CPU Modeling: First Order Latency and Data Dependencies - Building

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This post is a new episode in a miniseries focused on modeling the workings of a CPU to use for generating optimal code. This post focuses on taking a sequence of instructions and simulating their results while respecting the latency of each instruction execution and data dependencies between instructions.

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A Solution

Github PR

The initial solution is fairly simple: partition registers into two categories. The first categories are registers with data that's available. The instruction writing to that has completed its full latency and written to the memory (ignoring different caching for now). The second category are registers with data that's "in progress". The instruction writing data to that register hasn't completed yet, so an instruction that depends on the data can't use it yet.

Each in progress register is tagged with the cycle when it'll become available. This allows the CPU to block on the data dependency if the next instruction to process depends on data that isn't in the available registers.

Wide Registers

The first instruction to prototype this operation was just a 4 way element wise addition. This means that the wide registers of 4x 32bit floats aren't really that different from a single value; however, this won't always hold. Some SIMD operations perform custom logic or shuffling for values based on where they are in the batch.

To allow for this generalization, each operation defines a function for custom logic that returns a wide register with the correct output values. At the time the instruction "enters" the CPU and its data is available, the result is calculated and inserted into the in-progress registers with the correct latency applied.

Usage

With a model of latencies and register storage, the "CPU" can run a fixed list of instructions by continually iterating through the list until all instructions are consumed and all registers become available. The unsurprising but happy result of this baby CPU model is that we can see (with a very simple program) an increase in cycle count for a set of instructions with a data dependency vs one without a data dependency.

Missteps

Throughput

The one big miss for the initial algorithm is accounting for the throughput of instructions. In a pipeline-based architecture (which covers most CPUs today) multiple instructions can be in flight at once as long as their data is independent. The model as written takes a simpler approach and only processes one instruction at a time, which leads to an under-estimate of throughput for some cases.

Register Writing

If two subsequent instructions with different delays write to the same output register, whichever instruction enters the CPU second will overwrite the result of the first, potentially shortening or increasing the latency when the value will be ready and changing its value.

In practice a better approach would allow for different instructions to write to the same register without overwriting each other. This would allow for a third instruction to read the first available value and start computation before the second value gets written into the register.

There's also the chance that what I've written is actually how a processor is expected to behave, but for now I'll just leave it as a potential bug to revisit.