Add -dead_strip / --gc-sections for release builds

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

Author: keith

Sources/Build/BuildPlan.swift

@@ -1200,6 +1200,21 @@ public final class ProductBuildDescription {
         return args.filter({ !invalidArguments.contains($0) })
     }
 
+    private var deadStripArguments: [String] {
+        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 +1261,14 @@ 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 +1279,7 @@ 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.

Tests/BuildTests/BuildPlanTests.swift

@@ -466,7 +466,7 @@ 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",
             "-Xlinker", "-rpath", "-Xlinker", "/fake/path/lib/swift-5.5/macosx",
@@ -1029,7 +1029,7 @@ 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"),