While I was studying about the conditions for detecting hazards while forwarding, I found this sentence very confusing.
'In the event that an instruction in the pipeline has $0 as its destination (for example,
sll $0, $1, 2), we want to avoid forwarding its possibly nonzero result value.'
I thought register $0 cannot be written in a different value, so it's value is always 0. Then what is the use of sll $0, $1, 2, shifting left the value in $1 by 2 and storing it in $0, which cannot be changed? So how can $0 being as destination possibly be related to the need to forwarding?
"the need to forwarding" seems to indicate that you don't understand forwarding. It's an optimization. There is no "need" to do it.
Optimizations should not affect semantics. Storing a value to $0 and subsequently reading the value of $0 should return 0 per the usual semantics, not the previously-stored value. Yet forwarding would forward the stored value.
Now there's the question why $0 can be used as the store location. That's just the RISC logic of having orthogonal instruction sets. On non-MIPS architectures, this is somewhat more useful as these discarded-value operations can still set flags.