MOVUPD vs. MOVDQU (x86/x64 assembly)

Question

What's the difference between these instructions? MOVDQU is an unaligned double quadword move and MOVUPD is an unaligned two 64-bit float move. I mean they're both just moving unaligned 128-bit data.

MOVDQU is on page 948, MOVUPD is on page 995 of the intel x64 manual.

Answer 1

Agner Fog says:

The instructions MOVDQA, MOVDQU, MOVAPS, MOVUPS, MOVAPD and MOVUPD are all identical when used with [128 bit] register operands

Then he goes on to say (he's using the aligned versions in his examples, but I'm guessing the same applies for the unaligned variants):

On Intel Core 2 and earlier Intel processors, some floating point instructions are executed in the integer units. This includes XMM move instructions, Boolean, and some shuffle and pack instructions. These instructions have a bypass delay when mixed with instructions that use the floating point unit. On most other processors, the execution unit used is in accordance with the instruction name, e.g. MOVAPS XMM1,XMM2 uses the floating point unit, MOVDQA XMM1,XMM2 uses the integer unit.

---

Instructions that read or write memory use a separate unit. The bypass delay from the memory unit to the floating point unit may be longer than to the integer unit on some processors, but it doesn't depend on the type of the instruction. Thus, there is no difference in latency between MOVAPS XMM0,[MEM] and MOVDQA XMM0,[MEM] on current processors, but it cannot be ruled out that there will be a difference on future processors.

---

[Y]ou may use MOVAPS instead of MOVAPD or MOVDQA for moving data to or from memory or between registers. A bypass delay occurs in some processors when using MOVAPS for moving the result of an integer instruction to another register, but not when moving data to or from memory.