|
|
|
|
|
|
by BeeOnRope
2852 days ago
|
|
|
Well the 16-bit and 8-bit registers work this way, and actual hardware shows this is far from free. We've had partial register stalls, merging uops, and other weird behavior (for example, ah today works very differently from al). It's easy to say "think of them as separate registers" just as it's easy to say "think of them as a partial registers" - but the ISA definition is such that they have to appear as partial registers in the scenarios where it would be visible. So sure, you could make hardware that would rename and treat them as different registers (at it has been done on some x86 versions), but then when you read a wider portion you need to combine them, which won't be free (yes it happens "implicitly" but that doesn't somehow make it much easier for the hardware). There are also plenty of cases where you want zero-extension for functional reasons, especially in compiled code where things like casts to a larger size become free. Cleverly using both halves of a register and using the implicit combination into a full 64-bit value is much rarer that just wanting to store a 32-bit value and sometimes wanting to use it as a zero-extended 64-bit value. |
|
|
Can you elaborate on the last behavior, that " ah today works very differently from al"? How do they differ? I hadn't heard this before.