[stdlib][SR-4818] Add overflow assignment operators #15144

rockbruno · 2018-03-10T17:30:54Z

This adds the overflow assignment operators (&+= &-= &*=) that were missing from integer types.

To match how other operators are written, I moved the regular overflow operator's implementation to the new assignment operators. I am unsure if this will have performance implications, so please let me know if this is a bad idea! I also removed the trailing white space from it's declaration since the other operators did not have one.

Resolves SR-4818.
Resolves: rdar://problem/32035586

CodaFi · 2018-03-10T17:44:51Z

This seems like it requires an evolution proposal.

@airspeedswift

rockbruno · 2018-03-10T18:16:46Z

I wrote a pitch some days ago and my overall feeling from the comments was that their absence was merely a bug. But If you guys think it will be better if this takes the proposal route, I'll gladly arrange one

xwu

Glad you've gone ahead and looked into making the change. Some minor comments. I do suspect we'll probably need a Swift Evolution proposal, but it shouldn't be controversial.

xwu · 2018-03-10T18:47:44Z

stdlib/public/core/Integers.swift.gyb

@@ -2740,8 +2797,18 @@ ${unsafeOperationComment(x.operator)}
 ${operatorComment('&' + x.operator, True)}
  @_inlineable // FIXME(sil-serialize-all)
  @_transparent
-  public static func &${x.operator} (lhs: Self, rhs: Self) -> Self {
-    return lhs.${x.name}ReportingOverflow(${callLabel}rhs).partialValue


These implementations can be left alone. No need to have the compiler do more work than is necessary to arrive at the same result.

xwu · 2018-03-10T18:48:06Z

stdlib/public/core/Integers.swift.gyb

+  @_transparent
+  public static func &${x.operator}=(lhs: inout Self, rhs: Self) {
+    let result = lhs.${x.name}ReportingOverflow(${callLabel}rhs).partialValue
+    lhs = result


This can be a one-liner; no need for the temporary result.

xwu · 2018-03-10T18:49:13Z

utils/SwiftIntTypes.py

@@ -122,7 +122,7 @@ def all_integer_or_real_binary_operator_names():


 def all_integer_assignment_operator_names():
-    return ['%=', '<<=', '>>=', '&=', '^=', '|=']
+    return ['%=', '<<=', '>>=', '&*=', '&=', '&+=', '&-=', '^=', '|=']


I don't think these changes are required here. For example, &<<= isn't present.

xwu · 2018-03-10T18:51:16Z

stdlib/public/core/Integers.swift.gyb

@@ -588,6 +588,63 @@ def assignmentOperatorComment(operator, fixedWidth):
  /// - Parameters:
  ///   - lhs: The value to divide.
  ///   - rhs: The value to divide `lhs` by. `rhs` must not be zero.
+""",
+        '&+': """\
+  /// Stores the sum of the two given values in the left-hand-size variable, discarding any overflow.


Please wrap to 80 columns and follow the style of the documentation above; that is, "Adds the two values and stores..."

xwu · 2018-03-10T18:53:37Z

stdlib/public/core/Integers.swift.gyb

+        '&+': """\
+  /// Stores the sum of the two given values in the left-hand-size variable, discarding any overflow.
+  ///
+  /// The masking addition operator (`&+`) silently discards any overflow that


Please modify this text so that it talks about the assignment operator you're documenting.

rockbruno · 2018-03-10T19:42:36Z

Very well then. I'll arrange a formal proposal soon.
Thanks guys!

…their documentation

nuclearace · 2018-03-12T12:17:09Z

@rockbruno Per Ben Cohen this might not need a formal review. I would email the core team list to get a definite answer though.

natecook1000 · 2018-03-15T19:40:51Z

Was going to ask that you add tests for these alongside the tests for the non-assignment masking operators, but can't find those anywhere. Maybe validation-test/stdlib/FixedPointArithmeticTraps.swift.gyb, since that's where we're testing the ...ReportingOverflow methods. @moiseev?

rockbruno · 2018-03-15T21:19:26Z

@natecook1000 When I first implemented this I located operator tests in validation-test/stdlib/NumericDiagnostics.swift.gyb (which is why I had added the assignment operators in utils/SwiftIntTypes.py in the original commit), but as far as I could understand the boilerplate, it seemed to be more about integer types than the operators themselves which confused me a bit

If you guys point me in the right direction, I'll gladly write proper tests for them!

