Can __attribute__((packed)) affect the performance of a program?

user_131981 0
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I have a structure called log that has 13 chars in it. after doing a sizeof(log) I see that the size is not 13 but 16. I can use the __attribute__((packed)) to get it to the actual size of 13 but I wonder if this will affect the performance of the program. It is a structure that is used quite frequently.

I would like to be able to read the size of the structure (13 not 16). I could use a macro, but if this structure is ever changed ie fields added or removed, I would like the new size to be updated without changing a macro because I think this is error prone. Have any suggestion?

data-structures struct c c++
5 Answers

As with all other performance optimizations, you'll need to profile your code to find the right answer. The right answer will vary by architecture --- and how your use your structure.

If you're creating gigantic arrays the space savings from packing might mean the difference between fitting and not fitting in cache. Or your data might already fit into your cache, in which case it will make no difference. If you're allocating large numbers of the structures in an STL associative container that allocates the storage for your struct with operator new it might not matter at all --- operator new might round your storage up to something that's aligned anyway.

If most of your structures live on the stack the extra storage might already be optimized away anyway.

For a change this simple to test, I suggest building a timing rig and then trying things both ways. For further optimizations I suggest using a profiler to identify your bottlenecks and go from there.

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