Strange performance behavior

[Bill Baxter]

Bruce Adams wrote:
> Bill Baxter Wrote:
> 
>> Bruce Adams wrote:
>>> Bill Baxter Wrote:
>>>
>>>> Bill Baxter wrote:
>>>>> Marius Muja wrote:
>>>>>> I have noticed the following strange (at least for me) performance 
>>>>>> behavior with one of my programs. It is a program that does some 
>>>>>> scientific computations and while trying to optimize it I noticed that 
>>>>>> the code from case_B below executes faster (almost twice as fast) as 
>>>>>> the code in case_A. This is a bit counterintuitive for me, since in 
>>>>>> case _B there is also the cost of the function call (or should be the 
>>>>>> same if the function is inlined).
>>>>>> Can anybody shed some light on why it's behaving this way?
>>>>>>
>>>>>> case_A:
>>>>>> -------------------------------
>>>>>> foreach (i,index; indices) {
>>>>>>    foreach (k, inout value; centers[belongs_to[i]])
>>>>>>     value += vecs[index][k];
>>>>>> }
>>>>>> ----------------------------------
>>>>>>
>>>>>> case_B:
>>>>>> -------------------------------
>>>>>> void addTo(T,U)(T[] a, U[] b) {
>>>>>>    foreach(index, inout value; a) {
>>>>>>       value += b[index];
>>>>>>    }
>>>>>> }
>>>>>> ....
>>>>>> foreach (i,index; indices) {
>>>>>>    addTo(centers[belongs_to[i]],vecs[index]);
>>>>>> }
>>>>>> _______________________________
>>>>> My guess would be the compiler's failure to optimize[*] away the [index] 
>>>>> indexing in A.  So you do 2 lookups per iteration rather than just one. 
>>>>>  If so then this should be just as fast as case_B:
>>>>>
>>>>> foreach (i,index; indices) {
>>>>>    auto vecs_i = vecs[index];
>>>>>    foreach (k, inout value; centers[belongs_to[i]])
>>>>>       value += vecs_i[k];
>>>>> }
>>>>>
>>>> Forgot the footnote!:
>>>>
>>>> [*] It may not really be a failure of the optimizer -- maybe it's 
>>>> unreasonably difficult to determine absolutely that vecs won't changed 
>>>> by the operation '+= vecs[index][k]'.
>>>>
>>>> --bb
>>> <shields up>
>>> This is exactly the sort of thing where const would help :)
>>> </shields down>
>> Uh oh... you should have left the shields up. Now you're unprotected...
>>
>>
>> --bb
> 
> I was invariant but now I'm just const but you still can't modify me through this interface :p

Ooh, you better watch out.  Somebody's going to put you in a cast for 
making such volatile statements.  But still, if it's a pure literal 
expression of how you feel, then I won't take exception.

ugh. :-P
--bb