[swift-dev] deprecating -Ounchecked

[Erik Eckstein]

> On Nov 2, 2017, at 8:50 PM, Chris Lattner <clattner at nondot.org> wrote:
> 
> 
>> On Nov 2, 2017, at 9:52 AM, Erik Eckstein via swift-dev <swift-dev at swift.org> wrote:
>> 
>> Hi,
>> 
>> I’d like to propose to deprecate the -Ounchecked swift optimization mode.
>> 
>> The -Ounchecked mode actually contradicts one of the main goals of swift: to be a safe language.
>> In the past we didn’t see lot of significant performance differences compared to -O (there were some improvements but also some regressions).
>> Also, we want to reduce the effort of maintaining too many different optimization modes, especially because we recently added -Osize.
>> 
>> Deprecating would mean that we map -Ounchecked to -O.
>> 
>> If you have any comments or concerns, please let me know
> 
> What’s the motivation for this?  What problem does it solve?

There are several issues:
First, Ounchecked does not come for free. To really support this mode we would have to constantly track performance metrics for this, investigate performance/codesize regressions, etc. This is a lot of effort.

So if we replace Ounchecked with an option -unsafe-remove-checks (similar to -assume-single-threaded), as Johannes suggested, this is more like a “at your own risk” thing (regarding performance). For example, it might happen that users see perf regressions from one release to another, using this option.

The second thing is, IMO, that -Ounchecked suggests that it’s just another choice like -O. But I think we should encourage people to not use Ounchecked. I think it’s fine if experienced developers, who know what they are doing, use -unsafe-remove-checks, but we should not promote this mode in a prominent place like in the -O namespace. This is just my personal opinion and I’m sure some people have other opinions on this.

The third issue is that we now have -Osize. The -unsafe-remove-checks would be orthogonal to -O and -Osize.
BTW, we should rename -O to -Ospeed for consistency (keeping -O as an alias for -Ospeed).


> 
> Swift isn’t a "strict safety at all costs” language.  It is a pragmatic language which aims to give people tools to solve problems.  One of the nice things about -Ounchecked is that it provides an easy way to get a data point about the cost of runtime checks.
> 
> -Chris
> 

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