[lld] [LLD][COFF] Make /wholearchive thin-archive member identifiers consistent (PR #145487)

[via llvm-commits]

https://github.com/bd1976bris created https://github.com/llvm/llvm-project/pull/145487

A thin archive is an archive/library format where the archive itself contains only references to member object files on disk, rather than embedding the file contents.

For the non-/wholearchive case, we use the path to the archive member as the identifier for thin-archive members (see comments in `enqueueArchiveMember`). This patch modifies the /wholearchive path to behave the same way.

Apart from consistency, my motivation for fixing this is DTLTO (#126654), where having the member identifier be the path on disk allows distribution of bitcode members during ThinLTO.

>From 740d81ae961d7f14445d65c77b4881414ec2ae1c Mon Sep 17 00:00:00 2001
From: Dunbobbin <Ben.Dunbobbin at sony.com>
Date: Tue, 24 Jun 2025 10:20:35 +0100
Subject: [PATCH] [LLD][COFF] Make /wholearchive thin-archive member
 identifiers consistent

A thin archive is an archive/library format where the archive itself
contains only references to member object files on disk, rather than
embedding the file contents.

For the non-/wholearchive case, we use the path to the archive member
as the identifier for thin-archive members (see comments in
`enqueueArchiveMember`). This patch modifies the /wholearchive path to
behave the same way.

Apart from consistency, my motivation for fixing this is DTLTO
(https://github.com/llvm/llvm-project/pull/126654), where having the
member identifier be the path on disk allows distribution of bitcode
members during ThinLTO.
---
 lld/COFF/Driver.cpp          |  7 ++++++-
 lld/test/COFF/thin-archive.s | 20 ++++++++++++++------
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/lld/COFF/Driver.cpp b/lld/COFF/Driver.cpp
index f3240b22a1442..caff50d4bbae4 100644
--- a/lld/COFF/Driver.cpp
+++ b/lld/COFF/Driver.cpp
@@ -275,7 +275,12 @@ void LinkerDriver::addBuffer(std::unique_ptr<MemoryBuffer> mb,
 
       int memberIndex = 0;
       for (MemoryBufferRef m : getArchiveMembers(ctx, archive))
-        addArchiveBuffer(m, "<whole-archive>", filename, memberIndex++);
+        if (!archive->isThin())
+          addArchiveBuffer(m, "<whole-archive>", filename, memberIndex++);
+        else
+          // Pass empty string as archive name so that the original filename is
+          // used as the buffer identifier.
+          addArchiveBuffer(m, "<whole-archive>", "", /*OffsetInArchive=*/0);
       return;
     }
     addFile(make<ArchiveFile>(ctx, mbref));
diff --git a/lld/test/COFF/thin-archive.s b/lld/test/COFF/thin-archive.s
index 55d71ea635673..930c7488a7f56 100644
--- a/lld/test/COFF/thin-archive.s
+++ b/lld/test/COFF/thin-archive.s
@@ -22,12 +22,20 @@
 # SYMTAB:        ?f@@YAHXZ in
 # NO-SYMTAB-NOT: ?f@@YAHXZ in
 
-# RUN: lld-link /entry:main %t.main.obj %t.lib /out:%t.exe 2>&1 | \
-# RUN:     FileCheck --allow-empty %s
-# RUN: lld-link /entry:main %t.main.obj %t_thin.lib /out:%t.exe 2>&1 | \
-# RUN:     FileCheck --allow-empty %s
-# RUN: lld-link /entry:main %t.main.obj /wholearchive:%t_thin.lib /out:%t.exe 2>&1 | \
-# RUN:     FileCheck --allow-empty %s
+# RUN: echo /entry:main %t.main.obj /out:%t.exe /verbose > %t.rsp
+
+# RUN: lld-link @%t.rsp %t.lib 2>&1 | \
+# RUN:     FileCheck --allow-empty %s --check-prefixes=CHECK,LOAD_NON_THIN
+# RUN: lld-link @%t.rsp %t_thin.lib 2>&1 | \
+# RUN:     FileCheck --allow-empty %s --check-prefixes=CHECK,LOAD_THIN_SYM
+# RUN: lld-link @%t.rsp /wholearchive:%t_thin.lib 2>&1 | \
+# RUN:     FileCheck --allow-empty %s --check-prefixes=CHECK,LOAD_THIN_WHOLE
+# RUN: lld-link @%t.rsp /wholearchive %t_thin.lib 2>&1 | \
+# RUN:     FileCheck --allow-empty %s --check-prefixes=CHECK,LOAD_THIN_WHOLE
+
+# LOAD_NON_THIN:   Loaded thin-archive{{.*}}.lib(thin-archive{{.*}}.obj) for int __cdecl f(void)
+# LOAD_THIN_SYM:   Loaded {{.*}}thin-archive{{.*}}.obj for int __cdecl f(void)
+# LOAD_THIN_WHOLE: Loaded {{.*}}thin-archive{{.*}}.obj for <whole-archive>
 
 # RUN: rm %t.lib.obj
 # RUN: lld-link /entry:main %t.main.obj %t.lib /out:%t.exe 2>&1 | \