Module finite_wasm::gas

Analysis of the amount of time (gas) a function takes to execute.

The gas analysis is a two-pass linear-time algorithm. This algorithm first constructs a table containing instrumentation offsets (represented as instructions in the example below), their costs and kinds:

| Instructions | Cost | Kind |
| ============ | ==== | ==== |
| i32.const 0  | 1    | Pure |
| i32.const 1  | 1    | Pure |
| i32.add      | 1    | Pure |

In this table the instructions with certain instrumentation kind combinations can then be coalesced in order to reduce the number of gas instrumentation points. For example all instrumentation can be merged together across all instructions considered Pure to produce a table like this:

| Instructions                           | Cost | Kind |
| ====================================== | ==== | ==== |
| (i32.add (i32.const 0) (i32.const 1))  | 3    | Pure |

Instrumentation can then, instead of inserting a gas charge before each of the 3 instructions, insert a charge of 3 gas before the entire sequence, all without any observable difference in execution semantics.

Why two passes? A short answer is – branching. As the algorithm goes through the function code for the first time, it can mark certain instructions as being a branch or a branch target. For example end in block…end can be either pure or a branch target. Table entries for the instructions that participate in control flow cannot be merged together if an eventually accurate gas count is desired.

Structs

• FunctionState
  The per-function state used by the Visitor.
• Visitor
  The core algorihtm of the gas analysis.

Enums

• Error
• InstrumentationKind
  The type of a particular instrumentation point (as denoted by its offset.)

Traits

• Config
  The configuration for the gas analysis.