feat: Light and Heavy operation gas costs (#622)

* Light and Heavy gas costs * Update * Changelog * Update fuel-tx/src/transaction/consensus_parameters/gas.rs * Update fuel-tx/src/transaction/consensus_parameters/gas.rs Co-authored-by: Green Baneling <XgreenX9999@gmail.com> --------- Co-authored-by: Green Baneling <XgreenX9999@gmail.com>
FuelLabs · Nov 8, 2023 · c419c9b · c419c9b
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 #### Breaking
 
+- [#622](https://github.com/FuelLabs/fuel-vm/pull/622): Divide `DependentCost` into "light" and "heavy" operations: Light operations consume `0 < x < 1` gas per unit, while heavy operations consume `x` gas per unit. This distinction provides more precision when calculating dependent costs.
 - [#618](https://github.com/FuelLabs/fuel-vm/pull/618): Transaction fees for `Create` now include the cost of metadata calculations, including: contract root calculation, state root calculation, and contract id calculation.
 - [#613](https://github.com/FuelLabs/fuel-vm/pull/613): Transaction fees now include the cost of signature verification for each input. For signed inputs, the cost of an EC recovery is charged. For predicate inputs, the cost of a BMT root of bytecode is charged. 
 - [#607](https://github.com/FuelLabs/fuel-vm/pull/607): The `Interpreter` expects the third generic argument during type definition that specifies the implementer of the `EcalHandler` trait for `ecal` opcode.

diff --git a/fuel-tx/src/transaction/consensus_parameters/gas.rs b/fuel-tx/src/transaction/consensus_parameters/gas.rs
@@ -324,18 +324,29 @@ pub struct GasCostsValues {
 }
 
 /// Dependent cost is a cost that depends on the number of units.
-/// The cost starts at the base and grows by `dep_per_unit` for every unit.
-///
-/// For example, if the base is 10 and the `dep_per_unit` is 2,
-/// then the cost for 0 units is 10, 1 unit is 12, 2 units is 14, etc.
-#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
-pub struct DependentCost {
-    /// The minimum that this operation can cost.
-    pub base: Word,
-    /// The amount that this operation costs per
-    /// increase in unit.
-    pub dep_per_unit: Word,
+pub enum DependentCost {
+    /// When an operation is dependent on the magnitude of its inputs, and the
+    /// time per unit of input is less than a single no-op operation
+    LightOperation {
+        /// The minimum that this operation can cost.
+        base: Word,
+        /// How many elements can be processed with a single gas. The
+        /// higher the `units_per_gas`, the less additional cost you will incur
+        /// for a given number of units, because you need more units to increase
+        /// the total cost.
+        units_per_gas: Word,
+    },
+
+    /// When an operation is dependent on the magnitude of its inputs, and the
+    /// time per unit of input is greater than a single no-op operation
+    HeavyOperation {
+        /// The minimum that this operation can cost.
+        base: Word,
+        /// How much gas is required to process a single unit.
+        gas_per_unit: Word,
+    },
 }
 
 #[cfg(feature = "alloc")]
@@ -584,22 +595,66 @@ impl GasCostsValues {
 impl DependentCost {
     /// Create costs that are all set to zero.
     pub fn free() -> Self {
-        Self {
+        Self::HeavyOperation {
             base: 0,
-            dep_per_unit: 0,
+            gas_per_unit: 0,
         }
     }
 
     /// Create costs that are all set to one.
     pub fn unit() -> Self {
-        Self {
+        Self::HeavyOperation {
             base: 1,
-            dep_per_unit: 0,
+            gas_per_unit: 0,
         }
     }
 
+    pub fn from_units_per_gas(base: Word, units_per_gas: Word) -> Self {
+        debug_assert!(
+            units_per_gas > 0,
+            "Cannot create dependent gas cost with per-0-gas ratio"
+        );
+        DependentCost::LightOperation {
+            base,
+            units_per_gas,
+        }
+    }
+
+    pub fn from_gas_per_unit(base: Word, gas_per_unit: Word) -> Self {
+        DependentCost::HeavyOperation { base, gas_per_unit }
+    }
+
+    pub fn base(&self) -> Word {
+        match self {
+            DependentCost::LightOperation { base, .. } => *base,
+            DependentCost::HeavyOperation { base, .. } => *base,
+        }
+    }
+
+    pub fn set_base(&mut self, value: Word) {
+        match self {
+            DependentCost::LightOperation { base, .. } => *base = value,
+            DependentCost::HeavyOperation { base, .. } => *base = value,
+        };
+    }
+
     pub fn resolve(&self, units: Word) -> Word {
-        self.base + units.saturating_div(self.dep_per_unit)
+        let base = self.base();
+        let dependent_value = match self {
+            DependentCost::LightOperation { units_per_gas, .. } => {
+                // Apply the linear transformation f(x) = 1/m * x = x/m = where:
+                //   x is the number of units
+                //   1/m is the gas_per_unit
+                units.saturating_div(*units_per_gas)
+            }
+            DependentCost::HeavyOperation { gas_per_unit, .. } => {
+                // Apply the linear transformation f(x) = mx, where:
+                //   x is the number of units
+                //   m is the gas per unit
+                units.saturating_mul(*gas_per_unit)
+            }
+        };
+        base + dependent_value
     }
 }
 
