements _Unwind_FindEnclosingFunction() for AIX by obtaining the function start address from its traceback table.

[compnerd]

libunwind/src/UnwindLevel1-gcc-ext.c

92

Should we use uintptr_t here instead of uint32_t?

93

Should we protect against _Unwind_FindEnclosingFunction(NULL)?

110

Just drop the else, we return a value if the traceback table has offsets or we will return NULL.

[xingxue]

Addressed comment.

• _Unwind_FindEnclosingFunction() returns NULL if argument pc is NULL;
• dropped else for returning NULL as suggested.

[xingxue]

libunwind/src/UnwindLevel1-gcc-ext.c

92

p is for searching for word 0 from instruction pc and the length of PowerPC instructions is one word (32-bit) regardless of whether it is in 32-bit or 64-bit mode, so we need to use uint32_t.

93

Good idea, thanks!

110

Removed else as suggested, thanks!

[MaskRay]

libunwind/src/UnwindLevel1-gcc-ext.c

100

llvm style prefers pre-increment ++p

[xingxue]

libunwind/src/UnwindLevel1-gcc-ext.c

100

Sorry, changed both instances as suggested, thanks!

libunwind/src/UnwindLevel1-gcc-ext.c

 #include <string.h>
 
 #include "config.h"
 #include "libunwind_ext.h"
 #include "libunwind.h"
 #include "Unwind-EHABI.h"
 #include "unwind.h"
 
+#if defined(_AIX)
+#include <sys/debug.h>
+#endif
+
 #if defined(_LIBUNWIND_BUILD_ZERO_COST_APIS)
 
 #if defined(_LIBUNWIND_SUPPORT_SEH_UNWIND)
 #define PRIVATE_1 private_[0]
 #elif defined(_LIBUNWIND_ARM_EHABI)
 #define PRIVATE_1 unwinder_cache.reserved1
 #else
 #define PRIVATE_1 private_1
_Unwind_GetTextRelBase(struct _Unwind_Context *context) {
   _LIBUNWIND_ABORT("_Unwind_GetTextRelBase() not implemented");
 }
 
 
 /// Scans unwind information to find the function that contains the
 /// specified code address "pc".
 _LIBUNWIND_EXPORT void *_Unwind_FindEnclosingFunction(void *pc) {
   _LIBUNWIND_TRACE_API("_Unwind_FindEnclosingFunction(pc=%p)", pc);
+#if defined(_AIX)
+  if (pc == NULL)
+    return NULL;
+
+  // Get the start address of the enclosing function from the function's
+  // traceback table.
+  uint32_t *p = (uint32_t *)pc;
+
+  // Keep looking forward until a word of 0 is found. The traceback
+  // table starts at the following word.
+  while (*p)
+    ++p;
+  struct tbtable *TBTable = (struct tbtable *)(p + 1);
+
+  // Get the address of the traceback table extension.
+  p = (uint32_t *)&TBTable->tb_ext;
+
+  // Skip field parminfo if it exists.
+  if (TBTable->tb.fixedparms || TBTable->tb.floatparms)
+    ++p;
+
+  if (TBTable->tb.has_tboff)
+    // *p contains the offset from the function start to traceback table.
+    return (void *)((uintptr_t)TBTable - *p - sizeof(uint32_t));
+  return NULL;
+#else
   // This is slow, but works.
   // We create an unwind cursor then alter the IP to be pc
   unw_cursor_t cursor;
   unw_context_t uc;
   unw_proc_info_t info;
   __unw_getcontext(&uc);
   __unw_init_local(&cursor, &uc);
   __unw_set_reg(&cursor, UNW_REG_IP, (unw_word_t)(intptr_t)pc);
   if (__unw_get_proc_info(&cursor, &info) == UNW_ESUCCESS)
     return (void *)(intptr_t) info.start_ip;
   else
     return NULL;
+#endif
 }
 
 /// Walk every frame and call trace function at each one.  If trace function
 /// returns anything other than _URC_NO_REASON, then walk is terminated.
 _LIBUNWIND_EXPORT _Unwind_Reason_Code
 _Unwind_Backtrace(_Unwind_Trace_Fn callback, void *ref) {
   unw_cursor_t cursor;
   unw_context_t uc;

libunwind/src/Unwind_AIXExtras.cpp

char *getFuncNameFromTBTable(uintptr_t Pc, uint16_t &NameLen,
 
   // Get to the name of the function.
   p = reinterpret_cast<uint32_t *>(&TBTable->tb_ext);
 
   // Skip field parminfo if it exists.
   if (TBTable->tb.fixedparms || TBTable->tb.floatparms)
     p++;
 
-  // If the tb_offset field exisits, get the offset from the start of
+  // If the tb_offset field exists, get the offset from the start of
   // the function to pc. Skip the field.
   if (TBTable->tb.has_tboff) {
     unw_word_t StartIp =
         reinterpret_cast<uintptr_t>(TBTable) - *p - sizeof(uint32_t);
     *Offset = Pc - StartIp;
     p++;
   }