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[syscalls] Update mod exp compute cost #2807

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Sep 2, 2024
Merged

[syscalls] Update mod exp compute cost #2807

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6f267cb
update mod exp compute cost
samkim-crypto Aug 31, 2024
2806d5a
re-organize modular exponentiation costs
samkim-crypto Sep 2, 2024
99ef62f
rename multiplicative factor to divisor
samkim-crypto Sep 2, 2024
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11 changes: 8 additions & 3 deletions compute-budget/src/compute_budget.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -103,8 +103,12 @@ pub struct ComputeBudget {
/// + alt_bn128_pairing_one_pair_cost_other * (num_elems - 1)
pub alt_bn128_pairing_one_pair_cost_first: u64,
pub alt_bn128_pairing_one_pair_cost_other: u64,
/// Big integer modular exponentiation cost
pub big_modular_exponentiation_cost: u64,
/// Big integer modular exponentiation base cost
pub big_modular_exponentiation_base_cost: u64,
/// Big integer moduler exponentiation cost divisor
/// The modular exponentiation cost is computed as
/// `input_length`/`big_modular_exponentiation_cost_divisor` + `big_modular_exponentiation_base_cost`
pub big_modular_exponentiation_cost_divisor: u64,
/// Coefficient `a` of the quadratic function which determines the number
/// of compute units consumed to call poseidon syscall for a given number
/// of inputs.
Expand Down Expand Up @@ -180,7 +184,8 @@ impl ComputeBudget {
alt_bn128_multiplication_cost: 3_840,
alt_bn128_pairing_one_pair_cost_first: 36_364,
alt_bn128_pairing_one_pair_cost_other: 12_121,
big_modular_exponentiation_cost: 33,
big_modular_exponentiation_base_cost: 190,
big_modular_exponentiation_cost_divisor: 2,
poseidon_cost_coefficient_a: 61,
poseidon_cost_coefficient_c: 542,
get_remaining_compute_units_cost: 100,
Expand Down
12 changes: 8 additions & 4 deletions programs/bpf_loader/src/syscalls/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1710,13 +1710,15 @@ declare_builtin_function!(
let input_len: u64 = std::cmp::max(input_len, params.modulus_len);

let budget = invoke_context.get_compute_budget();
// the compute units are calculated by the quadratic equation `0.5 input_len^2 + 190`
consume_compute_meter(
invoke_context,
budget.syscall_base_cost.saturating_add(
input_len
.saturating_mul(input_len)
.checked_div(budget.big_modular_exponentiation_cost)
.unwrap_or(u64::MAX),
.checked_div(budget.big_modular_exponentiation_cost_divisor)
.unwrap_or(u64::MAX)
.saturating_add(budget.big_modular_exponentiation_base_cost),
),
)?;

Expand Down Expand Up @@ -4719,7 +4721,8 @@ mod tests {
let budget = invoke_context.get_compute_budget();
invoke_context.mock_set_remaining(
budget.syscall_base_cost
+ (MAX_LEN * MAX_LEN) / budget.big_modular_exponentiation_cost,
+ (MAX_LEN * MAX_LEN) / budget.big_modular_exponentiation_cost_divisor
+ budget.big_modular_exponentiation_base_cost,
);

let result = SyscallBigModExp::rust(
Expand Down Expand Up @@ -4760,7 +4763,8 @@ mod tests {
let budget = invoke_context.get_compute_budget();
invoke_context.mock_set_remaining(
budget.syscall_base_cost
+ (INV_LEN * INV_LEN) / budget.big_modular_exponentiation_cost,
+ (INV_LEN * INV_LEN) / budget.big_modular_exponentiation_cost_divisor
+ budget.big_modular_exponentiation_base_cost,
);

let result = SyscallBigModExp::rust(
Expand Down