core/vm: avoid unnecessary stack memory copies during gas calc

[karalabe]

This PR removes two batches of unnecessary memory allocations and copies with regard to EVM stack and gas calculation:

• Whenever a CALL* opcode is executed, the gas allowance on the EVM stack is replaced according to the 63/64 fork rule. I don't see a reason to allocate a new big.Int for the new stack item. If we're overwriting anyway, we can overwrite the existing content.
• Due to modifying the stack for CALL* opcodes, the EVM tracer currently copied the entire stack for every single opcode. This is insanely wasteful since we're only changing 1 value very rarely, and the tracer copied all values all the time to handle it. The PR contains a very very selective logic for saving the original stack item only if it will actually change. This saves us about 27% of tracing time.

[fjl]

The copy was introduced for trace compatibility in #15035. I already complained there that this is a super annoying hack. This PR makes the hack more complicated.

The real solution is to add a struct field, e.g. callGas, that will hold the gas value calculated according to the 63/64 rule. This would eliminate the need to track this value on the stack. The copy could then be removed entirely.

[holiman]

The real solution is to add a struct field, e.g. callGas that will hold the gas value calculate according to the 63/64 rule.

I agree, but I also thinks that this change -- which only touches code within in.cfg.Debug- scopes -- is a lot less critical...

[fjl]

#15563 implements my suggestion.

[karalabe]

Closing in favor of #15563.