core/vm: hoist jumptable nil-check out of the interpreter loop

[omerfirmak]

simple enough. benchmark of a dummy EVM loop shows ~%2 speed up on my machine.

[fjl]

I wonder why it's a pointer at all

[omerfirmak]

I guess to avoid 2KB copies on each interpreter creation.

[fjl]

Please post benchstat comparison.

[omerfirmak]

goos: linux
goarch: amd64
pkg: github.com/ethereum/go-ethereum/core/vm
cpu: AMD Ryzen 7 5700U with Radeon Graphics         
        │   old.txt   │              new.txt               │
        │   sec/op    │   sec/op     vs base               │
Loop-16   193.9n ± 4%   184.3n ± 2%  -4.95% (p=0.000 n=10)

        │  old.txt   │            new.txt             │
        │    B/op    │    B/op     vs base            │
Loop-16   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

        │  old.txt   │            new.txt             │
        │ allocs/op  │ allocs/op   vs base            │
Loop-16   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

"op" here is a 100 gas worth of execution.