[android] Basic support for backtraces.

Android doesn't support the GNU backtrace call, but unwind.h is there to
help. The code is a little bit more complicated than a simple backtrace
call, but it is just a callback and the context struct.

Sadly, it seems that -funwind-tables is needed in every piece of code
that the backtrace needs to traverse, which means that at least the
runtime and the stubs need -funwind-tables. However, the flag has been
enabled for all targets (only on Android), just in case some other code
ends up being in the stack. This might only be necessary for ARM
targets, but currently we only build ARM targets in Android. The Swift
parts of the stack seems to have unwind information and can be unwind.

Another problem at the moment is the symbols from the executable are
normally missing. To get symbol names two things need to happen: the
symbol has to be public (so it is an exported symbol, and not just a
local symbol that dladdr seems to not provide the name), and the linker
has to be invoked with -export-dynamic. Without those two pieces, the
addresses are returned (and match the binary just fine), but no names
are printed.

Author: drodriguez

cmake/modules/AddSwift.cmake

@@ -303,6 +303,11 @@ function(_add_variant_c_compile_flags)
                        "-fcoverage-mapping")
   endif()
 
+  if((CFLAGS_ARCH STREQUAL "armv7" OR CFLAGS_ARCH STREQUAL "aarch64") AND
+     (CFLAGS_SDK STREQUAL "LINUX" OR CFLAGS_SDK STREQUAL "ANDROID"))
+     list(APPEND result -funwind-tables)
+  endif()
+
   if("${CFLAGS_SDK}" STREQUAL "ANDROID")
     swift_android_libcxx_include_paths(CFLAGS_CXX_INCLUDES)
     swift_android_include_for_arch("${CFLAGS_ARCH}" "${CFLAGS_ARCH}_INCLUDE")

stdlib/public/runtime/Errors.cpp

@@ -14,7 +14,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(__CYGWIN__) || defined(__ANDROID__) || defined(__HAIKU__)
+#if defined(__CYGWIN__) || defined(__HAIKU__)
 #define SWIFT_SUPPORTS_BACKTRACE_REPORTING 0
 #else
 #define SWIFT_SUPPORTS_BACKTRACE_REPORTING 1
@@ -58,6 +58,10 @@
 #include <android/log.h>
 #endif
 
+#if defined(__ELF__)
+#include <unwind.h>
+#endif
+
 namespace FatalErrorFlags {
 enum: uint32_t {
   ReportBacktrace = 1 << 0
@@ -188,13 +192,46 @@ void swift::dumpStackTraceEntry(unsigned index, void *framePC,
 #endif
 }
 
+#if defined(__ELF__)
+struct UnwindState {
+  void **current;
+  void **end;
+};
+
+static _Unwind_Reason_Code SwiftUnwindFrame(struct _Unwind_Context *context, void *arg) {
+  struct UnwindState *state = static_cast<struct UnwindState *>(arg);
+  if (state->current == state->end) {
+    return _URC_END_OF_STACK;
+  }
+
+  uintptr_t pc;
+#if defined(__arm__)
+  // ARM r15 is PC.  UNW_REG_PC is *not* the same value, and using that will
+  // result in abnormal behaviour.
+  _Unwind_VRS_Get(context, _UVRSC_CORE, 15, _UVRSD_UINT32, &pc);
+  // Clear the ISA bit during the reporting.
+  pc &= ~(uintptr_t)0x1;
+#else
+  pc = _Unwind_GetIP(context);
+#endif
+  if (pc) {
+    *state->current++ = reinterpret_cast<void *>(pc);
+  }
+  return _URC_NO_REASON;
+}
+#endif
+
 LLVM_ATTRIBUTE_NOINLINE
 void swift::printCurrentBacktrace(unsigned framesToSkip) {
 #if SWIFT_SUPPORTS_BACKTRACE_REPORTING
   constexpr unsigned maxSupportedStackDepth = 128;
   void *addrs[maxSupportedStackDepth];
 #if defined(_WIN32)
   int symbolCount = CaptureStackBackTrace(0, maxSupportedStackDepth, addrs, NULL);
+#elif defined(__ELF__)
+  struct UnwindState state = {&addrs[0], &addrs[maxSupportedStackDepth]};
+  _Unwind_Backtrace(SwiftUnwindFrame, &state);
+  int symbolCount = state.current - addrs;
 #else
   int symbolCount = backtrace(addrs, maxSupportedStackDepth);
 #endif