@@ -623,3 +678,48 @@ impl From<GasCosts> for GasCostsValues {
         (*i.0).clone()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::DependentCost;
+
+    #[test]
+    fn light_operation_gas_cost_resolves_correctly() {
+        // Create a linear gas cost function with a slope of 1/10
+        let cost = DependentCost::from_units_per_gas(0, 10);
+        let total = cost.resolve(0);
+        assert_eq!(total, 0);
+
+        let total = cost.resolve(5);
+        assert_eq!(total, 0);
+
+        let total = cost.resolve(10);
+        assert_eq!(total, 1);
+
+        let total = cost.resolve(100);
+        assert_eq!(total, 10);
+
+        let total = cost.resolve(721);
+        assert_eq!(total, 72);
+    }
+
+    #[test]
+    fn heavy_operation_gas_cost_resolves_correctly() {
+        // Create a linear gas cost function with a slope of 10
+        let cost = DependentCost::from_gas_per_unit(0, 10);
+        let total = cost.resolve(0);
+        assert_eq!(total, 0);
+
+        let total = cost.resolve(5);
+        assert_eq!(total, 50);
+
+        let total = cost.resolve(10);
+        assert_eq!(total, 100);
+
+        let total = cost.resolve(100);
+        assert_eq!(total, 1_000);
+
+        let total = cost.resolve(721);
+        assert_eq!(total, 7_210);
+    }
+}
diff --git a/fuel-tx/src/transaction/consensus_parameters/gas/default_gas_costs.rs b/fuel-tx/src/transaction/consensus_parameters/gas/default_gas_costs.rs
@@ -92,82 +92,82 @@ pub fn default_gas_costs() -> GasCostsValues {
         wqmm: 3,
         xor: 1,
         xori: 1,
-        k256: DependentCost {
+        k256: DependentCost::LightOperation {
             base: 11,
-            dep_per_unit: 214,
+            units_per_gas: 214,
         },
-        s256: DependentCost {
+        s256: DependentCost::LightOperation {
             base: 2,
-            dep_per_unit: 214,
+            units_per_gas: 214,
         },
-        call: DependentCost {
+        call: DependentCost::LightOperation {
             base: 144,
-            dep_per_unit: 214,
+            units_per_gas: 214,
         },
-        ccp: DependentCost {
+        ccp: DependentCost::LightOperation {
             base: 15,
-            dep_per_unit: 103,
+            units_per_gas: 103,
         },
-        csiz: DependentCost {
+        csiz: DependentCost::LightOperation {
             base: 17,
-            dep_per_unit: 790,
+            units_per_gas: 790,
         },
-        ldc: DependentCost {
+        ldc: DependentCost::LightOperation {
             base: 15,
-            dep_per_unit: 272,
+            units_per_gas: 272,
         },
-        logd: DependentCost {
+        logd: DependentCost::LightOperation {
             base: 26,
-            dep_per_unit: 64,
+            units_per_gas: 64,
         },
-        mcl: DependentCost {
+        mcl: DependentCost::LightOperation {
             base: 1,
-            dep_per_unit: 3333,
+            units_per_gas: 3333,
         },
-        mcli: DependentCost {
+        mcli: DependentCost::LightOperation {
             base: 1,
-            dep_per_unit: 3333,
+            units_per_gas: 3333,
         },
-        mcp: DependentCost {
+        mcp: DependentCost::LightOperation {
             base: 1,
-            dep_per_unit: 2000,
+            units_per_gas: 2000,
         },
-        mcpi: DependentCost {
+        mcpi: DependentCost::LightOperation {
             base: 3,
-            dep_per_unit: 2000,
+            units_per_gas: 2000,
         },
-        meq: DependentCost {
+        meq: DependentCost::LightOperation {
             base: 1,
-            dep_per_unit: 2500,
+            units_per_gas: 2500,
         },
         rvrt: 13,
-        smo: DependentCost {
+        smo: DependentCost::LightOperation {
             base: 209,
-            dep_per_unit: 55,
+            units_per_gas: 55,
         },
-        retd: DependentCost {
+        retd: DependentCost::LightOperation {
             base: 29,
-            dep_per_unit: 62,
+            units_per_gas: 62,
         },
-        srwq: DependentCost {
+        srwq: DependentCost::LightOperation {
             base: 47,
-            dep_per_unit: 5,
+            units_per_gas: 5,
         },
-        scwq: DependentCost {
+        scwq: DependentCost::LightOperation {
             base: 13,
-            dep_per_unit: 5,
+            units_per_gas: 5,
         },
-        swwq: DependentCost {
+        swwq: DependentCost::LightOperation {
             base: 44,
-            dep_per_unit: 5,
+            units_per_gas: 5,
         },
-        contract_root: DependentCost {
+        contract_root: DependentCost::LightOperation {
             base: 75,
-            dep_per_unit: 1,
+            units_per_gas: 1,
         },
-        state_root: DependentCost {
+        state_root: DependentCost::LightOperation {
             base: 412,
-            dep_per_unit: 1,
+            units_per_gas: 1,
         },
     }
 }
diff --git a/fuel-vm/src/interpreter/blockchain.rs b/fuel-vm/src/interpreter/blockchain.rs
@@ -106,8 +106,8 @@ where
         let mut gas_cost = self.gas_costs().ldc;
         // Charge only for the `base` execution.
         // We will charge for the contracts size in the `load_contract_code`.
-        self.gas_charge(gas_cost.base)?;
-        gas_cost.base = 0;
+        self.gas_charge(gas_cost.base())?;
+        gas_cost.set_base(0);
         let contract_max_size = self.contract_max_size();
         let current_contract =
             current_contract(&self.context, self.registers.fp(), self.memory.as_ref())?
@@ -262,8 +262,8 @@ where
         let mut gas_cost = self.gas_costs().csiz;
         // Charge only for the `base` execution.
         // We will charge for the contracts size in the `code_size`.
-        self.gas_charge(gas_cost.base)?;
-        gas_cost.base = 0;
+        self.gas_charge(gas_cost.base())?;
+        gas_cost.set_base(0);
         let current_contract =
             current_contract(&self.context, self.registers.fp(), self.memory.as_ref())?
                 .copied();

