Reg file vs. forwarding network

By: Peter (, June 23, 2008 3:44 am
Room: Moderated Discussions
>>I suspect that given the cost of wires today a wide issue >processor with lots of
>>execution resource would run into timing issues if it >tried to do a lot of forwarding.
>What do you think of as a 'wide' processor? If you are thinking of something like
>a GPU, which has >100 parallel ALUs, then I'd totally agree. If you were thinking
>of something more like an 8 issue CPU, I'm not sure I'd agree.

Depends on the micro-architecture of the processor.

The micro-architecture of the execution pipelines in a P6 style processor is quite simple so I can see why you don't agree.

However, a few years ago I built the forwarding on a dual-issue in-order superscalar processor with multiple stages in its execution pipelines. The forwarding was, lets say, challenging to get right both in terms of timing and routability. If we had gone triple-issue or more I reckon a mental breakdown would have been on the cards!

The micro-architecture of this particular processor made the right decisions as a whole it is just that one of the side effects was a complex forwarding system.

Therefore I would say that designing an architecture that explicitly requires forwarding to achieve good performance may not be the best idea as it would constrain the sorts of micro-architectures that could efficiently implement the architecture.

>>Also, designing a micro-architecture that relies on >forwarding operands from multiple
>>execution resources to all other execution resources is >not going to particularly
>>power efficient as wire power is so high.
>Absolutely, this is the classic shared nothing versus shared everything argument.
>Shared everything is always easier to program.
>Shared nothing is always higher performance/cheaper/lower power.
>CPUs fall into the former category, GPUs into the latter. However, for the right
>sort of workloads the disadvantages of the GPU's shared nothing architecture are mitigated.

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