The O3CPU is our new detailed model for the v2.0 release. It is an out of order CPU model loosely based on the Alpha 21264. This page will give you a general overview of the O3CPU model, the pipeline stages and the pipeline resources. We have made efforts to keep the code well documented, so please browse the code for exact details on how each part of the O3CPU works.
The O3CPU has the following pipeline stages:
- Fetches instructions each cycle, selecting which thread to fetch from based on the policy selected. This stage is where the DynInst is first created. Also handles branch prediction.
- Decodes instructions each cycle. Also handles early resolution of PC-relative unconditional branches.
- Renames instructions using a physical register file with a free list. Will stall if there are not enough registers to rename to, or if back-end resources have filled up. Also handles any serializing instructions at this point by stalling them in rename until the back-end drains.
- Our simulator model handles both execute and writeback when the execute() function is called on an instruction, so we have combined these three stages into one stage. This stage (IEW) handles dispatching instructions to the instruction queue, telling the instruction queue to issue instruction, and executing and writing back instructions.
- Commits instructions each cycle, handling any faults that the instructions may have caused. Also handles redirecting the front-end in the case of a branch misprediction.
Additionally it has the following structures:
- Branch predictor
- Allows for selection between several branch predictors, including a local predictor, a global predictor, and a tournament predictor. Also has a branch target buffer and a return address stack.
- Reorder buffer
- Holds instructions that have reached the back-end. Handles squashing instructions and keeping instructions in program order.
- Instruction queue
- Handles dependencies between instructions and scheduling ready instructions. Uses the memory dependence predictor to tell when memory operations are ready.
- Load-store queue
- Holds loads and stores that have reached the back-end. It hooks up to the d-cache and initiates accesses to the memory system once memory operations have been issued and executed. Also handles forwarding from stores to loads, replaying memory operations if the memory system is blocked, and detecting memory ordering violations.
- Functional units
- Provides timing for instruction execution. Used to determine the latency of an instruction executing, as well as what instructions can issue each cycle.
- Memory dependence prediction using store sets
- Informs the IQ which memory instructions it predicts as ready to issue (in terms of memory ordering). In the Alpha models, memory operations have been atomic operations where the address calculation and memory access are bundled as one instruction. Because the effective addresses are not calculated separately, memory dependence prediction is necessary in order to give some idea of the order in which memory operations can execute.
For the O3CPU, we've made efforts to make it highly timing accurate. In order to do this, we use a model that actually executes instructions at the execute stage of the pipeline. Most simulator models will execute instructions either at the beginning or end of the pipeline; SimpleScalar and our old detailed CPU model both execute instructions at the beginning of the pipeline and then pass it to a timing backend. This presents two potential problems: first, there is the potential for error in the timing backend that would not show up in program results. Second, by executing at the beginning of the pipeline, the instructions are all executed in order and out-of-order load interaction is lost. Our model is able to avoid these deficiencies and provide an accurate timing model.
The O3CPU makes heavy use of template policies to obtain a level of polymorphism without having to use virtual functions. It uses template policies to pass in an "Impl" to almost all of the classes used within the O3CPU. This Impl has defined within it all of the important classes for the pipeline, such as the specific Fetch class, Decode class, specific DynInst types, the CPU class, etc. It allows any class that uses it as a template parameter to be able to obtain full type information of any of the classes defined within the Impl. By obtaining full type information, there is no need for the traditional virtual functions/base classes which are normally used to provide polymorphism. The main drawback is that the CPU must be entirely defined at compile time, and that the templated classes require manual instantiation. See src/cpu/o3/alpha/impl.hh and src/cpu/o3/cpu_policy.hh for example Impl classes.
The O3CPU has been designed to try to separate code that is ISA dependent and code that is ISA independent. The pipeline stages and resources are all mainly ISA independent, as well as the lower level CPU code. The ISA dependent code implements ISA-specific functions. For example, the AlphaO3CPU implements Alpha-specific functions, such as hardware return from error interrupt (hwrei()) or reading the interrupt flags. The lower level CPU, the FullO3CPU, handles orchestrating all of the pipeline stages and handling other ISA-independent actions. We hope this separation makes it easier to implement future ISAs, as hopefully only the high level classes will have to be redefined.
Interaction with ThreadContext
The ThreadContext provides interface for external objects to access thread state within the CPU. However, this is slightly complicated by the fact that the O3CPU is an out-of-order CPU. While it is well defined what the architectural state is at any given cycle, it is not well defined what happens if that architectural state is changed. Thus it is feasible to do reads to the ThreadContext without much effort, but doing writes to the ThreadContext and altering register state requires the CPU to flush the entire pipeline. This is because there may be in flight instructions that depend on the register that has been changed, and it is unclear if they should or should not view the register update. Thus accesses to the ThreadContext have the potential to cause slowdown in the CPU simulation.