[llvm-ar] Use COFF archive format for COFF targets. (#82898)

Detect COFF files by default and allow specifying it with --format
argument.

This is important for ARM64EC, which uses a separated symbol map for EC
symbols. Since K_COFF is mostly compatible with K_GNU, this shouldn't
really make a difference for other targets.

This originally landed as #82642, but was reverted due to test failures
in tests using no symbol table. Since COFF symbol can't express it,
fallback to GNU format in that case.

1 files changed, 15 insertions, 13 deletions

diff --git a/llvm/lib/Object/ArchiveWriter.cpp b/llvm/lib/Object/ArchiveWriter.cpp
index e062974..aa57e55 100644
--- a/llvm/lib/Object/ArchiveWriter.cpp
+++ b/llvm/lib/Object/ArchiveWriter.cpp
@@ -62,12 +62,16 @@ object::Archive::Kind NewArchiveMember::detectKindFromObject() const {
   Expected<std::unique_ptr<object::ObjectFile>> OptionalObject =
       object::ObjectFile::createObjectFile(MemBufferRef);
 
-  if (OptionalObject)
-    return isa<object::MachOObjectFile>(**OptionalObject)
-               ? object::Archive::K_DARWIN
-               : (isa<object::XCOFFObjectFile>(**OptionalObject)
-                      ? object::Archive::K_AIXBIG
-                      : object::Archive::K_GNU);
+  if (OptionalObject) {
+    if (isa<object::MachOObjectFile>(**OptionalObject))
+      return object::Archive::K_DARWIN;
+    if (isa<object::XCOFFObjectFile>(**OptionalObject))
+      return object::Archive::K_AIXBIG;
+    if (isa<object::COFFObjectFile>(**OptionalObject) ||
+        isa<object::COFFImportFile>(**OptionalObject))
+      return object::Archive::K_COFF;
+    return object::Archive::K_GNU;
+  }
 
   // Squelch the error in case we had a non-object file.
   consumeError(OptionalObject.takeError());
@@ -80,10 +84,7 @@ object::Archive::Kind NewArchiveMember::detectKindFromObject() const {
             MemBufferRef, file_magic::bitcode, &Context)) {
       auto &IRObject = cast<object::IRObjectFile>(**ObjOrErr);
       auto TargetTriple = Triple(IRObject.getTargetTriple());
-      return TargetTriple.isOSDarwin()
-                 ? object::Archive::K_DARWIN
-                 : (TargetTriple.isOSAIX() ? object::Archive::K_AIXBIG
-                                           : object::Archive::K_GNU);
+      return object::Archive::getDefaultKindForTriple(TargetTriple);
     } else {
       // Squelch the error in case this was not a SymbolicFile.
       consumeError(ObjOrErr.takeError());
@@ -976,10 +977,12 @@ static Error writeArchiveToStream(raw_ostream &Out,
   SmallString<0> StringTableBuf;
   raw_svector_ostream StringTable(StringTableBuf);
   SymMap SymMap;
+  bool ShouldWriteSymtab = WriteSymtab != SymtabWritingMode::NoSymtab;
 
   // COFF symbol map uses 16-bit indexes, so we can't use it if there are too
-  // many members.
-  if (isCOFFArchive(Kind) && NewMembers.size() > 0xfffe)
+  // many members. COFF format also requires symbol table presence, so use
+  // GNU format when NoSymtab is requested.
+  if (isCOFFArchive(Kind) && (NewMembers.size() > 0xfffe || !ShouldWriteSymtab))
     Kind = object::Archive::K_GNU;
 
   // In the scenario when LLVMContext is populated SymbolicFile will contain a
@@ -1008,7 +1011,6 @@ static Error writeArchiveToStream(raw_ostream &Out,
   uint64_t LastMemberHeaderOffset = 0;
   uint64_t NumSyms = 0;
   uint64_t NumSyms32 = 0; // Store symbol number of 32-bit member files.
-  bool ShouldWriteSymtab = WriteSymtab != SymtabWritingMode::NoSymtab;
 
   for (const auto &M : Data) {
     // Record the start of the member's offset