[Vm-dev] Direct object pointers vs indirect ones pros and cons

[Levente Uzonyi]

On Sat, 13 Nov 2010, Igor Stasenko wrote:

>
> On 12 November 2010 17:44, Bert Freudenberg <bert at freudenbergs.de> wrote:
>>
>> On 12.11.2010, at 14:41, Igor Stasenko wrote:
>>
>>> I'm inviting you to make own version of benchmark
>>
>> I don't think this can be realistically simulated inside Squeak. But possibly you could change the macros in sqMemoryAccess.h to fake an object table access?
>>
>> I just tried that. Using tinyBenchmarks, byte code performance drops to 63% and sends to 78%.
>>
>> Now declaring that variable volatile might be overkill as it prevents all caching, but I couldn't quite figure out a more realistic declaration.
>>
> you mean that non-volatile like:
>
> int FakeObjTable = 0;
>
> could be optimized away by compiler?
> Well, since compiler compiles module by module (a separate C files),
> if you remove 'static'
> it can no longer able to optimize it to no-op, since it can't guess
> what may happen to this variable in another object file,
> since even if in one module there only a read-only access to it, some
> other module could contain a code which modifying it.
>
> So, i think this is the worst case performance slowdown. :)
>
> If we take into account that to get object location you need to do
> object table look only once,
> and then any consequent read/write operations on object won't require
> table lookup, this can be improved.

You can't do that if you want O(1) time for #become:.


Levente

> Consider, for example, that to read ivar, interpreter reads & checks
> header, and only then ivar slot, so it should cost:
> 1 table lookup and 2 reads at object location.
> instead of 2 table lookups + 2 reads at object location.
>
>> - Bert -
>>
>> #else
>> # ifndef FAKE_OBJ_TABLE
>> # define FAKE_OBJ_TABLE
>>  static volatile int FakeObjTable= 0;
>> # define OBJTABLELOOKUP(oop) (oop + FakeObjTable)
>> # endif
>>  /* Use macros when static inline functions aren't efficient. */
>> # define byteAtPointer(ptr)             ((sqInt)(*((unsigned char *)(OBJTABLELOOKUP(ptr)))))
>> # define byteAtPointerput(ptr, val)     ((sqInt)(*((unsigned char *)(OBJTABLELOOKUP(ptr)))= (unsigned char)(val)))
>> # define shortAtPointer(ptr)            ((sqInt)(*((short *)(OBJTABLELOOKUP(ptr)))))
>> # define shortAtPointerput(ptr, val)    ((sqInt)(*((short *)(OBJTABLELOOKUP(ptr)))= (short)(val)))
>> # define intAtPointer(ptr)              ((sqInt)(*((unsigned int *)(OBJTABLELOOKUP(ptr)))))
>> # define intAtPointerput(ptr, val)      ((sqInt)(*((unsigned int *)(OBJTABLELOOKUP(ptr)))= (int)(val)))
>> # define longAtPointer(ptr)             ((sqInt)(*((sqInt *)(OBJTABLELOOKUP(ptr)))))
>> # define longAtPointerput(ptr, val)     ((sqInt)(*((sqInt *)(OBJTABLELOOKUP(ptr)))= (sqInt)(val)))
>> # define oopAtPointer(ptr)              (sqInt)(*((sqInt *)OBJTABLELOOKUP(ptr)))
>> # define oopAtPointerput(ptr, val)      (sqInt)(*((sqInt *)OBJTABLELOOKUP(ptr))= (sqInt)val)
>> # define pointerForOop(oop)             ((char *)(sqMemoryBase + ((usqInt)(oop))))
>> # define oopForPointer(ptr)             ((sqInt)(((char *)(ptr)) - (sqMemoryBase)))
>> # define byteAt(oop)                    byteAtPointer(pointerForOop(oop))
>> # define byteAtput(oop, val)            byteAtPointerput(pointerForOop(oop), (val))
>> # define shortAt(oop)                   shortAtPointer(pointerForOop(oop))
>> # define shortAtput(oop, val)           shortAtPointerput(pointerForOop(oop), (val))
>> # define longAt(oop)                    longAtPointer(pointerForOop(oop))
>> # define longAtput(oop, val)            longAtPointerput(pointerForOop(oop), (val))
>> # define intAt(oop)                     intAtPointer(pointerForOop(oop))
>> # define intAtput(oop, val)             intAtPointerput(pointerForOop(oop), (val))
>> # define oopAt(oop)                     oopAtPointer(pointerForOop(oop))
>> # define oopAtput(oop, val)             oopAtPointerput(pointerForOop(oop), (val))
>> #endif
>>
>>
>
>
>
> -- 
> Best regards,
> Igor Stasenko AKA sig.
>