[stdlib] Resolving some FIXME comments on Set type. (#20631)

* Fixing some fixmes on stdlib Set

* Adding @inline attr

* Fixing spaces

* Adding isEmpty as fast path in other places where is possible.

* Quotes on variable name on comment.

* Update stdlib/public/core/Set.swift

Co-Authored-By: LucianoPAlmeida <passos.luciano@outlook.com>

* Adding benchmark for isDisjoint Set method

* Adding empty sets to benchmark

* Fixing the factor on benchmarks and naming warnings for empty 5 words

benchmark/single-source/SetTests.swift

@@ -16,6 +16,10 @@ let size = 400
 let half = size / 2
 let quarter = size / 4
 
+// Construction of empty sets.
+let setE: Set<Int> = []
+let setOE: Set<Box<Int>> = []
+
 // Construction kit for sets with 25% overlap
 let setAB = Set(0 ..< size)                              //   0 ..< 400
 let setCD = Set(size ..< 2 * size)                       // 400 ..< 800
@@ -53,6 +57,11 @@ let setQ: Set<Int> = {
 
 public let SetTests = [
   // Mnemonic: number after name is percentage of common elements in input sets.
+  BenchmarkInfo(
+    name: "Set.Empty.IsSubsetInt0",
+    runFunction: { n in run_SetIsSubsetInt(setE, setAB, true, 5000 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setE, setAB]) }),
   BenchmarkInfo(
     name: "SetIsSubsetInt0",
     runFunction: { n in run_SetIsSubsetInt(setAB, setCD, false, 5000 * n) },
@@ -84,6 +93,47 @@ public let SetTests = [
     tags: [.validation, .api, .Set],
     setUpFunction: { blackHole([setP, setQ]) }),
 
+  BenchmarkInfo(
+    name: "Set.Empty.IsDisjointInt0",
+    runFunction: { n in run_SetIsDisjointInt(setE, setAB, true, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setE, setAB]) }),
+  BenchmarkInfo(
+    name: "Set.Empty.IsDisjointBox0",
+    runFunction: { n in run_SetIsDisjointBox(setOE, setOAB, true, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setOE, setOAB]) }),
+  BenchmarkInfo(
+    name: "SetIsDisjointInt0",
+    runFunction: { n in run_SetIsDisjointInt(setAB, setCD, true, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setAB, setCD]) }),
+  BenchmarkInfo(
+    name: "SetIsDisjointBox0",
+    runFunction: { n in run_SetIsDisjointBox(setOAB, setOCD, true, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setOAB, setOCD]) }),
+  BenchmarkInfo(
+    name: "SetIsDisjointInt25",
+    runFunction: { n in run_SetIsDisjointInt(setB, setAB, false, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setB, setAB]) }),
+  BenchmarkInfo(
+    name: "SetIsDisjointBox25",
+    runFunction: { n in run_SetIsDisjointBox(setOB, setOAB, false, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setOB, setOAB]) }),
+  BenchmarkInfo(
+    name: "SetIsDisjointInt50",
+    runFunction: { n in run_SetIsDisjointInt(setY, setXY, false, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setY, setXY]) }),
+  BenchmarkInfo(
+    name: "SetIsDisjointInt100",
+    runFunction: { n in run_SetIsDisjointInt(setP, setQ, false, 50 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setP, setQ]) }),
+  
   BenchmarkInfo(
     name: "SetSymmetricDifferenceInt0",
     runFunction: { n in run_SetSymmetricDifferenceInt(setAB, setCD, countABCD, 10 * n) },
@@ -177,6 +227,16 @@ public let SetTests = [
     tags: [.validation, .api, .Set],
     setUpFunction: { blackHole([setP, setQ]) }),
 
