[PATCH] [AArch64] Properly extract the reference to a return value from x8

[Luis Machado <luis.machado@linaro.org>]

When running gdb.cp/non-trivial-retval.exp, the following shows up for
AArch64-Linux:

Breakpoint 3, f1 (i1=23, i2=100) at src/gdb/testsuite/gdb.cp/non-trivial-retval.cc:35
35        A a;
(gdb) finish
Run till exit from #0  f1 (i1=23, i2=100) at src/gdb/testsuite/gdb.cp/non-trivial-retval.cc:35
main () at /src/gdb/testsuite/gdb.cp/non-trivial-retval.cc:163
163       B b = f2 (i1, i2);
Value returned is $6 = {a = -11952}
(gdb)

The return value should be {a = 123} instead. This happens because the AArch64
backend doesn't extract the return value from the correct location. GDB should
fetch a pointer to the memory location from X8.

With the patch, gdb.cp/non-trivial-retval.exp has full passes on
AArch64-Linux Ubuntu 20.04/18.04.

The problem only shows up with the "finish" command. The "call" command
works correctly and displays the correct return value.

This is also related to PR gdb/28681
(https://sourceware.org/bugzilla/show_bug.cgi?id=28681).
---
 gdb/aarch64-tdep.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 63d626f90ac..0efb3834584 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -2362,7 +2362,8 @@ aarch64_return_in_memory (struct gdbarch *gdbarch, struct type *type)
       return 0;
     }
 
-  if (TYPE_LENGTH (type) > 16)
+  if (TYPE_LENGTH (type) > 16
+      || !language_pass_by_reference (type).trivially_copyable)
     {
       /* PCS B.6 Aggregates larger than 16 bytes are passed by
 	 invisible reference.  */
@@ -2474,8 +2475,24 @@ aarch64_return_value (struct gdbarch *gdbarch, struct value *func_value,
     {
       if (aarch64_return_in_memory (gdbarch, valtype))
 	{
+	  /* From the AAPCS64's Result Return section:
+
+	     "Otherwise, the caller shall reserve a block of memory of
+	      sufficient size and alignment to hold the result.  The address
+	      of the memory block shall be passed as an additional argument to
+	      the function in x8.  */
+
 	  aarch64_debug_printf ("return value in memory");
-	  return RETURN_VALUE_STRUCT_CONVENTION;
+
+	  if (readbuf)
+	    {
+	      CORE_ADDR addr;
+
+	      regcache->cooked_read (AARCH64_STRUCT_RETURN_REGNUM, &addr);
+	      read_memory (addr, readbuf, TYPE_LENGTH (valtype));
+	    }
+
+	  return RETURN_VALUE_ABI_RETURNS_ADDRESS;
 	}
     }
 
-- 
2.25.1