Re: [Qemu-devel] exec: About DISAS_JUMP and DISAS_UPDATE

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 23.10.2015 00:28, Peter Maydell wrote:
> On 22 October 2015 at 19:28, Sergey Fedorov <serge.fdrv@gmail.com> wrote:
>> Hi all,
>>
>> I am trying to understand what the difference should be between
>> DISAS_JUMP and DISAS_UPDATE. Actually, these macros have comments in
>> include/exec/exec-all.h which say that DISAS_JUMP should be used when
>> only PC was modified dynamically whereas DISAS_UPDATE should be used
>> when some other CPU state was (in addition to PC?) modified dynamically.
>> In fact, every target except ARM AArch64 does not distinguish between
>> them. As I can see ARM AArch64 seems to suppose that: (1) PC was not
>> modified when DISAS_UPDATE is used and should be updated with dc->pc
>> when finishing translation; (2) DISAS_JUMP can be used to indicate that
>> a new PC value was set and it should be preserved when finishing
>> translation.
> As Richard says, (a) the semantics for these values are really
> private to each translator (b) the general idea is how AArch64
> uses them. I think the 32-bit ARM code does something a bit odd
> because it has to handle conditional execution (some things we
> might have otherwise done immediately in the decode function
> get postponed to the end of the loop). Mostly I haven't messed
> around too much with that bit of the code because it works
> and it's kind of complicated to understand. But the AArch64
> stuff we wrote from scratch so it does things in the straightforward
> way.

Thanks, Peter. What if I am going to modify DISAS_JUMP and DISAS_UPDATE
usage in 32-bit ARM code and apply AArch64 semantics to them?

Best,
Sergey