Add -dead_strip / --gc-sections for release builds (#4135)

motivation: In the past it was unsafe to pass -dead_strip because there was some
swift metadata that could be stripped. It seems that at this point all
of those cases are either fixed or marked as @llvm.used, so passing
these flags should safely reduce binary size in some cases. If there are
other cases where this isn't safe we should likely annotate them as
@llvm.used anyways.

Related: https://bugs.swift.org/browse/SR-521
#215

changes: 
* Add -dead_strip / --gc-sections for release builds
* Add --disable-dead-strip flag
* Add CHANGELOG entry

Author: keith

CHANGELOG.md

@@ -11,6 +11,10 @@ Swift 5.7
 
   Update to manifest API to make it impossible to create an invalid build setttings condition.
 
+* [#4135]
+
+  Enable linker dead stripping for all platforms. This can be disabled with `--disable-dead-strip`
+
 Swift 5.6
 -----------
 * [SE-0332]
@@ -222,4 +226,5 @@ Swift 3.0
 [#3942]: https://github.com/apple/swift-package-manager/pull/3942
 [#4119]: https://github.com/apple/swift-package-manager/pull/4119
 [#4131]: https://github.com/apple/swift-package-manager/pull/4131
+[#4135]: https://github.com/apple/swift-package-manager/pull/4135
 

Sources/Build/BuildPlan.swift

@@ -1200,6 +1200,25 @@ public final class ProductBuildDescription {
         return args.filter({ !invalidArguments.contains($0) })
     }
 
+    private var deadStripArguments: [String] {
+        if !buildParameters.linkerDeadStrip {
+            return []
+        }
+
+        switch buildParameters.configuration {
+        case .debug:
+            return []
+        case .release:
+            if buildParameters.triple.isDarwin() {
+                return ["-Xlinker", "-dead_strip"]
+            } else if buildParameters.triple.isWindows() {
+                return ["-Xlinker", "/OPT:REF"]
+            } else {
+                return ["-Xlinker", "--gc-sections"]
+            }
+        }
+    }
+
     /// The arguments to link and create this product.
     public func linkArguments() throws -> [String] {
         var args = [buildParameters.toolchain.swiftCompiler.pathString]
@@ -1246,12 +1265,14 @@ public final class ProductBuildDescription {
             case .manifest:
                 args += ["-emit-executable"]
             }
+            args += deadStripArguments
         case .library(.dynamic):
             args += ["-emit-library"]
             if buildParameters.triple.isDarwin() {
                 let relativePath = "@rpath/\(buildParameters.binaryRelativePath(for: product).pathString)"
                 args += ["-Xlinker", "-install_name", "-Xlinker", relativePath]
             }
+            args += deadStripArguments
         case .executable, .snippet:
             // Link the Swift stdlib statically, if requested.
             if buildParameters.shouldLinkStaticSwiftStdlib {
@@ -1262,6 +1283,7 @@ public final class ProductBuildDescription {
                 }
             }
             args += ["-emit-executable"]
+            args += deadStripArguments
 
             // If we're linking an executable whose main module is implemented in Swift,
             // we rename the `_<modulename>_main` entry point symbol to `_main` again.

Sources/Commands/Options.swift

@@ -375,6 +375,12 @@ public struct SwiftToolOptions: ParsableArguments {
     @Option(name: .customLong("resolver-fingerprint-checking"))
     var resolverFingerprintCheckingMode: FingerprintCheckingMode = .warn
 
+    @Flag(
+        name: .customLong("dead-strip"),
+        inversion: .prefixedEnableDisable,
+        help: "Disable/enable dead code stripping by the linker")
+    var linkerDeadStrip: Bool = true
+
     @Flag(name: .customLong("netrc"), help: .hidden)
     var _deprecated_netrc: Bool = false
 

Sources/Commands/SwiftTool.swift

@@ -842,6 +842,7 @@ public class SwiftTool {
                 isXcodeBuildSystemEnabled: options.buildSystem == .xcode,
                 printManifestGraphviz: options.printManifestGraphviz,
                 forceTestDiscovery: options.enableTestDiscovery, // backwards compatibility, remove with --enable-test-discovery
+                linkerDeadStrip: options.linkerDeadStrip,
                 isTTY: isTTY
             )
         })

Sources/SPMBuildCore/BuildParameters.swift

@@ -172,6 +172,9 @@ public struct BuildParameters: Encodable {
     // What strategy to use to discover tests
     public var testDiscoveryStrategy: TestDiscoveryStrategy
 
+    /// Whether to disable dead code stripping by the linker
+    public var linkerDeadStrip: Bool
+
     public var isTTY: Bool
 
     public init(
@@ -200,6 +203,7 @@ public struct BuildParameters: Encodable {
         printManifestGraphviz: Bool = false,
         enableTestability: Bool? = nil,
         forceTestDiscovery: Bool = false,
+        linkerDeadStrip: Bool = true,
         isTTY: Bool = false
     ) {
         let triple = destinationTriple ?? .getHostTriple(usingSwiftCompiler: toolchain.swiftCompiler)
@@ -237,6 +241,7 @@ public struct BuildParameters: Encodable {
         self.enableTestability = enableTestability ?? (.debug == configuration)
         // decide if to enable the use of test manifests based on platform. this is likely to change in the future
         self.testDiscoveryStrategy = triple.isDarwin() ? .objectiveC : .manifest(generate: forceTestDiscovery)
+        self.linkerDeadStrip = linkerDeadStrip
         self.isTTY = isTTY
     }
 