+  BenchmarkInfo(
+    name: "Set.Empty.SubtractingInt0",
+    runFunction: { n in run_SetSubtractingInt(setE, setAB, 0, 10 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setE, setAB]) }),
+  BenchmarkInfo(
+    name: "Set.Empty.SubtractingBox0",
+    runFunction: { n in run_SetSubtractingBox(setOE, setOAB, 0, 10 * n) },
+    tags: [.validation, .api, .Set],
+    setUpFunction: { blackHole([setOE, setOAB]) }),
   BenchmarkInfo(
     name: "SetSubtractingInt0",
     runFunction: { n in run_SetSubtractingInt(setAB, setCD, countAB, 10 * n) },
@@ -296,6 +356,18 @@ public func run_SetSubtractingInt(
   }
 }
 
+@inline(never)
+public func run_SetIsDisjointInt(
+    _ a: Set<Int>,
+    _ b: Set<Int>,
+    _ r: Bool,
+    _ n: Int) {
+    for _ in 0 ..< n {
+        let isDisjoint = a.isDisjoint(with: identity(b))
+        CheckResults(isDisjoint == r)
+    }
+}
+
 class Box<T : Hashable> : Hashable {
   var value: T
 
@@ -371,3 +443,15 @@ func run_SetSubtractingBox(
     CheckResults(and.count == r)
   }
 }
+
+@inline(never)
+func run_SetIsDisjointBox(
+    _ a: Set<Box<Int>>,
+    _ b: Set<Box<Int>>,
+    _ r: Bool,
+    _ n: Int) {
+    for _ in 0 ..< n {
+        let isDisjoint = a.isDisjoint(with: identity(b))
+        CheckResults(isDisjoint == r)
+    }
+}

stdlib/public/core/Set.swift

@@ -712,7 +712,8 @@ extension Set: SetAlgebra {
   @inlinable
   public func isSubset<S: Sequence>(of possibleSuperset: S) -> Bool
   where S.Element == Element {
-    // FIXME(performance): isEmpty fast path, here and elsewhere.
+    guard !isEmpty else { return true }
+    
     let other = Set(possibleSuperset)
     return isSubset(of: other)
   }
@@ -763,10 +764,12 @@ extension Set: SetAlgebra {
   @inlinable
   public func isSuperset<S: Sequence>(of possibleSubset: __owned S) -> Bool
     where S.Element == Element {
-    // FIXME(performance): Don't build a set; just ask if every element is in
-    // `self`.
-    let other = Set(possibleSubset)
-    return other.isSubset(of: self)
+    for member in possibleSubset {
+      if !contains(member) {
+        return false
+      }
+    }
+    return true
   }
 
   /// Returns a Boolean value that indicates whether the set is a strict
@@ -811,9 +814,7 @@ extension Set: SetAlgebra {
   @inlinable
   public func isDisjoint<S: Sequence>(with other: S) -> Bool
   where S.Element == Element {
-    // FIXME(performance): Don't need to build a set.
-    let otherSet = Set(other)
-    return isDisjoint(with: otherSet)
+    return _isDisjoint(with: other)
   }
 
   /// Returns a new set with the elements of both this set and the given
@@ -920,6 +921,10 @@ extension Set: SetAlgebra {
   @inlinable
   internal mutating func _subtract<S: Sequence>(_ other: S)
   where S.Element == Element {
+    // If self is empty we don't need to iterate over `other` because there's
+    // nothing to remove on self.
+    guard !isEmpty else { return }
+
     for item in other {
       remove(item)
     }
@@ -1121,8 +1126,16 @@ extension Set {
   ///   otherwise, `false`.
   @inlinable
   public func isDisjoint(with other: Set<Element>) -> Bool {
-    for member in self {
-      if other.contains(member) {
+    return _isDisjoint(with: other)
+  }
+    
+  @inlinable
+  internal func _isDisjoint<S: Sequence>(with other: S) -> Bool
+  where S.Element == Element {
+    guard !isEmpty else { return true }
+
+    for member in other {
+      if contains(member) {
         return false
       }
     }