> I'd assume it would ... be ... whatever the previous scalar was (i.e. untouched) It is a fair assumption these days that no instruction leaves bits in the re

> I'd assume it would ... be ... whatever the previous scalar was (i.e. untouched) It is a fair assumption these days that no instruction leaves bits in the register "untouched" unless doing so is a core part of what's needed for the instruction to work. On architectures with register renaming (which is all high-performance architectures) it can be surprisingly expensive. The very least, it forces a dependency on all previous instructions that write that register, and adds another source operand to the instruction. And on most real architectures, this source operand would be required to be present before the instruction itself can start. This is particularly bad for load instructions, if it pushes back when the load can be submitted to cache. This can wipe out all your memory parallelism, and effectively cut your performance to a fraction of what it would othewise be. The basic mental model you need to have is that every instruction that writes to a register always gets a new register that starts with all bits zeroed. To have anything else, you have to do work.