Re: [Qemu-devel] [PATCH 09/13] target-openrisc: Add CPU which neglects Carry and Overflow Flag

[Sebastian Macke <sebastian@macke.de>]

On 30/10/2013 11:14 AM, Richard Henderson wrote:
> On 10/29/2013 12:04 PM, Sebastian Macke wrote:
>> The carry and overflag and the instructions l.addc and l.addic
>> are never used in the toolchain. Linux and gcc compiled software
>> don't need them.
> Really?  That's quite surprising.

Yes, Really.  :)

>> To speed up the emulation a cpu was added which
>> neglects the flags for l.addi, l.add, l.sub and
>> generates an illegal instruction error for l.addic and l.addc
> I'm somewhat shocked that l.addc is never used.  To me that points
> to a missed opportunity in the compiler.
Yes

> It would be much better to simply improve handling of these bits.
> In a previous patch set you broke out SR[F] to its own variable;
> I suggest that you do the same for SR[CY] and SR[OV].
>
> If you can implement add et al without branches, the TCG optimizer
> will be able to do a good job eliminating shadowed computation.
>
> A good example to follow here is the ARM implementation.  Have a
> look at the gen_add_CC and gen_sub_CC functions especially.  Note
> that the overflow bit is stored in bit 31 of cpu_VF and the other
> bits of cpu_VF are ignored.
>
>
> r~

I would like to implement lazy flags or at least change the big bunch of 
code which is there right now.
But for this patchset I want to keep it that way and change the whole 
flag handling in one of the next patches.

There is almost nothing available to test the flags right now. And I 
have to keep an eye on the delayed slot. So this is more work than you 
might expect.