[llvm-lib] Don't rewrite paths for members in non-thin archives (#123416)

This matches what MS lib.exe does (and llvm-ar too); when adding files
to an archive, MS lib.exe stores the file name as it was given on the
command line, whereas llvm-lib rewrote it into a relative path name,
relative to the archive location. Such a rewrite makes sense for thin
archives, but not for regular archives. (MS lib.exe doesn't support
producing thin archives; that's an LLVM extension - see the
thin-relative.test testcase.)

The behaviour to rewrite these paths was added in
451c2ef199e9c5163007ac32e2d426fbfb37e664; it is unclear why it was
chosen to do the rewriting for non-thin archives as well. This quirk is
even pointed out in a code comment - but neither the code review at
https://reviews.llvm.org/D57842 nor the linked bug report at
https://crbug.com/41440160 mentions why this is done for all archives,
not only thin ones.

Therefore, assume that this only was done out of convenience, and change
llvm-lib to not adjust the paths for non-thin archives.

Normally, the actual member names doesn't matter for non-thin archives;
however for short import libraries, where each member is named e.g.
"foo.dll", the names do matter. If using llvm-lib to merge two import
libraries (as a non-thin library), preserve the original names rather
than making the member names relative.

1 files changed, 10 insertions, 11 deletions

diff --git a/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp b/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp
index 6ce06b4..15d959d 100644
--- a/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp
+++ b/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp
@@ -503,23 +503,22 @@ int llvm::libDriverMain(ArrayRef<const char *> ArgsArr) {
       return 1;
     }
   }
-  // llvm-lib uses relative paths for both regular and thin archives, unlike
-  // standard GNU ar, which only uses relative paths for thin archives and
-  // basenames for regular archives.
-  for (NewArchiveMember &Member : Members) {
-    if (sys::path::is_relative(Member.MemberName)) {
-      Expected<std::string> PathOrErr =
-          computeArchiveRelativePath(OutputPath, Member.MemberName);
-      if (PathOrErr)
-        Member.MemberName = Saver.save(*PathOrErr);
+
+  bool Thin = Args.hasArg(OPT_llvmlibthin);
+  if (Thin) {
+    for (NewArchiveMember &Member : Members) {
+      if (sys::path::is_relative(Member.MemberName)) {
+        Expected<std::string> PathOrErr =
+            computeArchiveRelativePath(OutputPath, Member.MemberName);
+        if (PathOrErr)
+          Member.MemberName = Saver.save(*PathOrErr);
+      }
     }
   }
 
   // For compatibility with MSVC, reverse member vector after de-duplication.
   std::reverse(Members.begin(), Members.end());
 
-  bool Thin = Args.hasArg(OPT_llvmlibthin);
-
   auto Symtab = Args.hasFlag(OPT_llvmlibindex, OPT_llvmlibindex_no,
                              /*default=*/true)
                     ? SymtabWritingMode::NormalSymtab