(By the way, I sent an e-mail to the core team asking about the review-needs for this feature.)

airspeedswift · 2018-03-15T23:31:05Z

Discussed this amongst the core team and the agreement is the omission of these was an oversight and there's no need to go through a review to add them. Thanks!

moiseev · 2018-03-16T00:45:03Z

@natecook1000, @rockbruno https://github.com/apple/swift/blob/master/test/stdlib/Integers.swift.gyb looks OK for the tests, does it not?

bruno-rocha-movile · 2018-03-16T22:39:34Z

@moiseev That looks like a good place! But since it's my first time doing this, before I pushed anything I wanted to ask: What should be the test coverage for a feature like this?

Looking at the tests from this file, it seems like this could be written in two ways:

Through a single simple arithmetic test, like:

tests.test("Simple-Masked-Overflow-Arithmetic") {
  expectEqual(Int8.max &+ 1, Int8.min)
  var x = Int8.max
  x &+= 1
  expectEqual(x, Int8.min)
  //and so on for other operators
}

By multiple tests that bounds check each integer type, like it's already happening with the regular operators in this class:

tests.test("${Type}/Add/Overflow") {
  func f(_ x: ${Type}) -> ${Type} {
    return x + 1
  }
  expectCrashLater()
  _ = f(${Type}.max)
} //but with masked operators instead

What's usually expected?

moiseev · 2018-03-19T18:54:25Z

Something like this should be OK:

tests.test("${Type}/MaskingAdd") {
  expectEqual(${Type}.max &+ 1, ${Type}.min)
  do {
    var x = ${Type}.max
    x &+= 1
    expectEqual(x, ${Type}.min)
  }
}

The trick with an f function used in your second snippet is there to avoid compile time overflow check, as in, if you write Int.max + 1 it will just not compile, but we do want to make sure that it would fail at runtime, hence the extra function call.

rockbruno · 2018-03-19T20:50:26Z

Thanks a lot @moiseev. I took the liberty of separating the operators into pairs of tests (non-assignment/assignment) to match the style of the previous tests in that file, but if that's not ideal just give me a heads up and I'll do them like in your example.

(There were no tests for *, so that one ended up in a single test.)

moiseev · 2018-03-19T21:16:36Z

test/stdlib/Integers.swift.gyb

+tests.test("${Type}/MaskingMultiply") {
+  var x = ${Type}.max
+  x &*= 2
+  expectEqual(x, ${Type}.max &* 2)


Since these operators are defined in terms of one another, it makes the test ... not testing anything =)
How about something like expectEquals(${Type}.min, (${Type}.max / 2 + 1) &* 2)?

Good catch, I couldn't think of a guaranteed single way to test multiplication overflow across all types 🙂

What about this?

tests.test("${Type}/Multiply/MaskingOverflow") { expectEqual((${Type}.max / 2 + 1) &* 2, ${Type}.min) } tests.test("${Type}/MultiplyInPlace/MaskingOverflow") { var x = ${Type}.max / 2 + 1 x &*= 2 expectEqual(x, ${Type}.min) }

in expectXXX functions, the expected value is the first parameter.

Sorry, I did not notice it in the first pass through the code. Can you please update the usages?

moiseev · 2018-03-19T22:28:37Z

@swift-ci Please smoke test

moiseev · 2018-03-19T23:06:13Z

Thanks @rockbruno for your contribution!

xwu · 2018-03-19T23:10:15Z

This is fantastic. Glad it's finally done. Thanks @rockbruno!

bruno-rocha-movile · 2018-03-19T23:20:11Z

Awesome. Thanks a lot guys!

[stdlib][SR-4818] Add overflow assignment operators

9750689

xwu reviewed Mar 10, 2018

View reviewed changes

CodaFi added the swift evolution pending discussion Flag → feature: A feature that has a Swift evolution proposal currently in review label Mar 10, 2018

[stdlib] Revert overflow operators implementation change and improve …

3b07420

…their documentation

moiseev removed the swift evolution pending discussion Flag → feature: A feature that has a Swift evolution proposal currently in review label Mar 16, 2018

airspeedswift assigned moiseev Mar 19, 2018

[stdlib] Add overflow operator tests

16d793a

moiseev reviewed Mar 19, 2018

View reviewed changes

[stdlib] Improve masked operator tests

08610df

moiseev merged commit d50f60c into swiftlang:master Mar 19, 2018

rockbruno deleted the overflow-assignment-2 branch August 31, 2018 22:30

rockbruno restored the overflow-assignment-2 branch August 31, 2018 22:30

swift-ci mentioned this pull request Mar 12, 2018

[SR-4818] We have masking shift-assignment operators, but not other masking assignment operators #47395

Closed

[stdlib][SR-4818] Add overflow assignment operators #15144

[stdlib][SR-4818] Add overflow assignment operators #15144

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