cmd/evm: Add --bench flag for benchmarking

[chfast]

The --bench flag uses the testing.B to execute the EVM bytecode many times and get the average exeuction time out of it.

[karalabe]

A suggestion that would make this a lot better. Instead of simply running once and measuring it (unstable), lets use Go's built in benchmark runner (testing.B). You can specify how much time a run should take and it will iteratively increase the loop counts to your benchmark takes that long. The idea being, that if you try to benchmark something taking milliseconds, a single measurement is useless, but Go's benchmarker will hone in on 1000 iterations (1 second), making outlier execution times irrelevant.

[chfast]

Yes, I was going this direction. I was considering two options:

1. Add --iterations N option and then run some many times.
2. Using benchmarking from testing. As you suggested this one, I will pick it up. Should I add --bench flag for it?

[chfast]

@karalabe, this is ready now for review.

[holiman]

This generally LGTM, but could you provide (on the PR description) some sample/example output from running it -- so we get a feel for how to use it and how the output looks?

[chfast]

The output is unchanged, just the evm execution time is more accurate, and other stats are disturbed because the EVM execution is done multiple times.

With --bench:

build/bin/evm --codefile ~/Projects/ethereum/evmone/test/benchmarks/sha1_shifts.evm --statdump --input 1605782b00000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000000 --bench run
evm execution time: 473.312µs
heap objects:       9490
allocations:        3158024
total allocations:  145871912
GC calls:           63
Gas used:           35234

Without:

build/bin/evm --codefile ~/Projects/ethereum/evmone/test/benchmarks/sha1_shifts.evm --statdump --input 1605782b00000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000000 run        
evm execution time: 667.701µs
heap objects:       5664
allocations:        1525080
total allocations:  3207648
GC calls:           1
Gas used:           35234

[chfast]

Oh, thanks :)