Archives require a symbol table on Solaris, even if empty.

lib/Object/ArchiveWriter.cpp

@@ -294,6 +294,11 @@ writeSymbolTable(raw_fd_ostream &Out, object::Archive::Kind Kind,
   if (Kind == object::Archive::K_BSD)
     print32(Out, Kind, StringTable.size()); // byte count of the string table
   Out << StringTable;
+  // If there are no symbols, emit an empty symbol table, to satisfy Solaris
+  // tools, older versions of which expect a symbol table in a non-empty
+  // archive, regardless of whether there are any symbols in it.
+  if (StringTable.size() == 0)
+    print32(Out, Kind, 0);
 
   // ld64 requires the next member header to start at an offset that is
   // 4 bytes aligned.

test/Object/Inputs/solaris-nosymbols.yaml

@@ -0,0 +1,7 @@
+--- !ELF
+FileHeader:      
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+...

test/Object/archive-format.test

@@ -65,3 +65,15 @@ THIN-PATH-NEXT: /65             0           0     0     644     4         `
 
 RUN: not llvm-ar --format=bsd rcT bad.a 0123456789abcde 0123456789abcdef 2>&1 | FileCheck --check-prefix=BSD-THIN %s
 BSD-THIN: Only the gnu format has a thin mode.
+
+If an archive has an object with no symbols, the linker and some other
+tools on some versions of Solaris will abort operations if there is no
+symbol table.  Create such an object, put it into an archive, and check to
+see that there is an empty symbol table.
+RUN: mkdir -p %t
+RUN: yaml2obj %S/Inputs/solaris-nosymbols.yaml > %t/foo.o
+RUN: llvm-ar rs %t/foo.a %t/foo.o
+RUN: cat -v %t/foo.a | FileCheck -strict-whitespace --check-prefix=SOLARIS %s
+SOLARIS:      !<arch>
+SOLARIS-NEXT: /               0           0     0     0       8         `
+SOLARIS-NEXT: ^@^@^@^@^@^@^@^@foo.o/