How does a zero register improve performance?

Question

In the MIPS ISA, there's a zero register ($r0) which always gives a value of zero. This allows the processor to:

1. Any instruction which produces result that is to be discarded can direct its target to this register
2. To be a source of 0

It is said in this source that this improved the speed of the CPU. How does it improve performance? And what are the reasons why not all ISA adopt this zero register?

$r0 is not general purpose. It is hardwired to 0. No matter what you do to this register, it always has a value of 0. You might wonder why such a register is needed in MIPS.

The designers of MIPS used benchmarks (programs used to determine the performance of a CPU), which convinced them that having a register hardwired to 0 would improve the performance (speed) of the CPU as opposed to not having it. Not everyone agrees a register hardwired to 0 is essential, so not all ISAs have a zero register.

Answer 1

There's a few potential ways that this can improve performance; it's not clear which ones apply to that particular processor, but I've listed them roughly in order from most to least likely.

1. It avoids spurious pipeline stalls. Without an explicit zero register, it's necessary to take a register, zero it out, and use its value. This means that the zero-using operation is dependent on the zeroing operation, and (depending on how powerful the pipeline forwarding system is) possibly on the zeroed register's previous value. Architectures like x86, which have quite small register files and basically virtualize their registers to keep that from causing problems, have extremely powerful hazard analysis tools. The same is not generally true of RISC processors.
2. Certain operations may be more pipelineable if they can avoid a register read. If an explicit zero register is used, the fact that the operand will be zero is known at the instruction decode stage, rather than later on in the register fetch stage. Thus, the register read stage can be skipped.
3. Similarly, the ability to explicitly discard results avoids the need for a register write stage.
4. Certain operations may generate simpler microcode when one of their operands is known to be zero, or when the result is known to be discarded.
5. An explicit zero register takes some pressure off the compiler's optimizer, as it doesn't need to be as careful with its register assignment (no need to identify a register which won't cause a stall on read or write).