refactor(contracts): specify gas cost in SemaphoreVerifier precompile calls (#883)

* chore(contracts): remove sub() from gas() in staticcall at SemaphoreVerifier

re #871

* refactor(contracts): using fixed gas cost on precompile calls

re #871

* refactor(contracts): increease fixed gas cost with safety buffer

Author: jimmychu0807

packages/contracts/contracts/base/SemaphoreVerifier.sol

@@ -59,7 +59,10 @@ contract SemaphoreVerifier {
                 mstore(add(mIn, 32), y)
                 mstore(add(mIn, 64), s)
 
-                success := staticcall(sub(gas(), 2000), 7, mIn, 96, mIn, 64)
+                // ecMul gas cost is fixed at 6000. Add 33.3% gas for safety buffer.
+                // Last checked in 2024 Oct, evm codename Cancun
+                // ref: https://www.evm.codes/precompiled?fork=cancun#0x07
+                success := staticcall(8000, 7, mIn, 96, mIn, 64)
 
                 if iszero(success) {
                     mstore(0, 0)
@@ -69,7 +72,10 @@ contract SemaphoreVerifier {
                 mstore(add(mIn, 64), mload(pR))
                 mstore(add(mIn, 96), mload(add(pR, 32)))
 
-                success := staticcall(sub(gas(), 2000), 6, mIn, 128, pR, 64)
+                // ecAdd gas cost is fixed at 150. Add 33.3% gas for safety buffer.
+                // Last checked in 2024 Oct, evm codename Cancun
+                // ref: https://www.evm.codes/precompiled?fork=cancun#0x06
+                success := staticcall(200, 6, mIn, 128, pR, 64)
 
                 if iszero(success) {
                     mstore(0, 0)
@@ -149,7 +155,10 @@ contract SemaphoreVerifier {
                 mstore(add(_pPairing, 704), mload(add(vkPoints, 64)))
                 mstore(add(_pPairing, 736), mload(add(vkPoints, 96)))
 
-                let success := staticcall(sub(gas(), 2000), 8, _pPairing, 768, _pPairing, 0x20)
+                // ecPairing gas cost at 181000 given 768 bytes input. Add 33.3% gas for safety buffer.
+                // Last checked in 2024 Oct, evm codename Cancun
+                // ref: https://www.evm.codes/precompiled?fork=cancun#0x08
+                let success := staticcall(241333, 8, _pPairing, 768, _pPairing, 0x20)
 
                 isOk := and(success, mload(_pPairing))
             }