[PATCH 1/2] PowerPC, function ppc64_sysv_abi_return_value add missing return value convention

[Carl Love <cel@us.ibm.com>]

GDB maintainers:

The following PowerPC specific patch fixes an issue of GDB reporting a
bogus return value for functions that return a non-trivial value.  The
bogus return values result in five testcase failures for test
gdb.cp/non-trivial-retval.exp.  The issue is the function
ppc64_sysv_abi_return_value does not return the correct value when the
valtype->code() is TYPE_CODE_STRUCT and the language_pass_by_reference
is not trivially_copyable.  This patch adds the needed code to return
RETURN_VALUE_STRUCT_CONVENTION in these cases.  

The testcase gdb.cp/non-trivial-retval.exp still fails as gdb now
correctly reports "Cannot determine contents" instead of the expected
values, which is correct in this case.  The PowerPC ABI uses passes the
return buffer address in r3.  The value of r3 is valid on entry to the
function but the PowerPC ABI does not guarantee it will not be changed
in the function.  Hence the contents of r3 is not reliable on exit from
the function.  This issue will be addressed by the next patch in this
patch series.

The patch has been tested on PowerPC and on Intel X86-64 with no
regression failures.

Please let me know if this patch is acceptable for the GDB mainline. 
Thanks.

                       Carl Love

-----------------------------------------
PowerPC, function ppc64_sysv_abi_return_value add missing return value convention

This patch address five testcase failures in gdb.cp/non-trivial-retval.exp.
The following commit resulted in the five testcases failures on PowerPC.  The
value returned by the function is being reported incorrectly.

  commit b1718fcdd1d2a5c514f8ee504ba07fb3f42b8608
  Author: Andrew Burgess <aburgess@redhat.com>
  Date:   Mon Dec 13 16:56:16 2021 +0000

      gdb: on x86-64 non-trivial C++ objects are returned in memory

      Fixes PR gdb/28681.  It was observed that after using the `finish`
      command an incorrect value was displayed in some cases.  Specifically,
      this behaviour was observed on an x86-64 target.

The function:

  enum return_value_convention
  ppc64_sysv_abi_return_value (struct gdbarch *gdbarch, struct value *function,
                               struct type *valtype, struct regcache *regcache,
                               gdb_byte *readbuf, const gdb_byte *writebuf)

should return RETURN_VALUE_STRUCT_CONVENTION if the valtype->code() is
TYPE_CODE_STRUCT and if the language_pass_by_reference is not
trivially_copyable.

This patch adds the need code to return the value
RETURN_VALUE_STRUCT_CONVENTION in the case of this case.

With this patch, the five test cases still fail but with the message "Value
returned has type: A. Cannot determine contents".  The PowerPC ABI stores the
address of the buffer containing the function return value in register r3 on
entry to the function.  However, the PowerPC ABI does not guarentee that r3
will not be modified in the function.  So when the function returns, the return
buffer address cannot be reliably obtained from register r3.  Thus the message
"Cannot determine contents" is appropriate in this case.
---
 gdb/ppc-sysv-tdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/gdb/ppc-sysv-tdep.c b/gdb/ppc-sysv-tdep.c
index f57c261d9dc..14effb93210 100644
--- a/gdb/ppc-sysv-tdep.c
+++ b/gdb/ppc-sysv-tdep.c
@@ -2099,6 +2099,10 @@ ppc64_sysv_abi_return_value (struct gdbarch *gdbarch, struct value *function,
       return RETURN_VALUE_REGISTER_CONVENTION;
     }
 
+  if (!language_pass_by_reference (valtype).trivially_copyable
+      && valtype->code () == TYPE_CODE_STRUCT)
+    return RETURN_VALUE_STRUCT_CONVENTION;
+
   /* In the ELFv2 ABI, aggregate types of up to 16 bytes are
      returned in registers r3:r4.  */
   if (tdep->elf_abi == POWERPC_ELF_V2
-- 
2.31.1