Why a register sometimes gets skipped
The ABI requires a 64-bit argument's register pair to start on an odd-numbered register (r3, r5, r7, r9). Most of the time that's automatic. But when a narrower argument comes first, it can leave a register unused to keep the pair aligned.
Consider show_align(u32 x, u64 y):
x (a u32) takes r3.y would naturally fall into r4:r5 — but r4 is even, so it's skipped.y aligns up to r5:r6 instead, leaving r4 empty.
So return y + 10; operates on r5:r6, not the r4:r5 you might expect:
li r3, 10
li r0, 0
addc r4, r6, r3 # low: y_lo (r6) + 10
adde r3, r5, r0 # high: y_hi (r5) + 0 + carry
blr
This is a frequent source of confusion when reading real code: an argument register appears to go unused for no reason. The reason is alignment — the 64-bit value behind it had to bump up to the next odd register. Spot the skipped register and the signature suddenly makes sense.
Your task
Write aligned_64, taking a u32 then a u64, to reproduce the assembly above.