[libunwind][LoongArch] Restore $r1 before $r4 in `jumpto`

[SixWeining]

Summary

$ra should be restored before $a0, otherwise the baseaddress ($a0) would
be destroyed. See file UnwindRegistersSave.S for reference.

This also makes libcxx and libcxxabi regtest pass for the -DLIBCXXABI_USE_LLVM_UNWINDER=ON build.

[xen0n]

libunwind/src/UnwindRegistersRestore.S

1225

Ah now I know why no one has noticed this before. Someone mixed raw register names ($r1 r4) and ABI names ($ra $a0) so the flaw was not immediately noticeable otherwise.

So, in order to reduce unnecessary mental burden reading this, I'd suggest changing $r1 and $r4 to ABI names while at it. The code change is okay otherwise, thanks for the patch!

[SixWeining]

use register ABI names rather raw names

[SixWeining]

libunwind/src/UnwindRegistersRestore.S

1225

Thanks for your suggestion. I have changed them.

libunwind/src/UnwindRegistersRestore.S

# endif
   .irp i,1,2,3
     ld.d $r\i, $a0, (8 * \i)
   .endr
   // skip $a0 for now
   .irp i,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31
     ld.d $r\i, $a0, (8 * \i)
   .endr
 
-  ld.d    $r4,  $a0, (8 * 4)   // restore $a0 last
-  ld.d    $r1,  $a0, (8 * 32)  // load new pc into $ra
+  ld.d    $ra,  $a0, (8 * 32)  // load new pc into $ra
+  ld.d    $a0,  $a0, (8 * 4)   // restore $a0 last
 
   jr      $ra
 
 #endif
 
 #endif /* !defined(__USING_SJLJ_EXCEPTIONS__) */
 
 NO_EXEC_STACK_DIRECTIVE
 

libunwind/test/unw_resume.pass.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Ensure that unw_resume() resumes execution at the stack frame identified by
 // cursor.
 
-// FIXME: The return address register($ra/$r1) is restored with a destroyed base
-// address register($a0/$r4) in the assembly file `UnwindRegistersRestore.S` on
-// LoongArch. And we will fix this issue in the next commit.
-// XFAIL: target={{loongarch64-.+}}
-
 #include <libunwind.h>
 
 void test_unw_resume() {
   unw_context_t context;
   unw_cursor_t cursor;
 
   unw_getcontext(&context);
   unw_init_local(&cursor, &context);
   unw_step(&cursor);
   unw_resume(&cursor);
 }
 
 int main() {
   test_unw_resume();
   return 0;
 }