AutoCounter: Profiling with Out-of-Band Performance Counter Collection

FireSim can provide visibility into a simulated CPU’s architectural and microarchitectural state over the course of execution through the use of counters. These are similar to performance counters provided by processor vendors, and more general counters provided by architectural simulators.

This functionality is provided by the AutoCounter feature (introduced in our FirePerf paper at ASPLOS 2020), and can be used for profiling and debugging. Since AutoCounter injects counters only in simulation (unlike target-level performance counters), these counters do not affect the behavior of the simulated machine, no matter how often they are sampled.

Ad-hoc Performance Counters

AutoCounter enables the addition of ad-hoc counters using the PerfCounter function. The PerfCounter function takes 3 arguments: A boolean signal to be counted, a counter label, and the counter description. Here is an example counter declaration:

midas.targetutils.PerfCounter(s1_pc, "s1_pc", "stage 1 program counter")

Building a Design with AutoCounter

To enable AutoCounter when building a design, prepend the WithAutoCounter config to your PLATFORM_CONFIG. During compilation, FireSim will print the signals it is generating counters for. If AutoCounter has been enabled, the autocounter_t bridge driver will also be automatically instantiated.

Rocket Chip Cover Functions

The cover function is applied to various signals in the Rocket Chip generator repository to mark points of interest (i.e., interesting signals) in the RTL. Tools are free to provide their own implementation of this function to process these signals as they wish. In FireSim, these functions can be used as a hook for automatic generation of counters.

Since cover functions are embedded throughout the code of Rocket Chip (and possibly other code repositories), AutoCounter provides a filtering mechanism based on module granularity. As such, only cover functions that appear within selected modules will generate counters.

The filtered modules can be indicated using one of two methods:

  1. A module selection annotation within the top-level configuration implementation. To use this method, add the AutoCounterCoverModuleAnnotation annotation with the name of the module for which you want the cover functions to be turned into AutoCounters. The following example will generate counters from cover functions within the StreamWriter module:
 class FireSimDUT(implicit p: Parameters) extends Subsystem
  with HasHierarchicalBusTopology
  with CanHaveMasterAXI4MemPort
  with HasPeripheryBootROM
  with HasPeripherySerial
  with HasPeripheryUART
  with HasPeripheryIceNIC
  with HasPeripheryBlockDevice
  with HasTraceIO
  override lazy val module = new FireSimModuleImp(this)

  1. An input file with a list of module names. This input file is named autocounter-covermodules.txt, and includes a list of module names separated by new lines (no commas).

AutoCounter Runtime Parameters

AutoCounter currently takes a single runtime configurable parameter, defined under the [autocounter] section in the config_runtime.ini file. The readrate parameter defines the rate at which the counters should be read, and is measured in target-cycles of the base target-clock (clock 0 produced by the ClockBridge). Hence, if the read-rate is defined to be 100 and the tile frequency is 2x the base clock (ex., which may drive the uncore), the simulator will read and print the values of the counters every 200 core-clock cycles. If the core-domain clock is the base clock, it would do so every 100 cycles. By default, the read-rate is set to 0 cycles, which disables AutoCounter.

# read counters every 100 cycles

Upon setting this value, when you run a workload, an AutoCounter output file will be placed in the sim_slot_<slot #> directory on the F1 instance under the name AUTOCOUNTERFILE<N>, with one file generated per clock domain containing an AutoCounter event. The header of each output file indicates the associated clock domain and its frequency relative to the base clock.


AutoCounter is designed as a coarse-grained observability mechanism, as sampling each counter requires two (blocking) MMIO reads (each read takes O(100) ns on EC2 F1). As a result sampling at intervals less than O(10000) cycles may adversely affect simulation performance for large numbers of counters. If you intend on reading counters at a finer granularity, please consider using synthesizable printfs.

Using TracerV Trigger with AutoCounter

In order to collect AutoCounter results from only from a particular region of interest in the simulation, AutoCounter has been integrated with TracerV triggers. See the Setting a TracerV Trigger section for more information.

AutoCounter using Synthesizable Printfs

The AutoCounter transformation in the Golden Gate compiler includes an event-driven mode that uses Synthesizable Printfs (see Printf Synthesis: Capturing RTL printf Calls when Running on the FPGA) to export counter results as they are updated rather than sampling them periodically with a dedicated Bridge. This mode can be enabled by prepending the WithAutoCounterCoverPrintf config to your PLATFORM_CONFIG instead of WithAutoCounterCover. In this mode, the counter values and the local cycle count will be printed every time the counter is incremented using a synthesized printf (hence, you will observe a series of printfs incrementing by 1). This mode may be useful for fine-grained observation of counters. The counter values will be printed to the same output stream as other synthesizable printfs. This mode uses considerably more FPGA resources per counter, and may consume considerable amounts of DMA bandwidth (since it prints every cycle a counter increments), which may adversly affect simulation performance (increased FMR).