Support gas prices pre-EIP 150

[mratsim]

Some tests are failing because the environment is pre-EIP 150 which increased a lot of gas prices.

For example SLOAD opcode changed from 50 to 200 and impacts this test:

• expXY_success.json

Decompiling the code yields

import ../src/vm/code_stream, ../src/opcode_values, strformat

var c = newCodeStreamFromUnescaped("0x6000356000556020356001556001546000540a600255")

let opcodes = c.decompile()
for op in opcodes:
  echo &"[{op[0]}]\t{op[1]}\t{op[2]}"

# [1]     PUSH1   0x00
# [2]     CALLDATALOAD
# [4]     PUSH1   0x00
# [5]     SSTORE
# [7]     PUSH1   0x20
# [8]     CALLDATALOAD
# [10]    PUSH1   0x01
# [11]    SSTORE
# [13]    PUSH1   0x01
# [14]    SLOAD
# [16]    PUSH1   0x00
# [17]    SLOAD
# [18]    EXP
# [20]    PUSH1   0x02
# [21]    SSTORE
# [-1]    STOP

Expected gas balance is 39853, current resulting gas balance was 39553

Other almost certainly impacted (gas not matching in logs) includes:

• expXY
• blockhash257Block
• blockhashInRange
• coinbase
• blockhashOutOfRange
• blockhashMyBlock
• blockhashNotExistingBlock
• blockhash258Block
• smod5 (out of gas error)
• smod7 (out of gas error)
• signextend_Overflow_dj42

[mratsim]

Note that the current constants.nim was probably translated from py-evm while py-evm was also undergoing gas refactoring or many constants were introduced temporarily pending further refactoring:

The current py-evm does not have the following:

• GAS_SLOAD (200 post-EIP150) vs GAS_SLOAD_COST (20 pre-EIP150)
• GAS_EXT_CODE (20 pre-EIP150) vs GAS_EXT_CODE_COST (700 post-EIP150)
• GAS_BALANCE (20 pre-EIP150) vs GAS_COST_BALANCE (400 post-EIP150).
• ...

Py-EVM defaults to Genesis costs and then each VM forks load and amend the relevant constants.