[ARM] __cxa_end_cleanup: avoid clobbering r4

[rprichard]

Summary

The fix for D111703 clobbered r4 both to:

• Save/restore the original lr.
• Load the address of _Unwind_Resume for LIBCXXABI_BAREMETAL.

This patch saves and restores lr without clobbering any extra
registers.

For LIBCXXABI_BAREMETAL, it is still necessary to clobber one extra
register to hold the address of _Unwind_Resume, but it seems better to
use ip/r12 (intended for linker veneers/trampolines) than r4 for this
purpose.

The function also clobbers r0 for the _Unwind_Resume function's
parameter, but that is unavoidable.

[danielkiss]

LGTM, thanks for the patch.

libcxxabi/src/cxa_exception.cpp

 /*
 The routine to be called after the cleanup has been performed.  It will get the
 propagating __cxa_exception from __cxa_eh_globals, and continue the stack
 unwinding with _Unwind_Resume.
 
 According to ARM EHABI 8.4.1, __cxa_end_cleanup() should not clobber any
 register, thus we have to write this function in assembly so that we can save
 {r1, r2, r3}.  We don't have to save r0 because it is the return value and the
-first argument to _Unwind_Resume().  In addition, we are saving lr in order to
-align the stack to 16 bytes and lr will be used to identify the caller and its
-frame information. _Unwind_Resume never return and we need to keep the original
-lr so just branch to it.
+first argument to _Unwind_Resume().  The function also saves/restores r4 to
+keep the stack aligned and to provide a temp register.  _Unwind_Resume never
+returns and we need to keep the original lr so just branch to it.  When
+targeting bare metal, the function also clobbers ip/r12 to hold the address of
+_Unwind_Resume, which may be too far away for an ordinary branch.
 */
 __attribute__((used)) static _Unwind_Exception *
 __cxa_end_cleanup_impl()
 {
     __cxa_eh_globals* globals = __cxa_get_globals();
     __cxa_exception* exception_header = globals->propagatingExceptions;
     if (NULL == exception_header)
     {

 
 asm("  .pushsection  .text.__cxa_end_cleanup,\"ax\",%progbits\n"
     "  .globl  __cxa_end_cleanup\n"
     "  .type __cxa_end_cleanup,%function\n"
     "__cxa_end_cleanup:\n"
 #if defined(__ARM_FEATURE_BTI_DEFAULT)
     "  bti\n"
 #endif
-    "  push  {r1, r2, r3, lr}\n"
+    "  push  {r1, r2, r3, r4}\n"
+    "  mov r4, lr\n"
     "  bl  __cxa_end_cleanup_impl\n"
-    "  pop {r1, r2, r3, r4}\n"
     "  mov lr, r4\n"
 #if defined(LIBCXXABI_BAREMETAL)
     "  ldr r4, =_Unwind_Resume\n"
-    "  bx  r4\n"
+    "  mov ip, r4\n"
+#endif
+    "  pop {r1, r2, r3, r4}\n"
+#if defined(LIBCXXABI_BAREMETAL)
+    "  bx  ip\n"
 #else
-    "   b _Unwind_Resume\n"
+    "  b _Unwind_Resume\n"
 #endif
     "  .popsection");
 #endif // defined(_LIBCXXABI_ARM_EHABI)
 
 /*
 This routine can catch foreign or native exceptions.  If native, the exception
 can be a primary or dependent variety.  This routine may remain blissfully
 ignorant of whether the native exception is primary or dependent.