Make a hack for LTO work only when you are actually doing LTO.

[ruiu]

Summary

We allow an archive file without symbol table as a linker input as a
workaround for a very common error in LTO build. But that logic worked
even for an archive file containing non-bitcode files, which is not
expected. This patch limits that workaround to one that contains only
bitcode files.

[efriedma]

lld/ELF/Driver.cpp

220

"return"? It seems weird to intentionally ignore archive files without a symbol table.

[ruiu]

lld/ELF/Driver.cpp

220

But that is what other linkers do and is also a natural consequence of how linker works on archive files. When a linker visits an archive file, it reads its symbol table, and pulls out object files that defines symbols that are currently undefined. Therefore, if an archive doesn't contain a symbol table, nothing is pulled out, and the archive is effectively ignored.

[efriedma]

LGTM

lld/ELF/Driver.cpp

220

I guess that's right; it's effectively the same as the other codepath which calls make<ArchiveFile>(). But it looks weird at first glance; maybe add an explicit comment noting that.

lld/ELF/Driver.cpp

case file_magic::archive: {
     std::unique_ptr<Archive> File =
         CHECK(Archive::create(MBRef), Path + ": failed to parse archive");
 
     // If an archive file has no symbol table, it is likely that a user
     // is attempting LTO and using a default ar command that doesn't
     // understand the LLVM bitcode file. It is a pretty common error, so
     // we'll handle it as if it had a symbol table.
     if (!File->isEmpty() && !File->hasSymbolTable()) {
-      for (const auto &P : getArchiveMembers(MBRef))
+      // First, check if all members are bitcode files.
+      for (const std::pair<MemoryBufferRef, uint64_t> &P :
+           getArchiveMembers(MBRef))
+        if (identify_magic(P.first.getBuffer()) != file_magic::bitcode)
+          return;
+
+      for (const std::pair<MemoryBufferRef, uint64_t> &P :
+           getArchiveMembers(MBRef))
         Files.push_back(make<LazyObjFile>(P.first, Path, P.second));
       return;
     }
 
     // Handle the regular case.
     Files.push_back(make<ArchiveFile>(std::move(File)));
     return;
   }

lld/test/ELF/invalid/invalid-elf.test

 # REQUIRES: x86
 # RUN: rm -rf %t && mkdir -p %t
 # RUN: llvm-mc %s -o %t/simple.o -filetype=obj -triple x86_64-pc-linux
 # RUN: echo > %t/empty.o
 # RUN: llvm-ar --format=gnu cr %t/not-elf.a %t/empty.o
 
-# RUN: not ld.lld %t/simple.o %t/not-elf.a -o %t2 2>&1 | \
+# RUN: not ld.lld --whole-archive %t/not-elf.a -o %t2 2>&1 | \
 # RUN:   FileCheck --check-prefix=NOT-ELF %s
 # NOT-ELF: not-elf.a(empty.o): not an ELF file
 
 # RUN: not ld.lld %t/simple.o %p/Inputs/data-encoding.a -o %t2 2>&1 | \
 # RUN:   FileCheck --check-prefix=INVALID-DATA-ENC %s
 # INVALID-DATA-ENC: data-encoding.a(test.o): corrupted ELF file: invalid data encoding
 
 # RUN: not ld.lld %t/simple.o %p/Inputs/file-class.a -o %t2 2>&1 | \

lld/test/ELF/lto/archive-no-index.ll

 ; REQUIRES: x86
-; Tests that we suggest that LTO symbols missing from an archive index
-; may be the cause of undefined references, but only if we both
-; encountered an empty archive index and undefined references (to prevent
-; noisy false alarms).
+; Tests that we accept an archive file without symbol table
+; if all the member files are bitcode files.
 
 ; RUN: llvm-as -o %t1.o %s
 ; RUN: llvm-as -o %t2.o %S/Inputs/archive.ll
 
 ; RUN: rm -f %t1.a %t2.a
 ; RUN: llvm-ar crS %t1.a %t2.o
 ; RUN: llvm-ar crs %t2.a %t2.o
 
 ; RUN: ld.lld -o %t -emain %t1.o %t1.a
 ; RUN: ld.lld -o %t -emain %t1.o %t2.a
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 declare void @f()
 
 define i32 @main() {
   call void @f()
   ret i32 0
 }
+
+; RUN: echo 'f:' | llvm-mc -triple=x86_64-pc-linux -filetype=obj - -o %t3.o
+; RUN: rm -f %t3.a
+; RUN: llvm-ar crS %t3.a %t3.o
+; RUN: not ld.lld -o /dev/null -emain %t1.o %t3.a 2>&1 | FileCheck -check-prefix=ERR1 %s
+; ERR1: error: undefined symbol: f
+
+; RUN: rm -f %t4.a
+; RUN: llvm-ar cr %t4.a
+; RUN: not ld.lld -o /dev/null -emain %t1.o %t4.a 2>&1 | FileCheck -check-prefix=ERR2 %s
+; ERR2: error: undefined symbol: f