Tests/BuildTests/BuildPlanTests.swift

@@ -69,7 +69,8 @@ final class BuildPlanTests: XCTestCase {
         canRenameEntrypointFunctionName: Bool = false,
         destinationTriple: TSCUtility.Triple = hostTriple,
         indexStoreMode: BuildParameters.IndexStoreMode = .off,
-        useExplicitModuleBuild: Bool = false
+        useExplicitModuleBuild: Bool = false,
+        linkerDeadStrip: Bool = true
     ) -> BuildParameters {
         return BuildParameters(
             dataPath: buildPath,
@@ -82,7 +83,8 @@ final class BuildPlanTests: XCTestCase {
             shouldLinkStaticSwiftStdlib: shouldLinkStaticSwiftStdlib,
             canRenameEntrypointFunctionName: canRenameEntrypointFunctionName,
             indexStoreMode: indexStoreMode,
-            useExplicitModuleBuild: useExplicitModuleBuild
+            useExplicitModuleBuild: useExplicitModuleBuild,
+            linkerDeadStrip: linkerDeadStrip
         )
     }
 
@@ -462,6 +464,60 @@ final class BuildPlanTests: XCTestCase {
         let exe = try result.target(for: "exe").swiftTarget().compileArguments()
         XCTAssertMatch(exe, ["-swift-version", "4", "-O", "-g", .equal(j), "-DSWIFT_PACKAGE", "-module-cache-path", "/path/to/build/release/ModuleCache", .anySequence])
 
+      #if os(macOS)
+        XCTAssertEqual(try result.buildProduct(for: "exe").linkArguments(), [
+            "/fake/path/to/swiftc", "-g", "-L", "/path/to/build/release",
+            "-o", "/path/to/build/release/exe", "-module-name", "exe", "-emit-executable",
+            "-Xlinker", "-dead_strip", "-Xlinker", "-rpath", "-Xlinker", "@loader_path",
+            "@/path/to/build/release/exe.product/Objects.LinkFileList",
+            "-Xlinker", "-rpath", "-Xlinker", "/fake/path/lib/swift/macosx",
+            "-Xlinker", "-rpath", "-Xlinker", "/fake/path/lib/swift-5.5/macosx",
+            "-target", defaultTargetTriple,
+        ])
+      #else
+        XCTAssertEqual(try result.buildProduct(for: "exe").linkArguments(), [
+            "/fake/path/to/swiftc", "-g", "-L", "/path/to/build/release",
+            "-o", "/path/to/build/release/exe", "-module-name", "exe", "-emit-executable",
+            "-Xlinker", "--gc-sections", "-Xlinker", "-rpath=$ORIGIN",
+            "@/path/to/build/release/exe.product/Objects.LinkFileList",
+            "-target", defaultTargetTriple,
+        ])
+      #endif
+    }
+
+    func testBasicReleasePackageNoDeadStrip() throws {
+        let fs = InMemoryFileSystem(emptyFiles:
+            "/Pkg/Sources/exe/main.swift"
+        )
+
+        let observability = ObservabilitySystem.makeForTesting()
+        let graph = try loadPackageGraph(
+            fs: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    name: "Pkg",
+                    path: .init("/Pkg"),
+                    targets: [
+                        TargetDescription(name: "exe", dependencies: []),
+                    ]),
+            ],
+            observabilityScope: observability.topScope
+        )
+        XCTAssertNoDiagnostics(observability.diagnostics)
+
+        let result = try BuildPlanResult(plan: BuildPlan(
+            buildParameters: mockBuildParameters(config: .release, linkerDeadStrip: false),
+            graph: graph,
+            fileSystem: fs,
+            observabilityScope: observability.topScope
+        ))
+
+        result.checkProductsCount(1)
+        result.checkTargetsCount(1)
+
+        let exe = try result.target(for: "exe").swiftTarget().compileArguments()
+        XCTAssertMatch(exe, ["-swift-version", "4", "-O", "-g", .equal(j), "-DSWIFT_PACKAGE", "-module-cache-path", "/path/to/build/release/ModuleCache", .anySequence])
+
       #if os(macOS)
         XCTAssertEqual(try result.buildProduct(for: "exe").linkArguments(), [
             "/fake/path/to/swiftc", "-g", "-L", "/path/to/build/release",
@@ -1029,7 +1085,7 @@ final class BuildPlanTests: XCTestCase {
         XCTAssertEqual(try result.buildProduct(for: "exe").linkArguments(), [
             "/fake/path/to/swiftc", "-g", "-L", "/path/to/build/release",
             "-o", "/path/to/build/release/exe", "-module-name", "exe", "-emit-executable",
-            "-Xlinker", "-rpath", "-Xlinker", "@loader_path",
+            "-Xlinker", "-dead_strip", "-Xlinker", "-rpath", "-Xlinker", "@loader_path",
             "@/path/to/build/release/exe.product/Objects.LinkFileList",
             "-Xlinker", "-rpath", "-Xlinker", "/fake/path/lib/swift/macosx",
             "-target", hostTriple.tripleString(forPlatformVersion: "12.0"),