Reg file vs. forwarding network

By: hobold (hobold.delete@this.informatik.uni-bremen.de), June 22, 2008 12:39 pm
Room: Moderated Discussions
David Kanter (dkanter@realworldtech.com) on 6/22/08 wrote:
---------------------------
[...]
>Also, the reason that the P6 has a big register file is that the register file
>is rarely used an operand source. I've talked a bit with Andy Glew about this,
>and he's pretty insistent that in a P6-style microarchitecture, the vast majority
>of your input operands come from the forwarding network, rather than the >physical register file itself.
>
This is correct, the P6 microarchitecture is optimized for execution of serially dependent instructions. The designers managed to convert the weakness of the small 'x86 architected register file (which resulted in lots of (false) dependencies in most code) into a strength of the implementation.

Other architectures which aimed to express more parallelism in their ISA (not just EPIC, also RISCs with their large register files and three operand encoding) were not as lucky in realizing a benefit.

I sometimes wonder if we should not experiment more with ISAs that make dependencies explicit rather than parallelism. I am aware of only a single old academic project named "WM - Weird Machine". It encoded instructions as dependent pairs with implicit producer/consumer dataflow. You could say it generalized things like 'fused multiply add', or the complex 'load effective address' instructions.
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