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

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.

Reviewed by: compnerd, MaskRay, libunwind

Differential Revision: https://reviews.llvm.org/D131709

libunwind/src/UnwindLevel1-gcc-ext.c

@@ -22,6 +22,10 @@
 #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)
@@ -82,6 +86,32 @@ _Unwind_GetTextRelBase(struct _Unwind_Context *context) {
 /// 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;
@@ -94,6 +124,7 @@ _LIBUNWIND_EXPORT void *_Unwind_FindEnclosingFunction(void *pc) {
     return (void *)(intptr_t) info.start_ip;
   else
     return NULL;
+#endif
 }
 
 /// Walk every frame and call trace function at each one.  If trace function

libunwind/src/Unwind_AIXExtras.cpp

@@ -38,7 +38,7 @@ char *getFuncNameFromTBTable(uintptr_t Pc, uint16_t &NameLen,
   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 =