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0 pointslordnacho4y ago0 comments

How does the instruction fusion work? It seems to be mentioned in the article and by a couple of other commenters.

0 comments

The CPU executes the two (or more) dependent instructions "as if" they were one, e.g., in 1 cycle.

The CPU has a frontend, which has a decoder, which is the part that "reads" the program instructions. When it "sees" certain pattern, like "instruction x to register r followed by instruction y consuming r", it can treat this "as if" it was a single instruction if the CPU has hardware for executing that single instruction (even if the ISA doesn't have a name for that instruction).

This allows the people that build the CPU to choose whether this is something they want to add hardware for. If they don't, this runs in e.g. 2 cycles, but if they do then it runs in 1. A server CPU might want to pay the cost of running it in 1 cycle, but a micro controller CPU might not.

cpeterso4y ago

Do RISC-V specs document which instruction combinations they recommend be fused? Sounds like the fused instructions are an implementation detail that must be well-documented for compiler writers to know to emit the magic instruction combinations.

mlyle4y ago

For this specific case, yes, the RISC-V ISA document recommends instruction sequences for checking for overflow-- that are both amenable to fusion and are relatively high performing on implementations that don't fuse.

Section 2.4, https://riscv.org/wp-content/uploads/2017/05/riscv-spec-v2.2...

__init4y ago

It generally goes the other way around -- programmers and compilers settle on a few idiomatic ways to do something, and new cores are built to execute those quickly. Because RISC-V is RISC, it seems likely that those few ways would be less idiomatic and more 'the only real way to do x', which would aid in the applicability of the fusions.

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