[libcxx-commits] [PATCH] D72543: [libcxxabi] Insert padding in __cxa_exception struct for compatibility

[Steven Wu via Phabricator via libcxx-commits]

steven_wu created this revision.
steven_wu added reviewers: EricWF, mclow.lists, ldionne, jroelofs, dexonsmith, rjmccall, compnerd, phosek.
Herald added subscribers: ributzka, jkorous, christof, kristof.beyls.
Herald added a project: libc++.

Preserve the old ABI for __cxa_exception and __cxa_dependent_exception
on 64 bit platforms or ARM_EHABI platforms.

After r276215, libunwind in llvm-project labels _Unwind_Exception to be
double word aligned. That change implictly adds a padding before
unwindHeader field in __cxa_exception and __cxa_dependent_exception.
Preserve the same negative offsets in those struct by moving the padding
to the beginning of the field.

The assumption here is that if the ABI is not aware of the padding before
unwindHeader and put the referenceCount/primaryException in there, no padding
should exist before unwindHeader.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D72543

Files:
  libcxxabi/src/cxa_exception.cpp
  libcxxabi/src/cxa_exception.h


Index: libcxxabi/src/cxa_exception.h
===================================================================
--- libcxxabi/src/cxa_exception.h
+++ libcxxabi/src/cxa_exception.h
@@ -29,6 +29,11 @@
 
 struct _LIBCXXABI_HIDDEN __cxa_exception {
 #if defined(__LP64__) || defined(_LIBCXXABI_ARM_EHABI)
+    // Now _Unwind_Exception is marked with __attribute__((aligned)),
+    // which implies __cxa_exception is also aligned. Insert padding
+    // in the beginning of the struct, rather than before unwindHeader.
+    void *reserve;
+
     // This is a new field to support C++ 0x exception_ptr.
     // For binary compatibility it is at the start of this
     // struct which is prepended to the object thrown in
@@ -71,6 +76,7 @@
 // primaryException instead of referenceCount.
 struct _LIBCXXABI_HIDDEN __cxa_dependent_exception {
 #if defined(__LP64__) || defined(_LIBCXXABI_ARM_EHABI)
+    void* reserve; // padding.
     void* primaryException;
 #endif
 
Index: libcxxabi/src/cxa_exception.cpp
===================================================================
--- libcxxabi/src/cxa_exception.cpp
+++ libcxxabi/src/cxa_exception.cpp
@@ -36,6 +36,37 @@
 
 namespace __cxxabiv1 {
 
+// Verify the negative offsets of different fields;
+static_assert(sizeof(_Unwind_Exception) +
+                      offsetof(__cxa_exception, unwindHeader) ==
+                  sizeof(__cxa_exception),
+              "unwindHeader has wrong negative offsets");
+static_assert(sizeof(_Unwind_Exception) +
+                      offsetof(__cxa_dependent_exception, unwindHeader) ==
+                  sizeof(__cxa_dependent_exception),
+              "unwindHeader has wrong negative offsets");
+
+#if !defined(__LP64__) && !defined(_LIBCXXABI_ARM_EHABI)
+static_assert(offsetof(__cxa_exception, referenceCount) +
+                      sizeof(_Unwind_Exception) + sizeof(void*) ==
+                  sizeof(__cxa_exception),
+              "referenceCount has wrong negative offset");
+static_assert(offsetof(__cxa_dependent_exception, primaryException) +
+                      sizeof(_Unwind_Exception) + sizeof(void*) ==
+                  sizeof(__cxa_dependent_exception),
+              "primaryException has wrong negative offset");
+#else
+static_assert(offsetof(__cxa_exception, adjustedPtr) +
+                      sizeof(_Unwind_Exception) + sizeof(void*) ==
+                  sizeof(__cxa_exception),
+              "adjustedPtr has wrong negative offset");
+static_assert(offsetof(__cxa_dependent_exception, adjustedPtr) +
+                      sizeof(_Unwind_Exception) + sizeof(void*) ==
+                  sizeof(__cxa_dependent_exception),
+              "adjustedPtr has wrong negative offset");
+#endif
+
+
 //  Utility routines
 static
 inline


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