PowerPC, fix support for printing the function return value for non-trivial values.

Currently, a non-trivial return value from a function cannot currently be
reliably determined on PowerPC.  This is due to the fact that the PowerPC
ABI uses register r3 to store the address of the buffer containing the
non-trivial return value when the function is called.  The PowerPC ABI
does not guarantee the value in register r3 is not modified in the
function.  Thus the value in r3 cannot be reliably used to obtain the
return addreses on exit from the function.

This patch adds a new gdbarch method to allow PowerPC to access the value
of r3 on entry to a function. On PowerPC, the new gdbarch method attempts
to use the DW_OP_entry_value for the DWARF entries, when exiting the
function, to determine the value of r3 on entry to the function.  This
requires the use of the -fvar-tracking compiler option to compile the
user application thus generating the DW_OP_entry_value in the binary.  The
DW_OP_entry_value entries in the binary file allows GDB to resolve the
DW_TAG_call_site entries.  This new gdbarch method is used to get the
return buffer address, in the case of a function returning a nontrivial
data type, on exit from the function.  The GDB function should_stop checks
to see if RETURN_BUF is non-zero.  By default, RETURN_BUF will be set to
zero by the new gdbarch method call for all architectures except PowerPC.
The get_return_value function will be used to obtain the return value on
all other architectures as is currently being done if RETURN_BUF is zero.
On PowerPC, the new gdbarch method will return a nonzero address in
RETURN_BUF if the value can be determined.  The value_at function uses the
return buffer address to get the return value.

This patch fixes five testcase failures in gdb.cp/non-trivial-retval.exp.
The correct function return values are now reported.

Note this patch is dependent on patch: "PowerPC, function
ppc64_sysv_abi_return_value add missing return value convention".

This patch has been tested on Power 10 and x86-64 with no regressions.

1 files changed, 11 insertions, 0 deletions

diff --git a/gdb/gdbarch-gen.h b/gdb/gdbarch-gen.h
index e0d7a08..a663316 100644
--- a/gdb/gdbarch-gen.h
+++ b/gdb/gdbarch-gen.h
@@ -441,6 +441,17 @@ typedef enum return_value_convention (gdbarch_return_value_ftype) (struct gdbarc
 extern enum return_value_convention gdbarch_return_value (struct gdbarch *gdbarch, struct value *function, struct type *valtype, struct regcache *regcache, gdb_byte *readbuf, const gdb_byte *writebuf);
 extern void set_gdbarch_return_value (struct gdbarch *gdbarch, gdbarch_return_value_ftype *return_value);
 
+/* Return the address at which the value being returned from
+   the current function will be stored.  This routine is only
+   called if the current function uses the the "struct return
+   convention".
+
+   May return 0 when unable to determine that address. */
+
+typedef CORE_ADDR (gdbarch_get_return_buf_addr_ftype) (struct type *val_type, frame_info_ptr cur_frame);
+extern CORE_ADDR gdbarch_get_return_buf_addr (struct gdbarch *gdbarch, struct type *val_type, frame_info_ptr cur_frame);
+extern void set_gdbarch_get_return_buf_addr (struct gdbarch *gdbarch, gdbarch_get_return_buf_addr_ftype *get_return_buf_addr);
+
 /* Return true if the return value of function is stored in the first hidden
    parameter.  In theory, this feature should be language-dependent, specified
    by language and its ABI, such as C++.  Unfortunately, compiler may