https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64724
--- Comment #3 from Andrey Prokopenko <aprokop at sandia dot gov> --- Andrew, Could you please point out the rule? I cannot find it. I also see that Intel x86/64 reference manual states in section 4.1.1 that ~~~ Words, doublewords, and quadwords do not need to be aligned in memory on natural boundaries. The natural boundaries for words, double words, and quadwords are even-numbered addresses, addresses evenly divisible by four, and addresses evenly divisible by eight, respectively. However, to improve the performance of programs, data structures (especially stacks) should be aligned on natural boundaries whenever possible. The reason for this is that the processor requires two memory accesses to make an unaligned memory access; aligned accesses require only one memory access. A word or doubleword operand that crosses a 4-byte boundary or a quadword operand that crosses an 8-byte boundary is considered unaligned and requires two separate memory bus cycles for access. ~~~
