[Frontend] Turn symbols-missing-from-TBD validation on by default in debug builds on Apple platforms.

TBD validation is effectively an expensive assertion, and is currently only
tuned for Apple platforms. However, we don't want it to regress more, and it
would be nice to start getting validation from people using master
snapshots. Together, this means that turning it on by default for the cases
mentioned above is an appropriate course of action.

At the very least, this has the benefit of running validation across the stdlib,
the overlays and the whole testsuite on each build, so people making changes to
the compiler that change symbols are hopefully alerted.

One limitation here is that this is only validating that the TBD is a superset
of the true set of symbols: it could include spurious symbols that aren't
actually in the binary. This case is less problematic for Swift than symbols
missing from the TBD file, and so we've focused energy on this. Once we've fixed
the extra-symbols problems and are confident in it, this validation can be
upgraded to validate that too.

Half of rdar://problem/40431434.

Author: huonw

include/swift/Frontend/FrontendOptions.h

@@ -240,13 +240,14 @@ class FrontendOptions {
 
   /// The different modes for validating TBD against the LLVM IR.
   enum class TBDValidationMode {
+    Default,        ///< Do the default validation for the current platform.
     None,           ///< Do no validation.
     MissingFromTBD, ///< Only check for symbols that are in IR but not TBD.
     All, ///< Check for symbols that are in IR but not TBD and TBD but not IR.
   };
 
   /// Compare the symbols in the IR against the TBD file we would generate.
-  TBDValidationMode ValidateTBDAgainstIR = TBDValidationMode::None;
+  TBDValidationMode ValidateTBDAgainstIR = TBDValidationMode::Default;
 
   /// The install_name to use in the TBD file.
   std::string TBDInstallName;

lib/FrontendTool/FrontendTool.cpp

@@ -1144,9 +1144,20 @@ static bool validateTBDIfNeeded(CompilerInvocation &Invocation,
     return false;
 
   const auto &frontendOpts = Invocation.getFrontendOptions();
-  const auto mode = frontendOpts.ValidateTBDAgainstIR;
+  auto mode = frontendOpts.ValidateTBDAgainstIR;
   // Ensure all cases are covered by using a switch here.
   switch (mode) {
+  case FrontendOptions::TBDValidationMode::Default:
+#ifndef NDEBUG
+    // When a debug compiler is targeting an apple platform, we do some
+    // validation by default.
+    if (Invocation.getLangOptions().Target.getVendor() == llvm::Triple::Apple) {
+      mode = FrontendOptions::TBDValidationMode::MissingFromTBD;
+      break;
+    }
+#endif
+    // Otherwise, the default is to do nothing.
+    LLVM_FALLTHROUGH;
   case FrontendOptions::TBDValidationMode::None:
     return false;
   case FrontendOptions::TBDValidationMode::All:

stdlib/public/SwiftOnoneSupport/CMakeLists.txt

@@ -2,6 +2,10 @@ add_swift_library(swiftSwiftOnoneSupport ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_
   # This file should be listed the first.  Module name is inferred from the
   # filename.
   SwiftOnoneSupport.swift
-  SWIFT_COMPILE_FLAGS "-parse-stdlib" "-Xllvm" "-sil-inline-generics=false" "${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS}"
+  # We have to disable validation of TBD files, because this module is
+  # _explicitly_ special-cased to result in extra symbols generated by the
+  # optimizer, meaning TBDGen can't (and shouldn't: it has to run
+  # pre-optimization for performance) list them.
+  SWIFT_COMPILE_FLAGS "-parse-stdlib" "-Xllvm" "-sil-inline-generics=false" "-Xfrontend" "-validate-tbd-against-ir=none" "${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS}"
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
   INSTALL_IN_COMPONENT stdlib)