[libunwind][Android] Fix findUnwindSections for ARM EHABI Bionic

Fix the arm_section_length count. The meaning of the arm_section_length
field changed from num-of-elements to num-of-bytes when the
dl_unwind_find_exidx special case was removed (D30306 and D30681). The
special case was restored in D39468, but that patch didn't account for the
change in arm_section_length's meaning.

That patch worked when it was applied to the NDK's fork of libunwind,
because it never removed the special case in the first place, and the
special case is probably disabled in the Android platform's copy of
libunwind, because __ANDROID_API__ is greater than 21.

Turn the dl_unwind_find_exidx special case on unconditionally for Bionic.
Bionic's dl_unwind_find_exidx is much faster than using dl_iterate_phdr.
(e.g. Bionic stores exidx info on an internal soinfo object.)

Reviewers: thomasanderson, srhines, danalbert, ed, keith.walker.arm, mclow.lists, compnerd

Reviewed By: srhines, danalbert

Subscribers: srhines, kristof.beyls, christof, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D68972

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@375275 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/AddressSpace.hpp b/src/AddressSpace.hpp
index 6c13408..908c898 100644
--- a/src/AddressSpace.hpp
+++ b/src/AddressSpace.hpp
@@ -32,6 +32,13 @@
+#if defined(_LIBUNWIND_ARM_EHABI)
+struct EHABIIndexEntry {
+  uint32_t functionOffset;
+  uint32_t data;
 #ifdef __APPLE__
 #include <mach-o/getsect.h>
 namespace libunwind {
@@ -462,12 +469,13 @@
   return true;
-#elif defined(_LIBUNWIND_ARM_EHABI) && defined(__BIONIC__) &&                  \
-    (__ANDROID_API__ < 21)
+#elif defined(_LIBUNWIND_ARM_EHABI) && defined(__BIONIC__)
+  // For ARM EHABI, Bionic didn't implement dl_iterate_phdr until API 21. After
+  // API 21, dl_iterate_phdr exists, but dl_unwind_find_exidx is much faster.
   int length = 0;
   info.arm_section =
       (uintptr_t)dl_unwind_find_exidx((_Unwind_Ptr)targetAddr, &length);
-  info.arm_section_length = (uintptr_t)length;
+  info.arm_section_length = (uintptr_t)length * sizeof(EHABIIndexEntry);
   if (info.arm_section && info.arm_section_length)
     return true;
diff --git a/src/UnwindCursor.hpp b/src/UnwindCursor.hpp
index 488c317..b4d44e1 100644
--- a/src/UnwindCursor.hpp
+++ b/src/UnwindCursor.hpp
@@ -1222,11 +1222,6 @@
 #if defined(_LIBUNWIND_ARM_EHABI)
-struct EHABIIndexEntry {
-  uint32_t functionOffset;
-  uint32_t data;
 template<typename A>
 struct EHABISectionIterator {
   typedef EHABISectionIterator _Self;