Re: Extract the return value in finish_command on Xtensa.

[Chris Zankel <zankel@tensilica.com>]

Daniel Jacobowitz wrote:
> On Wed, Jul 14, 2004 at 03:38:34PM -0400, Andrew Cagney wrote:
> 
>>>I am working on a port of GDB to the Xtensa architecture.
>>>
>>>Unfortunately, it is impossible on Xtensa to extract the return value once 
>>>the 'callee' has returned to the 'caller' without analyzing the call 
>>>instruction (if there was one). Depending on the call instruction, the 
>>>return value can be in register a6, a10, oder a14.
>>
>>Puzzled expression.  The ABI should specify which return-value 
>>convention will be used?
>>
>>I suspect you'll need to explain a little more about your architecture.

What Daniel said is basically correct. Xtensa uses register windows much 
like Sparc but with the difference that is doesn't always shift 8 
registers but a variable number of registers (4, 8, or 12). That number 
is encoded in the call instruction (call4, call8, or call12).
The shift walue is also encoded in the return address, so the return 
instruction knows how many registers it has to shift backwards.

The 'callee' always returns the return value in a2. For the caller the 
return value thus is in either a6 (call4), a10 (call8), or a14 (call12).

The calling function obviously knows what call instruction it used to 
extract the return value. The easiest way for GDB to find the return 
register would be to look at the return address of the 'callee', but 
once that function has returned, the information is lost. Because this 
problem only arises for the finish command, GDB could save the return 
address (from the 'callee' frame) before it resumes the target and then 
use that value to extract the return value.

Thanks,
Chris