--optimize should not trigger compilation if no optimized output is requested

[cameel]

Abstract

Currently just using the --optimize option alone triggers optimization, even if the user does not request any outputs which would make the effect of optimization actually observable, like --bin or --ir-optimized. The compiler should not run the optimizer in such cases.

Motivation

The output of solc --optimize --ir is identical to solc --ir and it could be just as fast, but instead it takes as much time as solc --optimize --ir-optimized.

The reason it matters is two-stage compilation, which is a mechanism we're soon going to be recommending as a way to parallelize compilation. Passing in the --optimize flag to the first stage is necessary to get correct metadata in cases where the optimization will only be performed by the second stage. For example, if the output of solc --ir is passed as input to solc --strict-assembly --optimize --bin, the metadata will say that the contract was not optimized. To get correct metadata the first command must be changed to solc --optimize --ir but currently this results in both stages performing the optimization, which doubles the compilation time and defeats the purpose.

Steps to reproduce

This can be easily observed when compiling a contract like chains.sol, where optimization takes a lot of time:

time solc test/benchmarks/chains.sol --ir

real    0m0.272s
user    0m0.232s
sys     0m0.020s

time solc test/benchmarks/chains.sol --ir --optimize

real    0m22.785s
user    0m22.616s
sys     0m0.070s

time solc test/benchmarks/chains.sol --ir --optimize --ir-optimized

real    0m22.515s
user    0m22.346s
sys     0m0.043s

Implementation notes

I think that --ir is actually the only output which exposes unoptimized artifacts (though we should check to make sure). If that's the case, internally it would be enough to extend the mechanism triggering enableIRGeneration() to have separate flags for optimized and unoptimized IR.

Otherwise, we might consider splitting the "compilation successful" state into into distinct "codegen successful" and "optimization successful" states in CompilerStack::State.