Which assertions are retained with wholemodule optimization mode?

Question

All explanations of Swift assertions I can find explain that assert* gets thrown out with -O but precondition* are kept unless -Ounchecked is set.

However, we now have whole-module optimization -wmo as new default for release builds.

Are precondition* assertions retained when whole-module optimization is enabled?

Answer 1

Consider this small program:

func f(_ i: Int) -> Int {
    assert(i > 0, "needed positive number")
    return i
}

print(f(0))

Compiling it with xcrun swiftc [opt] and running the result gives:

• -Onone: assertion error
• -Onone -wmo: assertion error
• -O: prints 0
• -O -wmo: prints 0

And in comparison:

func f(_ i: Int) -> Int {
    precondition(i > 0, "needed positive number")
    return i
}

print(f(0))

• -Onone: precondition error
• -Onone -wmo: precondition error
• -O: Illegal instruction: 4
• -O -wmo: Illegal instruction: 4

I can't quite explain the last two results, but it seems clear that whole-module optimization does not influence how assertions are handled; only the optimization level matters.

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This is on

Apple Swift version 3.0.1 (swiftlang-800.0.58.6 clang-800.0.42.1)
Target: x86_64-apple-macosx10.9