[ddulaney]

The general cost over a several large codebases has been observed to be minimal. Yet, there are specific scenarios where the costs are real and observable. For those rare cases, an explicit opt-in to risky behavior makes sense.

[shultays]

  The general cost over a several large codebases has been observed to be minimal

Is this unexpected? A large code base has a lot of other things and it is normal that such changes will be a rounding error. There are lots of other bottlenecks that will just overwhelm such a such change. I don't think "it is not affecting large code bases as much", you can use that argument for pretty much anything that adds an overhead

Not to mention if you change every int a to int a=0 right now, in those code bases, a=0 part will likely to be optimized away since that value is not being (shouldn't be) used at all and likely will be overwritten in all code paths