Make --export-dynamic-symbol to pull out object files from archives.

[ruiu]

Summary

This is for compatiblity with GNU gold. GNU gold tries to resolve
symbols specified by --export-dynamic-symbol. So, if a symbol specified
by --export-dynamic-symbol is in an archive file, lld's result is
currently different from gold's.

Interestingly, that behavior is different for --dynamic-list.
I added a new test to ensure that.

[smeenai]

I had actually run into this issue, but in my use case the --export-dynamic-symbol should have arguably been a -u in the first place, so I just changed it there. I didn't realize the fix in LLD would be this straightforward though. LGTM.

-dynamic-list and -export-dynamic-symbol have completely different meanings for shared libraries, so this behavior difference isn't surprising. -dynamic-list controls symbol preemptibility, while -export-dynamic-symbol controls dynamic symbol table population. @espindola had fixed -dynamic-list a while back, and I'll put up a patch to update the comment above the if statement here.

ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   // Parses -dynamic-list and -export-dynamic-symbol. They make some
   // symbols private. Note that -export-dynamic takes precedence over them
   // as it says all symbols should be exported.
   if (!HasExportDynamic) {
     for (auto *Arg : Args.filtered(OPT_dynamic_list))
       if (Optional<MemoryBufferRef> Buffer = readFile(Arg->getValue()))
         readDynamicList(*Buffer);
 
-    for (auto *Arg : Args.filtered(OPT_export_dynamic_symbol))
+    for (auto *Arg : Args.filtered(OPT_export_dynamic_symbol)) {
       Config->DynamicList.push_back(
           {Arg->getValue(), /*IsExternCpp*/ false, /*HasWildcard*/ false});
+      Config->Undefined.push_back(Arg->getValue());
+    }
   }
 
   for (auto *Arg : Args.filtered(OPT_version_script))
     if (Optional<MemoryBufferRef> Buffer = readFile(Arg->getValue()))
       readVersionScript(*Buffer);
 }
 
 // Some Config members do not directly correspond to any particular

test/ELF/dynamic-list-archive.s

+# REQUIRES: x86
+
+# RUN: rm -f %t.a
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/archive2.s -o %t1.o
+# RUN: llvm-ar rcs %t.a %t1.o
+
+# RUN: echo "{ foo; };" > %t.list
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2.o
+# RUN: ld.lld -shared -o %t.so --dynamic-list %t.list %t.a %t2.o
+
+# RUN: llvm-readelf -dyn-symbols %t.so | FileCheck %s
+# CHECK-NOT: foo
+
+.global _start
+_start:
+  nop

test/ELF/export-dynamic-symbol.s

+# REQUIRES: x86
+
+# RUN: rm -f %t.a
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/archive2.s -o %t1.o
+# RUN: llvm-ar rcs %t.a %t1.o
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2.o
+# RUN: ld.lld -shared -o %t.so --export-dynamic-symbol foo %t.a %t2.o
+
+# RUN: llvm-readelf -dyn-symbols %t.so | FileCheck %s
+# CHECK: foo
+
+.global _start
+_start:
+  nop