diff --git a/fuel-vm/src/interpreter/blockchain/code_tests.rs b/fuel-vm/src/interpreter/blockchain/code_tests.rs
@@ -52,10 +52,7 @@ fn test_load_contract() -> IoResult<(), Infallible> {
         profiler: &mut Profiler::default(),
         input_contracts: InputContracts::new(input_contracts.iter(), &mut panic_context),
         current_contract: None,
-        gas_cost: DependentCost {
-            base: 13,
-            dep_per_unit: 1,
-        },
+        gas_cost: DependentCost::from_units_per_gas(13, 1),
         cgas: RegMut::new(&mut cgas),
         ggas: RegMut::new(&mut ggas),
         ssp: RegMut::new(&mut ssp),

diff --git a/fuel-vm/src/interpreter/blockchain/other_tests.rs b/fuel-vm/src/interpreter/blockchain/other_tests.rs
@@ -322,10 +322,7 @@ fn test_code_size() {
     let input = CodeSizeCtx {
         storage: &mut storage,
         memory: &mut memory,
-        gas_cost: DependentCost {
-            base: 0,
-            dep_per_unit: 0,
-        },
+        gas_cost: DependentCost::free(),
         profiler: &mut Profiler::default(),
         input_contracts: InputContracts::new(input_contract.iter(), &mut panic_context),
         current_contract: None,
@@ -343,10 +340,7 @@ fn test_code_size() {
     let input = CodeSizeCtx {
         storage: &mut storage,
         memory: &mut memory,
-        gas_cost: DependentCost {
-            base: 0,
-            dep_per_unit: 0,
-        },
+        gas_cost: DependentCost::free(),
         input_contracts: InputContracts::new(input_contract.iter(), &mut panic_context),
         profiler: &mut Profiler::default(),
         current_contract: None,
@@ -363,10 +357,7 @@ fn test_code_size() {
     let input = CodeSizeCtx {
         storage: &mut storage,
         memory: &mut memory,
-        gas_cost: DependentCost {
-            base: 0,
-            dep_per_unit: 0,
-        },
+        gas_cost: DependentCost::free(),
         input_contracts: InputContracts::new(iter::empty(), &mut panic_context),
         profiler: &mut Profiler::default(),
         current_contract: None,

diff --git a/fuel-vm/src/interpreter/flow.rs b/fuel-vm/src/interpreter/flow.rs
@@ -390,8 +390,8 @@ where
         let mut gas_cost = self.gas_costs().call;
         // Charge only for the `base` execution.
         // We will charge for the frame size in the `prepare_call`.
-        self.gas_charge(gas_cost.base)?;
-        gas_cost.base = 0;
+        self.gas_charge(gas_cost.base())?;
+        gas_cost.set_base(0);
         let current_contract =
             current_contract(&self.context, self.registers.fp(), self.memory.as_ref())?
                 .copied();

diff --git a/fuel-vm/src/interpreter/flow/tests.rs b/fuel-vm/src/interpreter/flow/tests.rs
@@ -52,10 +52,7 @@ impl Default for Input {
             input_contracts: vec![Default::default()],
             storage_balance: Default::default(),
             memory: vec![0u8; MEM_SIZE].try_into().unwrap(),
-            gas_cost: DependentCost {
-                base: 10,
-                dep_per_unit: 10,
-            },
+            gas_cost: DependentCost::from_units_per_gas(10, 10),
             storage_contract: vec![(ContractId::default(), vec![0u8; 10])],
             script: None,
         }