SHA256 workaround

[hwwhww]

ethereum/consensus-specs#612

Since Vyper doesn't support built-in function call to call precompile sha256, it's a workaround to call call the precompile with raw_call() function.

Gas cost

(Updated: 2019/03/07: rebased)

The first 10 deposit gas cost with sha3

tests/contracts/test_deposit.py::test_deposit_tree
deposit transaction consumes 171466 gas
deposit transaction consumes 142334 gas
deposit transaction consumes 141476 gas
deposit transaction consumes 143202 gas
deposit transaction consumes 141476 gas
deposit transaction consumes 142344 gas
deposit transaction consumes 141486 gas
deposit transaction consumes 144070 gas
deposit transaction consumes 141476 gas
deposit transaction consumes 142344 gas

The first 10 deposit gas cost with this PR (SHA256)

tests/contracts/test_deposit.py::test_deposit_tree
deposit transaction consumes 225808 gas
deposit transaction consumes 198311 gas
deposit transaction consumes 195818 gas
deposit transaction consumes 200814 gas
deposit transaction consumes 195818 gas
deposit transaction consumes 198321 gas
deposit transaction consumes 195828 gas
deposit transaction consumes 203317 gas
deposit transaction consumes 195818 gas
deposit transaction consumes 198321 gas

Rough calculation

Gas price

GAS_CALL = 700

GAS_SHA256 = 60
GAS_SHA256WORD = 12

GAS_SHA3 = 30
GAS_SHA3WORD = 6

Merkliezing two leaves (32 bytes * 2) * 64 times + hashing deposit data (528 bytes) 1 time
= 64 * ((GAS_SHA256 + (64 // 32) * GAS_SHA256WORD) / 2.0 + GAS_CALL) + ((GAS_SHA256 + (528 // 32 + 1) * GAS_SHA256WORD) / 2.0 + GAS_CALL)
= 64 * ((60 + ceil(64 / 32) * 12) / 2.0 + 700) + ((60 + ceil(528 / 32) * 12) / 2.0 + 700)
= 48320


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[hwwhww]

closing via #19