Re: [Qemu-devel] [PATCH 3/3] memory-device: break the loop if tmp exceed the hinted range

[David Hildenbrand]

On 29.07.19 10:30, Wei Yang wrote:
> On Mon, Jul 29, 2019 at 09:49:37AM +0200, David Hildenbrand wrote:
>> On 28.07.19 15:13, Wei Yang wrote:
>>> The memory-device list built by memory_device_build_list is ordered by
>>> its address, this means if the tmp range exceed the hinted range, all
>>> the following range will not overlap with it.
>>>
>>> Signed-off-by: Wei Yang <address@hidden>
>>> ---
>>>  hw/mem/memory-device.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/hw/mem/memory-device.c b/hw/mem/memory-device.c
>>> index 413b514586..aea47ab3e8 100644
>>> --- a/hw/mem/memory-device.c
>>> +++ b/hw/mem/memory-device.c
>>> @@ -180,7 +180,7 @@ static uint64_t 
>>> memory_device_get_free_addr(MachineState *ms,
>>>                  range_make_empty(&new);
>>>                  break;
>>>              }
>>> -        } else if (!hint) {
>>> +        } else if (!hint || range_lob(&tmp) > range_upb(&new)) {
>>>              break;
>>>          }
>>>      }
>>>
>>
>> Lower bound is inclusive, upper bound is exclusive. Shouldn't this be
>>
>> range_lob(&tmp) >= range_upb(&new)
>>
> 
> Per my understanding, a range with start = 0, size = 0x10000, is represented
> 
>     [0, 0xffff]
> 
> So if I have another range [0xffff, 0x1ffff], they seems to overlap. The range
> [0x10000, 0x1ffff] doesn't overlap with [0, 0xffff].
> 
> My original comparison looks right. Do I miss some point?

I guess you saw my other reply by now. :)

> 
>> Also, I wonder if patch #2 is now really needed?
>>
> 
> Hmm... I think you are right.
> 
> I am afraid without Patch #2, the condition check is not that intuitive. Would
> this bring some confusion for audience and maintenance?

Less checks, less confusion :)

> 
> I am not sure the percentage of occurrence when hint is provided, while the
> generated code for check NULL is less than compare two values.

Nobody should care about that performance difference here.

I guess it is fine to just drop patch #2.

Thanks!


-- 

Thanks,

David / dhildenb