Nil coalescing operator exponential compile times

Originator:keithbsmiley
Number:rdar://23695200 Date Originated:30-Nov-2015 02:45 PM
Status:Open Resolved:
Product:Developer Tools Product Version:Swift 2.1
Classification:Other Bug Reproducible:Always
 
Summary:

When using Swift's nil coalescing operator, compile time seems to be
exponential based on the number of uses in a statement.

Steps to reproduce:

1. Attempt to compile the attached example

OR

1. Create a swift file with a single function that returns an optional
2. Chain ~10 calls to the function separated by `??`

Expected results:

The file is compiled in a reasonable amount of time

Actual results:

The compile takes an insane amount of time (on my 2014 rMPB the process
was killed after 59 minutes with 9 calls to the function). Seemingly
this compile time is exponential.

Comments

case where the issue doesn't happen

If the right hand side of ?? is non-optional, there is no compile time issue: 

    _ = f(f(f(f(f(f(f(f(f("")
        ?? "")
        ?? "")
        ?? "")
        ?? "")
        ?? "")
        ?? "")
        ?? "")
        ?? "")
        ?? ""

So the workaround is only needed when the right-hand side of the operator is an optional.

By adigitalknight at Dec. 5, 2016, 4:57 p.m. (reply...)

workaround

Issue is still happening a year later with Xcode 8.1 and Swift 3.

Assigning each intermediate value to a constant will workaround compile speed issue. 

    let a1 = f()
    let a2 = a1 ?? f()
    let a3 = a2 ?? f()
    let a4 = a3 ?? f()
    let a5 = a4 ?? f()
    let a6 = a5 ?? f()
    let a7 = a6 ?? f()
    let a8 = a7 ?? f()
    let a9 = a8 ?? f()
    let _ = a9 ?? f()
By adigitalknight at Dec. 5, 2016, 3:37 p.m. (reply...)

https://github.com/keith/radars/tree/master/NilCoalescingOperatorExponentialCompile

By keithbsmiley at Nov. 30, 2015, 10:47 p.m. (reply...)

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