Add a -experimental-hermetic-seal-at-link flag that triggers aggressive LTO-based dead-stripping (VFE, WME, conditional runtime records, internalization) (#39793)

Author: kubamracek

include/swift/AST/DiagnosticsDriver.def

@@ -166,6 +166,14 @@ ERROR(error_darwin_only_supports_libcxx, none,
       "The only C++ standard library supported on Apple platforms is libc++",
       ())
 
+ERROR(error_hermetic_seal_cannot_have_library_evolution, none,
+      "Cannot use -experimental-hermetic-seal-at-link with -enable-library-evolution",
+      ())
+
+ERROR(error_hermetic_seal_requires_lto, none,
+      "-experimental-hermetic-seal-at-link requires -lto=llvm-full or -lto=llvm-thin",
+      ())
+
 WARNING(warn_drv_darwin_sdk_invalid_settings, none,
     "SDK settings were ignored because 'SDKSettings.json' could not be parsed",
     ())

include/swift/Option/Options.td

@@ -620,6 +620,11 @@ def requirement_machine_protocol_signatures_EQ : Joined<["-"], "requirement-mach
   Flags<[FrontendOption]>,
   HelpText<"Control usage of experimental protocol requirement signature minimization: 'on', 'off', or 'verify'">;
 
+def experimental_hermetic_seal_at_link:
+  Flag<["-"], "experimental-hermetic-seal-at-link">,
+  Flags<[FrontendOption, HelpHidden]>,
+  HelpText<"Library code can assume that all clients are visible at linktime, and aggressively strip unused code">;
+
 // Diagnostic control options
 def suppress_warnings : Flag<["-"], "suppress-warnings">,
   Flags<[FrontendOption]>,

lib/Driver/Driver.cpp

@@ -283,6 +283,28 @@ static void validateSearchPathArgs(DiagnosticEngine &diags,
   }
 }
 
+static void validateLinkArgs(DiagnosticEngine &diags, const ArgList &args) {
+  if (args.hasArg(options::OPT_experimental_hermetic_seal_at_link)) {
+    if (args.hasArg(options::OPT_enable_library_evolution)) {
+      diags.diagnose(SourceLoc(),
+                     diag::error_hermetic_seal_cannot_have_library_evolution);
+    }
+
+    bool ltoOk = false;
+    if (const Arg *A = args.getLastArg(options::OPT_lto)) {
+      StringRef name = A->getValue();
+      if (name == "llvm-thin" || name == "llvm-full") {
+        ltoOk = true;
+      }
+    }
+
+    if (!ltoOk) {
+      diags.diagnose(SourceLoc(),
+                     diag::error_hermetic_seal_requires_lto);
+    }
+  }
+}
+
 /// Perform miscellaneous early validation of arguments.
 static void validateArgs(DiagnosticEngine &diags, const ArgList &args,
                          const llvm::Triple &T) {
@@ -294,6 +316,7 @@ static void validateArgs(DiagnosticEngine &diags, const ArgList &args,
   validateCompilationConditionArgs(diags, args);
   validateSearchPathArgs(diags, args);
   validateVerifyIncrementalDependencyArgs(diags, args);
+  validateLinkArgs(diags, args);
 }
 
 std::unique_ptr<ToolChain>

lib/Driver/ToolChains.cpp

@@ -12,6 +12,7 @@
 
 #include "ToolChains.h"
 
+#include "swift/AST/DiagnosticsDriver.h"
 #include "swift/Basic/Dwarf.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/Platform.h"
@@ -198,6 +199,13 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
         inputArgs.MakeArgString(Twine("-stdlib=") + arg->getValue()));
   }
 
+  if (inputArgs.hasArg(options::OPT_experimental_hermetic_seal_at_link)) {
+    arguments.push_back("-enable-llvm-vfe");
+    arguments.push_back("-enable-llvm-wme");
+    arguments.push_back("-conditional-runtime-records");
+    arguments.push_back("-internalize-at-link");
+  }
+
   // Handle the CPU and its preferences.
   inputArgs.AddLastArg(arguments, options::OPT_target_cpu);
 

