fix compilation of ExceptionTracer on ARM

Summary:
On ARM gcc, the unwind header defines exception_class as a char[8] rather
than a uint64_t.  Consequently compilation of exception_tracer fails on ARM
with the following error:

    .../folly/experimental/exception_tracer/ExceptionTracer.cpp: In function 'bool folly::exception_tracer::{anonymous}::isAbiCppException(const __cxxabiv1::__cxa_exception*)':
    .../folly/experimental/exception_tracer/ExceptionTracer.cpp:107:45: error: invalid operands of types 'const char [8]' and 'unsigned int' to binary 'operator&'
        return (exc->unwindHeader.exception_class & 0xffffffff) == cppClass;
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~

Construct a uint64_t for this platform.

Reviewed By: yfeldblum

Differential Revision: D13763587

fbshipit-source-id: 011cf13cdfcd1fffdeab8c384f7274f20faecbe5

folly/experimental/exception_tracer/ExceptionTracer.cpp

@@ -23,6 +23,7 @@
 
 #include <glog/logging.h>
 
+#include <folly/Portability.h>
 #include <folly/String.h>
 #include <folly/experimental/exception_tracer/ExceptionAbi.h>
 #include <folly/experimental/exception_tracer/StackTrace.h>
@@ -99,12 +100,28 @@ namespace {
  * exc doesn't actually point to a __cxa_exception structure, but
  * the offset of unwindHeader is correct, so exc->unwindHeader actually
  * returns a _Unwind_Exception object.  Yeah, it's ugly like that.
+ *
+ * Type of exception_class depends on ABI: on some it is defined as a
+ * native endian uint64_t, on others a big endian char[8].
  */
-bool isAbiCppException(const __cxa_exception* exc) {
+struct ArmAbiTag {};
+struct AnyAbiTag {};
+
+bool isAbiCppException(ArmAbiTag, const char (&klazz)[8]) {
+  return klazz[4] == 'C' && klazz[5] == '+' && klazz[6] == '+' &&
+      klazz[7] == '\0';
+}
+
+bool isAbiCppException(AnyAbiTag, const uint64_t& klazz) {
   // The least significant four bytes must be "C++\0"
   static const uint64_t cppClass =
       ((uint64_t)'C' << 24) | ((uint64_t)'+' << 16) | ((uint64_t)'+' << 8);
-  return (exc->unwindHeader.exception_class & 0xffffffff) == cppClass;
+  return (klazz & 0xffffffff) == cppClass;
+}
+
+bool isAbiCppException(const __cxa_exception* exc) {
+  using tag = std::conditional_t<kIsArchArm, ArmAbiTag, AnyAbiTag>;
+  return isAbiCppException(tag{}, exc->unwindHeader.exception_class);
 }
 
 } // namespace