[PATCH] D131709: [libunwind][AIX] Implement _Unwind_FindEnclosingFunction() using traceback table on AIX

[Xing Xue via Phabricator via llvm-commits]

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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 TOP 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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D131709

Files:
  libunwind/src/UnwindLevel1-gcc-ext.c
  libunwind/src/Unwind_AIXExtras.cpp


Index: libunwind/src/Unwind_AIXExtras.cpp
===================================================================
--- libunwind/src/Unwind_AIXExtras.cpp
+++ libunwind/src/Unwind_AIXExtras.cpp
@@ -38,7 +38,7 @@
   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 =
Index: libunwind/src/UnwindLevel1-gcc-ext.c
===================================================================
--- libunwind/src/UnwindLevel1-gcc-ext.c
+++ 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 +86,30 @@
 /// specified code address "pc".
 _LIBUNWIND_EXPORT void *_Unwind_FindEnclosingFunction(void *pc) {
   _LIBUNWIND_TRACE_API("_Unwind_FindEnclosingFunction(pc=%p)", pc);
+#if defined(_AIX)
+  // 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));
+  else
+    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 +122,7 @@
     return (void *)(intptr_t) info.start_ip;
   else
     return NULL;
+#endif
 }
 
 /// Walk every frame and call trace function at each one.  If trace function


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