IRGen: consider deployment target when emitting $ld$previous symbols

Deployment target can be set before the introduction of a moved symbol. When that
happens, using the introductory version in the $ld$previous symbol could lead to ld
not redirecting the linkage path to the original dylib, which is a mistake when running
the App on newer OSs when the original dylib does exist.

rdar://105181824

Author: nkcsgexi

lib/IRGen/TBDGen.cpp

@@ -277,6 +277,18 @@ getInnermostIntroVersion(ArrayRef<Decl*> DeclStack, PlatformKind Platform) {
   return None;
 }
 
+/// Using the introducing version of a symbol as the start version to redirect
+/// linkage path isn't sufficient. This is because the executable can be deployed
+/// to OS versions that were before the symbol was introduced. When that happens,
+/// strictly using the introductory version can lead to NOT redirecting.
+static llvm::VersionTuple calculateLdPreviousVersionStart(ASTContext &ctx,
+                                                llvm::VersionTuple introVer) {
+  auto minDep = ctx.LangOpts.getMinPlatformVersion();
+  if (minDep < introVer)
+    return llvm::VersionTuple(1, 0);
+  return introVer;
+}
+
 void TBDGenVisitor::addLinkerDirectiveSymbolsLdPrevious(StringRef name,
                                                 llvm::MachO::SymbolKind kind) {
   if (kind != llvm::MachO::SymbolKind::GlobalSymbol)
@@ -323,7 +335,8 @@ void TBDGenVisitor::addLinkerDirectiveSymbolsLdPrevious(StringRef name,
     static auto getMinor = [](Optional<unsigned> Minor) {
       return Minor.has_value() ? *Minor : 0;
     };
-    OS << IntroVer->getMajor() << "." << getMinor(IntroVer->getMinor()) << "$";
+    auto verStart = calculateLdPreviousVersionStart(Ctx, *IntroVer);
+    OS << verStart.getMajor() << "." << getMinor(verStart.getMinor()) << "$";
     OS << Ver.Version.getMajor() << "." << getMinor(Ver.Version.getMinor()) << "$";
     OS << name << "$";
     addSymbolInternal(OS.str(), SymbolKind::GlobalSymbol,

test/IRGen/original-defined-attr-linker-directives-fail.swift

@@ -6,6 +6,8 @@
 // RUN: not %target-swift-frontend -swift-version 4 -enforce-exclusivity=checked %s -emit-ir -module-name CurrentModule -D CURRENT_MODULE -previous-module-installname-map-file %t/nil.json >& %t/error.txt
 // RUN: %FileCheck %s -check-prefix=CHECK-ERROR < %t/error.txt
 
+// RUN: %target-swift-frontend -target x86_64-apple-macosx10.6 -swift-version 4 -enforce-exclusivity=checked %s -emit-ir -module-name CurrentModule -D CURRENT_MODULE -previous-module-installname-map-file %t/install-name.json | %FileCheck %s -check-prefix=CHECK-SYMBOLS-LOW-TARGET
+
 @available(OSX 10.8, *)
 @_originallyDefinedIn(module: "OriginalModule", macOS 10.10)
 public struct Entity {
@@ -19,4 +21,10 @@ public struct Entity {
 // CHECK-SYMBOLS: $ld$previous$/System/OriginalModule.dylib$$1$10.8$10.10$_$s14OriginalModule6EntityVMa$
 // CHECK-SYMBOLS: $ld$previous$/System/OriginalModule.dylib$$1$10.8$10.10$_$s14OriginalModule6EntityV06removeC0yyACF$
 
+// CHECK-SYMBOLS-LOW-TARGET: $ld$previous$/System/OriginalModule.dylib$$1$1.0$10.10$_$s14OriginalModule6EntityVMn$
+// CHECK-SYMBOLS-LOW-TARGET: $ld$previous$/System/OriginalModule.dylib$$1$1.0$10.10$_$s14OriginalModule6EntityV03addC0yyACF$
+// CHECK-SYMBOLS-LOW-TARGET: $ld$previous$/System/OriginalModule.dylib$$1$1.0$10.10$_$s14OriginalModule6EntityVN$
+// CHECK-SYMBOLS-LOW-TARGET: $ld$previous$/System/OriginalModule.dylib$$1$1.0$10.10$_$s14OriginalModule6EntityVMa$
+// CHECK-SYMBOLS-LOW-TARGET: $ld$previous$/System/OriginalModule.dylib$$1$1.0$10.10$_$s14OriginalModule6EntityV06removeC0yyACF$
+
 // CHECK-ERROR: cannot open previous install name map