[glibc] struct _Unwind_Exception alignment should not depend on compiler flags

[Florian Weimer]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=30af7c7fa13e17d82c3f1f91536384715844f432

commit 30af7c7fa13e17d82c3f1f91536384715844f432
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Nov 11 11:59:11 2020 +0100

    struct _Unwind_Exception alignment should not depend on compiler flags
    
    __attribute__((__aligned__)) selects an alignment that depends on
    the micro-architecture selected by GCC flags.  Enabling vector
    extensions may increase the allignment.  This is a problem when
    building glibc as a collection of ELF multilibs with different
    GCC flags because ld.so and libc.so/libpthread.so/&c may end up
    with a different layout of struct pthread because of the
    changing offset of its struct _Unwind_Exception field.
    
    Tested-By: Matheus Castanho <msc@linux.ibm.com>

Diff:
---
 sysdeps/generic/unwind.h | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/sysdeps/generic/unwind.h b/sysdeps/generic/unwind.h
index b667a5b652..c229603af3 100644
--- a/sysdeps/generic/unwind.h
+++ b/sysdeps/generic/unwind.h
@@ -75,15 +75,21 @@ typedef void (*_Unwind_Exception_Cleanup_Fn) (_Unwind_Reason_Code,
 
 struct _Unwind_Exception
 {
-  _Unwind_Exception_Class exception_class;
-  _Unwind_Exception_Cleanup_Fn exception_cleanup;
-  _Unwind_Word private_1;
-  _Unwind_Word private_2;
-
-  /* @@@ The IA-64 ABI says that this structure must be double-word aligned.
-     Taking that literally does not make much sense generically.  Instead we
-     provide the maximum alignment required by any type for the machine.  */
-} __attribute__((__aligned__));
+  union
+  {
+    struct
+    {
+      _Unwind_Exception_Class exception_class;
+      _Unwind_Exception_Cleanup_Fn exception_cleanup;
+      _Unwind_Word private_1;
+      _Unwind_Word private_2;
+    };
+
+    /* The IA-64 ABI says that this structure must be double-word aligned.  */
+    _Unwind_Word unwind_exception_align[2]
+      __attribute__ ((__aligned__ (2 * sizeof (_Unwind_Word))));
+  };
+};
 
 
 /* The ACTIONS argument to the personality routine is a bitwise OR of one