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Number of register file ports in implementations
Author:  Date: 2017-08-28 17:05
Agner wrote:
Hubert Lamontagne wrote:
On FPGAs, which I think would make sense for a test implementation, register files with lots of ports are an issue... especially with multiple write ports with no limitations on which registers are concurrently written to.


What do you think of an implementation like this:


There is a permanent register file, but no temporary register files. Temporary register values are stored instead in the instructions that need them in the reservation station. Each instruction has its register operands replaced by their values immediately after the decode stage if the values are available in the permanent register file. Register operands that are not yet available are replaced by the tag of the pending instruction that will produce the value. Each ALU has a write port that writes the result to all pending instructions that needs it, as well as to the permanent register file. ForwardCom does not allow instructions with more than one output value.


So we will need four or five register read ports in the early stage in the pipeline for reading the permanent register file, but no read ports for a huge temporary register file.


Hmm, interesting. I think I've read that some Intel P2 family cores work something like this somewhere. Though I'm a bit worried that this might make the Reservation Station rather big, since each input operand of each instruction in the Reservation Station has to watch all the write ports all the time and multiplex it in... So your reservation station entry needs a register per operand, plus a multiplexer for each operand so that the appropriate result can be saved. This is kindof moving the hugely-multiple-write-ported register file into the reservation station.

I think in that case your permanent register file also still needs about as many ports as in a design where the same register file is used both for permanent and temporary values.

It might make sense to go for a design where registers are renamed to different physical registers depending on which unit produces them. For instance, ALUs and memory load units could have a different physical register pool internally (even though they look the same to programmers) so that they don't have to share write ports, and the register renamer can relatively easily keep track of what's in which pool. Generally for FPGAs, register files with many load ports and a single write port tend to be the better plan - the Verilog tool simply creates as many register file copies as it needs.

Vectors are implemented as follows. The pipeline is split into 64-bit lanes. Each lane has its own 64-bit register file, reservation station, pipeline, and 64-bit ALU. We can add as many 64-bit lanes as we need to support a certain vector length. There is one scheduler which controls all the lanes. This can run smoothly and fast as long as there is no exchange of data between the lanes. Instructions that move data between the lanes, such as broadcast and permute, are implemented in a separate unit with a single bidirectional data path to all the lanes. Such instructions will take one or two clock cycles extra, as they do in x86 processors.


Quite sensible and I'd bet dollars to pennies that most SIMD units are built this way ;3

The in-order front end has its own ALU's which can handle simple integer instructions. Simple instructions on general purpose registers are executed here if their operands are available (as I have proposed in a previous post). Loop conters and simple branches will be executed in the front end so that the instructions of the loop body are sent to the out-of-order back end with the loop counter replaced by its value. This will reduce the branch misprediction penalty. Registers used for memory pointers and indexes will also mostly be resolved in the front end so that the memory operands can be fetched early.
Hmm, I wonder if it's possible to track which instructions/operands/values are "front end" and tag them in the instruction cache, so that they can be separately issued? Using some kind of heuristic like "result used in address computation" or "result not coming out of memory load/sufficiently stale by the time it gets used". Like, a front/back-endness predictor ;3
 
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