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

re semaphore-protocol#871

Author: jimmychu0807

packages/contracts/contracts/base/SemaphoreVerifier.sol

@@ -59,7 +59,11 @@ contract SemaphoreVerifier {
                 mstore(add(mIn, 32), y)
                 mstore(add(mIn, 64), s)
 
-                success := staticcall(gas(), 7, mIn, 96, mIn, 64)
+                // ref: https://www.evm.codes/precompiled 0x07 ecMul
+                // If inputs are valid, fixed gas cost 6000 is consumed.
+                // If inputs are invalid, all gas is consumed but is limited to 6000 in this case.
+                // `iszero(success)` will be true afterwards.
+                success := staticcall(6000, 7, mIn, 96, mIn, 64)
 
                 if iszero(success) {
                     mstore(0, 0)
@@ -69,7 +73,11 @@ contract SemaphoreVerifier {
                 mstore(add(mIn, 64), mload(pR))
                 mstore(add(mIn, 96), mload(add(pR, 32)))
 
-                success := staticcall(gas(), 6, mIn, 128, pR, 64)
+                // ref: https://www.evm.codes/precompiled 0x06 ecAdd
+                // If inputs are valid, fixed gas cost 150 is consumed.
+                // If inputs are invalid, all gas is consumed but is limited to 150 in this case.
+                // `iszero(success)` will be true afterwards.
+                success := staticcall(150, 6, mIn, 128, pR, 64)
 
                 if iszero(success) {
                     mstore(0, 0)
@@ -149,7 +157,9 @@ contract SemaphoreVerifier {
                 mstore(add(_pPairing, 704), mload(add(vkPoints, 64)))
                 mstore(add(_pPairing, 736), mload(add(vkPoints, 96)))
 
-                let success := staticcall(gas(), 8, _pPairing, 768, _pPairing, 0x20)
+                // ref: https://www.evm.codes/precompiled 0x08 ecPairing
+                // Given the input size 768 bytes, gas cost is computed from the above web page.
+                let success := staticcall(181000, 8, _pPairing, 768, _pPairing, 0x20)
 
                 isOk := and(success, mload(_pPairing))
             }