Re: [PATCH 2/2 ver 3] PowerPC, fix support for printing the function return value for non-trivial values.

[Ulrich Weigand <Ulrich.Weigand@de.ibm.com>]

Carl Love <cel@us.ibm.com> wrote:

>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.

I believe all of Kevin's and Bruno's comments have been addressed
in this version.  I've reviewed it myself as well, and it looks
good to me.

This is OK.

Thanks,
Ulrich