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spek

Performance & Struct sizes

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Hey,
 
I'm curious about some todo's and not todo's regarding looping through, and passing around structs (usually as function parameters). I'm programming in Delphi, but I guess the general rules apply on most languages.
 
* Optimal struct size
On a 32bit CPU, are there advised sizes (32,64, 128, ...)? And, do these double on a 64bit CPU? When dealing with vectors and structs, the size often grows quickly.
 
* Filling records
Lets say an optimal size would be 64 bytes, but I only have 60. Is it wise to add some dummy bytes to fill (not concerned about memory usage)?
 
I guess this is what Delphi does by default btw, unless you declare a "packed record".
 
 
* Splitting in multiple records
In case 64bytes would be an optimal size, but I need at least 100 bytes... Would it be smart to divide the data in 2 records, each 64 bytes each?
 
* Dunno how caching works exactly, but I guess there rules apply especially for looping through arrays structs. But, does it also apply when passing just a single struct to a function?
 
For example, I have a whole list of variables that can be adjusted by a callback function. So, what I do now is packing all those variables in a single struct, and pass the struct pointer to that callback. Don't know if I need to care about the size, or eventually splitting in multiple smaller structs here...
 
Greets,
Rick
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I'm speaking from C++ experience, but I guess Delphi is similar.

There are not optimum struct sizes, but there are required alignments for data types. A 32 bit value will usually need to be aligned to a four byte boundary. This alignment affects the structure size, because it must be a multiple of the largest alignment inside the struct, and there may also be padding between members.

Usually, the only difference on a 64 bit CPU (if the program is compiled as 64 bit) is that pointers are larger. If a struct contains pointers, it will be a different size in a 64 bit environment.

Padding is added automatically by the compiler, and there is no need to split up structs. Especially if you are passing by pointer, when the data is not actually copied and the size doesn't matter.
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All right, seems I'm a bit too concerned then hehe, other than the struct size should be dividable through 4 on a 32bit system, to put it simple. So, whether a struct would be 28 or 32 bytes wouldn't matter in that case.

 

Yet, I've read (a long time ago) that the programmer chose for a specific (small) struct size in complex algorithms such as pathfinding, so it could be easily "cached"... I know why things are cached, but forgot how it exactly worked.

 

 

Delphi is not padding all structs in my case, since I specifically tell those structs to be "packed" in some cases. Usually in cases where the struct is also stored in a binary file to keep things compact and easier to predict.

 

Thanks

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If you are not using automatic padding, you should get a performance boost by keeping the size a multiple of four bytes, and putting members smaller than that at the end so they don't affect the alignment of the other members. The CPU will have to do two loads from memory to access a location that is not aligned.

Data layout is more important for efficient caching than exact struct size. You want to be storing data continuously in memory in the order that you access it. Smaller struct size means less data to cache, which may be faster, but not at the expense of proper alignment.
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It can be advantageous to declare the members of your struct in order of decreasing size. This leads to the compiler inserting the least padding between members.
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This kind of strikes me as micro optimization, as has been said unless you're working with time critical code and feel you're passing structs and such that are actually causing a hangup you're probably gonna waste more time trying to tweak this then you will possibly net in performance gains.

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Not working with critical code (except for pathfinding and such), but I find it comfortable to know some general easy-to-implement hints. Like some explained here, ordering the struct contents from big to small is very easy to do, so why not do it? Of course, tiny optimizations shouldn't obstruct the code readability. If the code gets messy I shouldn't make optimizations, unless really needed or at least in a later stadium when everything works stable already.

 

Well, my head is too beer-ed to process all the info right now, But thanks a lot for the hints, explanation and the PDF!

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