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

Question

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?

Answer 1

Yes, it will affect the performance of the program. Adding the padding means the compiler can use integer load instructions to read things from memory. Without the padding, the compiler must load things separately and do bit shifting to get the entire value. (Even if it's x86 and this is done by the hardware, it still has to be done).

Consider this: Why would compilers insert random, unused space if it was not for performance reasons?

Answer 2

Don't use __attribute__((packed)). If your data structure is in-memory, allow it to occupy its natural size as determined by the compiler. If it's for reading/writing to/from disk, write serialization and deserialization functions; do not simply store cpu-native binary structures on disk. "Packed" structures really have no legitimate uses (or very few; see the comments on this answer for possible disagreeing viewpoints).