test/Driver/hermetic-seal.swift

@@ -0,0 +1,19 @@
+// RUN: %swiftc_driver -driver-print-jobs -target x86_64-apple-macosx10.9 %s \
+// RUN:   -experimental-hermetic-seal-at-link -lto=llvm-full 2>&1 | %FileCheck %s
+
+// CHECK: swift
+// CHECK: -enable-llvm-vfe
+// CHECK: -enable-llvm-wme
+// CHECK: -conditional-runtime-records
+// CHECK: -internalize-at-link
+// CHECK: -lto=llvm-full
+
+// RUN: not %swiftc_driver -driver-print-jobs -target x86_64-apple-macosx10.9 %s \
+// RUN:   -experimental-hermetic-seal-at-link -enable-library-evolution 2>&1 | %FileCheck %s --check-prefix CHECK-LE
+
+// CHECK-LE: error: Cannot use -experimental-hermetic-seal-at-link with -enable-library-evolution
+
+// RUN: not %swiftc_driver -driver-print-jobs -target x86_64-apple-macosx10.9 %s \
+// RUN:   -experimental-hermetic-seal-at-link 2>&1 | %FileCheck %s --check-prefix CHECK-NOLTO
+
+// CHECK-NOLTO: error: -experimental-hermetic-seal-at-link requires -lto=llvm-full or -lto=llvm-thin

test/IRGen/hermetic-seal-exec.swift

@@ -0,0 +1,72 @@
+// End-to-end test of -experimental-hermetic-seal-at-link flag.
+
+// RUN: %empty-directory(%t)
+
+// (1) Build library swiftmodule
+// RUN: %target-build-swift %s -DLIBRARY -module-name Library -experimental-hermetic-seal-at-link -lto=llvm-full %lto_flags \
+// RUN:     -Xfrontend -disable-reflection-metadata -Xfrontend -disable-reflection-names -Xfrontend -disable-objc-interop \
+// RUN:     -emit-library -static -o %t/libLibrary.a \
+// RUN:     -emit-module -emit-module-path %t/Library.swiftmodule
+
+// (2) Check that libLibrary.a does actually provide all its public interfaces
+// RUN: %llvm-nm %t/libLibrary.a | %FileCheck %s --check-prefix CHECK-NM-LIB
+
+// (3) Build client
+// RUN: %target-build-swift %s -DCLIENT -parse-as-library -module-name Main -experimental-hermetic-seal-at-link -lto=llvm-full %lto_flags \
+// RUN:     -Xfrontend -disable-reflection-metadata -Xfrontend -disable-reflection-names -Xfrontend -disable-objc-interop \
+// RUN:     -I%t -L%t -lLibrary -o %t/main
+
+// (4) Check that unused symbols are not present in final executable
+// RUN: %llvm-nm %t/main | %FileCheck %s --check-prefix CHECK-NM-EXEC
+
+// (5) Execute
+// RUN: %target-run %t/main | %FileCheck %s
+
+// REQUIRES: executable_test
+
+// Test disabled until LLVM GlobalDCE supports conditional references.
+// REQUIRES: rdar81868900
+
+#if LIBRARY
+
+  // The only symbol that's actually used by client code
+  public func used_func() {
+    print("used_func")
+  }
+
+  public class MyUnusedClass {}
+  public class MyUnusedSubClass: MyUnusedClass {}
+  public protocol MyUnusedProtocol {}
+  public struct MyUnusedStruct {}
+  public struct MyUnusedStruct2: MyUnusedProtocol {}
+  public enum MyUnusedEnum {}
+  public func MyUnusedFunc() {}
+
+  // (2) In libLibrary.a, all exported symbols are present...
+  // CHECK-NM-LIB-DAG: MyUnusedClass
+  // CHECK-NM-LIB-DAG: MyUnusedSubClass
+  // CHECK-NM-LIB-DAG: MyUnusedProtocol
+  // CHECK-NM-LIB-DAG: MyUnusedStruct
+  // CHECK-NM-LIB-DAG: MyUnusedStruct2
+  // CHECK-NM-LIB-DAG: MyUnusedEnum
+  // CHECK-NM-LIB-DAG: MyUnusedFunc
+
+  // (4) ... but after linking the main executable, they are removed.
+  // CHECK-NM-EXEC-NOT: MyUnused
+
+#endif  // LIBRARY
+
+#if CLIENT
+
+  import Library
+
+  @_cdecl("main")
+  func main() -> Int32 {
+     used_func()
+     print("Done")
+     // CHECK: used_func
+     // CHECK-NEXT: Done
+     return 0
+  }
+
+#endif  // CLIENT