Why doesn't GCC optimize structs?

Question

Systems demand that certain primitives be aligned to certain points within the memory (ints to bytes that are multiples of 4, shorts to bytes that are multiples of 2, etc.).

Answer 1

C compilers don't automatically pack structs precisely because of alignment issues like you mention. Accesses not on word boundaries (32-bit on most CPUs) carry heavy penalty on x86 and cause fatal traps on RISC architectures.