Re: [Qemu-devel] [PATCH 2/2] PPC: Fail on leaking temporaries

[Peter Maydell]

On 19 January 2014 20:55, Alexander Graf <address@hidden> wrote:
> On 19.01.2014, at 21:52, Peter Maydell <address@hidden> wrote:
>> Longer term I was wondering if we should define the concept
>> of a 'scope object' for TCG temporaries, so you create a scope
>> object and then we have versions of tcg_temp_new_*() that
>> take a scope object to effectively define the lifetime of that
>> temp. Destroying the scope object frees every TCG temp in it.
>> Then we could just have the target frontends create a scope
>> for each instruction, and they wouldn't need to worry about
>> manually freeing TCG temporaries within it at all. That seems
>> better than the current approach where every frontend rolls
>> its own auto-free mechanism, and would render this sort of
>> "check for bugs in manual temp freeing" unnecessary too.
>>
>> (We could also make the tcg_gen_brcond* functions do a
>> "free all temps in all scope objects" and then we'd catch
>> use-of-temp-after-branch bugs, especially if we also got
>> TCG to assert on use of a dead temporary rather than only
>> later when it was doing regalloc on it...)
>
> I agree - that would be awesome :). We could even go as far
> as defining a "fallback scope" that's instruction wide

I was thinking the fallback scope would have to be 'whole
basic block' (both because that makes more sense as a TCG
API and to avoid having to fix every frontend at once), though
you're right that this sits a bit awkwardly with the fact that in
practice the scope you want 99% of the time is going to be
"single guest insn".

thanks
-- PMM