Insert padding before the __cxa_exception header to ensure the thrown

object is sufficiently aligned.

r303175 annotated field unwindHeader of __cxa_exception with attribute
'aligned' to ensure the thrown object following the __cxa_exception
header was sufficiently aligned. This caused changes in the field
offsets of __cxa_exception relative to the start of the thrown object,
which was an ABI breaking change for some clients.

Instead of annotating field unwindHeader, this commit inserts extra
space before the header. This ensures the thrown object following the
header is sufficiently aligned without changing the field offsets, thus
avoiding any ABI breakages.

rdar://problem/25364625
rdar://problem/35556163

llvm-svn: 319123

1 files changed, 36 insertions, 4 deletions

diff --git a/libcxxabi/src/cxa_exception.cpp b/libcxxabi/src/cxa_exception.cpp
index e6ed0931cebf..d5230cdc7d95 100644
--- a/libcxxabi/src/cxa_exception.cpp
+++ b/libcxxabi/src/cxa_exception.cpp
@@ -137,6 +137,28 @@ static _LIBCXXABI_NORETURN void failed_throw(__cxa_exception* exception_header)
     std::__terminate(exception_header->terminateHandler);
 }
 
+// Return the offset of the __cxa_exception header from the start of the
+// allocated buffer. If __cxa_exception's alignment is smaller than the maximum
+// useful alignment for the target machine, padding has to be inserted before
+// the header to ensure the thrown object that follows the header is
+// sufficiently aligned. This happens if _Unwind_exception isn't double-word
+// aligned (on Darwin, for example).
+static size_t get_cxa_exception_offset() {
+  struct S {
+  } __attribute__((aligned));
+
+  // Compute the maximum alignment for the target machine.
+  constexpr size_t alignment = std::alignment_of<S>::value;
+  constexpr size_t excp_size = sizeof(__cxa_exception);
+  constexpr size_t aligned_size =
+      (excp_size + alignment - 1) / alignment * alignment;
+  constexpr size_t offset = aligned_size - excp_size;
+  static_assert((offset == 0 ||
+                 std::alignment_of<_Unwind_Exception>::value < alignment),
+                "offset is non-zero only if _Unwind_Exception isn't aligned");
+  return offset;
+}
+
 extern "C" {
 
 //  Allocate a __cxa_exception object, and zero-fill it.
@@ -146,10 +168,16 @@ extern "C" {
 //  user's exception object.
 void *__cxa_allocate_exception(size_t thrown_size) throw() {
     size_t actual_size = cxa_exception_size_from_exception_thrown_size(thrown_size);
-    __cxa_exception *exception_header =
-        static_cast<__cxa_exception *>(__aligned_malloc_with_fallback(actual_size));
-    if (NULL == exception_header)
+
+    // Allocate extra space before the __cxa_exception header to ensure the
+    // start of the thrown object is sufficiently aligned.
+    size_t header_offset = get_cxa_exception_offset();
+    char *raw_buffer =
+        (char *)__aligned_malloc_with_fallback(header_offset + actual_size);
+    if (NULL == raw_buffer)
         std::terminate();
+    __cxa_exception *exception_header =
+        static_cast<__cxa_exception *>((void *)(raw_buffer + header_offset));
     std::memset(exception_header, 0, actual_size);
     return thrown_object_from_cxa_exception(exception_header);
 }
@@ -157,7 +185,11 @@ void *__cxa_allocate_exception(size_t thrown_size) throw() {
 
 //  Free a __cxa_exception object allocated with __cxa_allocate_exception.
 void __cxa_free_exception(void *thrown_object) throw() {
-    __aligned_free_with_fallback(cxa_exception_from_thrown_object(thrown_object));
+    // Compute the size of the padding before the header.
+    size_t header_offset = get_cxa_exception_offset();
+    char *raw_buffer =
+        ((char *)cxa_exception_from_thrown_object(thrown_object)) - header_offset;
+    __aligned_free_with_fallback((void *)raw_buffer);
 }