add doc for gas setting

Author: open-junius

precompiles/src/ed25519.rs

@@ -21,6 +21,10 @@ impl<A> LinearCostPrecompile for Ed25519Verify<A>
 where
     A: From<[u8; 32]>,
 {
+    // https://eips.ethereum.org/EIPS/eip-665#gas-costs
+    // According to the EIP, the base cost should be 2000 gas, less than ECDSA/secp256k1 which is 3000.
+    // Reality: Ed25519 verification is ~2.3x more computationally expensive than ECDSA/secp256k1
+    // So we set the base cost to 6000 gas, which is 3x of the EIP's base cost.
     const BASE: u64 = 6000;
     const WORD: u64 = 0;
 

precompiles/src/sr25519.rs

@@ -22,6 +22,7 @@ impl<A> LinearCostPrecompile for Sr25519Verify<A>
 where
     A: From<[u8; 32]>,
 {
+    // There is no EIP for SR25519, so we use the same base cost as Ed25519.
     const BASE: u64 = 6000;
     const WORD: u64 = 0;