Create Pack Specialisation pass

Author: aidan-hall

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/CMakeLists.txt

@@ -38,4 +38,5 @@ swift_compiler_sources(Optimizer
   StripObjectHeaders.swift
   TempLValueElimination.swift
   TempRValueElimination.swift
+  PackSpecialization.swift
 )

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/PackSpecialization.swift

@@ -108,6 +108,7 @@ private func registerSwiftPasses() {
   registerPass(closureSpecialization, { closureSpecialization.run($0) })
   registerPass(autodiffClosureSpecialization, { autodiffClosureSpecialization.run($0) })
   registerPass(loopInvariantCodeMotionPass, { loopInvariantCodeMotionPass.run($0) })
+  registerPass(packSpecialization, { packSpecialization.run($0) })
 
   // Instruction passes
   registerForSILCombine(BeginBorrowInst.self,      { run(BeginBorrowInst.self, $0) })

SwiftCompilerSources/Sources/Optimizer/PassManager/PassRegistration.swift

@@ -139,6 +139,7 @@ set(SWIFT_BENCH_MODULES
     single-source/ObserverUnappliedMethod
     single-source/OpaqueConsumingUsers
     single-source/OpenClose
+    single-source/ParameterPacks
     single-source/Phonebook
     single-source/PointerArithmetics
     single-source/PolymorphicCalls

benchmark/CMakeLists.txt

@@ -0,0 +1,53 @@
+//===--- ParameterPacks.swift ---------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+// This benchmark serves to test the overhead of
+
+import TestsUtils
+
+public let benchmarks = [
+  BenchmarkInfo(
+    name: "ParameterPacks.VariadicFibonacci",
+    runFunction: run_ParameterPacksVariadicFibonacci,
+    tags: [.validation])
+]
+
+@inline(never)
+func numericLoop<each T: BinaryInteger>(xs: repeat each T) -> (repeat each T) {
+  return
+    (repeat { x in
+      // Recursive "Fibonacci" function prevents inlining
+      if x <= 1 {
+        return x
+      } else {
+        let (a, b) = numericLoop(xs: x - 1, x - 2)
+        return a + b
+      }
+    }(each xs)
+
+  )
+}
+
+let expectedResult = (610, 987, 1597)
+
+@inline(never)
+public func run_ParameterPacksVariadicFibonacci(_ n: Int) {
+  var result = (0, 0, 0)
+  for _ in 1...n {
+    result = numericLoop(xs: 15, 16, 17)
+    if result != expectedResult {
+      break
+    }
+  }
+
+  check(result == expectedResult)
+}

benchmark/single-source/ParameterPacks.swift

@@ -143,6 +143,7 @@ import ObserverPartiallyAppliedMethod
 import ObserverUnappliedMethod
 import OpaqueConsumingUsers
 import OpenClose
+import ParameterPacks
 import Phonebook
 import PointerArithmetics
 import PolymorphicCalls
@@ -343,6 +344,7 @@ register(ObserverPartiallyAppliedMethod.benchmarks)
 register(ObserverUnappliedMethod.benchmarks)
 register(OpaqueConsumingUsers.benchmarks)
 register(OpenClose.benchmarks)
+register(ParameterPacks.benchmarks)
 register(Phonebook.benchmarks)
 register(PointerArithmetics.benchmarks)
 register(PolymorphicCalls.benchmarks)

benchmark/utils/main.swift

@@ -1360,6 +1360,7 @@ Some kinds need arguments, which precede ``Tf``.
   PASSID ::= '5'                             // GenericSpecializer,
   PASSID ::= '6'                             // MoveDiagnosticInOutToOut,
   PASSID ::= '7'                             // AsyncDemotion,
+  PASSID ::= '8'                             // PackSpecialization,
 
   FRAGILE ::= 'q'
 

docs/ABI/Mangling.rst

@@ -152,6 +152,8 @@ PASS(TempLValueElimination, "temp-lvalue-elimination",
      "Remove short-lived immutable temporary l-values")
 PASS(LoopInvariantCodeMotion, "loop-invariant-code-motion",
      "New Loop Invariant Code Motion")
+PASS(PackSpecialization, "pack-specialization",
+     "Specialize uses of parameter packs")
 
 PASS(ClosureSpecialization, "closure-specialization",
      "Specialize functions with closure arguments")

include/swift/SILOptimizer/PassManager/Passes.def

@@ -525,6 +525,7 @@ void addFunctionPasses(SILPassPipelinePlan &P,
   // of embedded Swift.
   if (!P.getOptions().EmbeddedSwift) {
     P.addGenericSpecializer();
+    P.addPackSpecialization();
     // Run devirtualizer after the specializer, because many
     // class_method/witness_method instructions may use concrete types now.
     P.addDevirtualizer();