swift-autolink-extract should look inside .a files.

Previously it just gave up when it saw anything other than an ELF .o file. We
could have it ignore .a files, but we might as well do the right thing.

<rdar://problem/23045632>

test/AutolinkExtract/empty_archive.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swiftc_driver -c %s -o %t
+// RUN: rm -f %t.a
+// RUN: ar cr %t.a %t
+// RUN: %target-swift-autolink-extract %t.a -o - | FileCheck --check-prefix CHECK-%target-object-format %s
+
+// REQUIRES: autolink-extract
+
+// CHECK-elf: -lswiftCore

test/AutolinkExtract/import_archive.swift

@@ -0,0 +1,13 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %target-swiftc_driver -emit-library -emit-module -emit-module-path %t/empty.swiftmodule -module-name empty -module-link-name empty %S/empty.swift
+// RUN: %target-swiftc_driver -c %s -I %t -o %t/import_experimental.o
+// RUN: ar cr %t/import_experimental.a %t/import_experimental.o
+// RUN: %target-swift-autolink-extract %t/import_experimental.a -o - | FileCheck --check-prefix CHECK-%target-object-format %s
+
+// REQUIRES: autolink-extract
+
+// CHECK-elf-DAG: -lswiftCore
+// CHECK-elf-DAG: -lempty
+
+import empty

tools/driver/autolink_extract_main.cpp

@@ -29,6 +29,7 @@
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/Option.h"
+#include "llvm/Object/Archive.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Object/ELFObjectFile.h"
 
@@ -104,6 +105,61 @@ class AutolinkExtractInvocation {
     return 0;
   }
 };
+
+// Look inside the binary 'Bin' and append any linker flags found in its
+// ".swift1_autolink_entries" section to 'LinkerFlags'. If 'Bin' is an archive,
+// recursively look inside all children within the archive. Return 'true' if
+// there was an error, and 'false' otherwise.
+static bool extractLinkerFlags(const llvm::object::Binary *Bin,
+                               CompilerInstance &Instance,
+                               StringRef BinaryFileName,
+                               std::set<std::string> &LinkerFlags) {
+  if (auto *ObjectFile = llvm::dyn_cast<llvm::object::ELFObjectFileBase>(Bin)) {
+    // Search for the section we hold autolink entries in
+    for (auto &Section : ObjectFile->sections()) {
+      llvm::StringRef SectionName;
+      Section.getName(SectionName);
+      if (SectionName == ".swift1_autolink_entries") {
+        llvm::StringRef SectionData;
+        Section.getContents(SectionData);
+
+        // entries are null-terminated, so extract them and push them into
+        // the set.
+        llvm::SmallVector<llvm::StringRef, 4> SplitFlags;
+        SectionData.split(SplitFlags, llvm::StringRef("\0", 1),
+          -1, /*KeepEmpty=*/false);
+        for (const auto &Flag : SplitFlags) {
+          LinkerFlags.insert(Flag);
+        }
+      }
+    }
+    return false;
+  } else if (auto *Archive = llvm::dyn_cast<llvm::object::Archive>(Bin)) {
+    for (const auto &Child : Archive->children()) {
+      auto ChildBinary = Child.getAsBinary();
+      // FIXME: BinaryFileName below should instead be ld-style names for
+      // object files in archives, e.g. "foo.a(bar.o)".
+      if (!ChildBinary) {
+        Instance.getDiags().diagnose(SourceLoc(), diag::error_open_input_file,
+                                     BinaryFileName,
+                                     ChildBinary.getError().message());
+        return true;
+      }
+      if (extractLinkerFlags(ChildBinary->get(), Instance, BinaryFileName,
+                             LinkerFlags)) {
+        return true;
+      }
+    }
+    return false;
+  } else {
+    Instance.getDiags().diagnose(SourceLoc(), diag::error_open_input_file,
+                                 BinaryFileName,
+                                 "Don't know how to extract from object file"
+                                 "format");
+    return true;
+  }
+}
+
 int autolink_extract_main(ArrayRef<const char *> Args, const char *Argv0,
                           void *MainAddr) {
   CompilerInstance Instance;
@@ -123,41 +179,18 @@ int autolink_extract_main(ArrayRef<const char *> Args, const char *Argv0,
   // Store each linker flag only once
   std::set<std::string> LinkerFlags;
 
-  for (const auto &ObjectFileName : Invocation.getInputFilenames()) {
-    auto ObjectFileOwner =
-      llvm::object::ObjectFile::createObjectFile(ObjectFileName);
-    if (!ObjectFileOwner) {
+  // Extract the linker flags from the objects.
+  for (const auto &BinaryFileName : Invocation.getInputFilenames()) {
+    auto BinaryOwner = llvm::object::createBinary(BinaryFileName);
+    if (!BinaryOwner) {
       Instance.getDiags().diagnose(SourceLoc(), diag::error_open_input_file,
-                                   ObjectFileName,
-                                   ObjectFileOwner.getError().message());
+                                   BinaryFileName,
+                                   BinaryOwner.getError().message());
       return 1;
     }
 
-    auto ObjectFile = ObjectFileOwner->getBinary();
-    if (llvm::isa<llvm::object::ELFObjectFileBase>(ObjectFile)) {
-      // Search for the section we hold autolink entries in
-      for (auto &Section : ObjectFileOwner->getBinary()->sections()) {
-        llvm::StringRef SectionName;
-        Section.getName(SectionName);
-        if (SectionName == ".swift1_autolink_entries") {
-          llvm::StringRef SectionData;
-          Section.getContents(SectionData);
-
-          // entries are null-terminated, so extract them and push them into
-          // the set.
-          llvm::SmallVector<llvm::StringRef, 4> SplitFlags;
-          SectionData.split(SplitFlags, llvm::StringRef("\0", 1),
-            -1, /*KeepEmpty=*/false);
-          for (const auto &Flag : SplitFlags) {
-            LinkerFlags.insert(Flag);
-          }
-        }
-      }
-    } else {
-      Instance.getDiags().diagnose(SourceLoc(), diag::error_open_input_file,
-                                   ObjectFileName,
-                                   "Don't know how to extract from object file"
-                                   "format");
+    if (extractLinkerFlags(BinaryOwner->getBinary(), Instance, BinaryFileName,
+                           LinkerFlags)) {
       return 1;
     }
   }