chore: added benchmarks documentation

Author: Eagle941

docs/BENCHMARK.md

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+# Addition of `VisitedPcs` trait
+
+Current blockifier doesn't store the complete vector of visited program counters
+for each entry point call in an invoke transaction. Instead, visited program
+counters are pushed in a `HashSet`. This is a limiting factor to perform
+profiling operations which require record of the full trace returned by
+`cairo-vm`. More flexibility is added to the blockifier with the introduction of
+trait `VisitedPcs` which allows the user to process visited program counters in
+the most suitable way for the task.
+
+## Existing code
+
+Visited program counters are kept in the `CachedState` structure as shown below:
+
+```rust
+#[derive(Debug)]
+pub struct CachedState<S: StateReader> {
+    pub state: S,
+    // Invariant: read/write access is managed by CachedState.
+    // Using interior mutability to update caches during `State`'s immutable getters.
+    pub(crate) cache: RefCell<StateCache>,
+    pub(crate) class_hash_to_class: RefCell<ContractClassMapping>,
+    /// A map from class hash to the set of PC values that were visited in the class.
+    pub visited_pcs: HashMap<ClassHash, HashSet<usize>>,
+}
+```
+
+## New code
+
+`VisitedPcs` is an additional generic parameter of `CachedState`.
+
+```rust
+#[derive(Debug)]
+pub struct CachedState<S: StateReader, V: VisitedPcs> {
+    pub state: S,
+    // Invariant: read/write access is managed by CachedState.
+    // Using interior mutability to update caches during `State`'s immutable getters.
+    pub(crate) cache: RefCell<StateCache>,
+    pub(crate) class_hash_to_class: RefCell<ContractClassMapping>,
+    /// A map from class hash to the set of PC values that were visited in the class.
+    pub visited_pcs: V,
+}
+```
+
+An implementation of the trait `VisitedPcs` is included in the blockifier with
+the name `VisitedPcsSet` and it mimics the existing `HashSet<usize>`. Also, for
+test purposes, `CachedState` is instantiated using `VisitedPcsSet`.
+
+## Performance considerations
+
+Given the importance of the blockifier in the Starknet ecosystem, we want to
+measure the performance impact of adding the trait `VisitedPcs`. The existing
+bechmark `transfers` doesn't cover operations with `CachedState` therefore we
+need to design new ones. We have created two new benchmarks:
+
+- `cached_state`: this benchmark tests the performance impact of populating
+  `visited_pcs` (implemented using `VisitedPcsSet`) with a realistic amount of
+  visited program counters. The size of the sets is taken from transaction
+  `0x0177C9365875CAA840EA8F03F97B0E3A8EE8851A8B952BF157B5DBD4FECCB060` in the
+  mainnet. This transaction has been chosen randomly, but there is no assurance
+  that it's representative of the most common Starknet invoke transaction. This
+  benchmark tests the write performance of visited program counters in the state
+  struct.
+- `execution`: this benchmark simulates a whole invoke transaction using a dummy
+  contract.
+
+## Performance impact
+
+A script `bench.sh` has been added to benchmark the performance impact of these
+changes: it is called as
+`bash scripts/bench.sh 14e6a87722c1d0c757b1aa2756ffabe3f248fd7d e39ae0be4cec31938399199e0a1070279b4a78ed`.
+The computer running the benchmark is: Debian VM over Windows 10 with VMWare
+Workstation 17, i9-9900K, 64GB RAM, Samsung 990 Pro NVME SSD.
+
+Noise threshold and confidence intervals are kept as per default Criterion.rs
+configuration.
+
+The results are shown in the following table:
+
+| Benchmark    | Time (ms) | Time change (%) | Criterion.rs report           |
+| ------------ | --------- | --------------- | ----------------------------- |
+| transfers    | 94.448    | +0.1080         | No change in performance      |
+| execution    | 1.2882    | -1.7216         | Change within noise threshold |
+| cached_state | 5.2330    | -0.8703         | No change in performance      |
+
+The analysis of Criterion.rs determines that there isn't statistically
+significant performance decrese.