Fix problem where gold fails to issue an undefined symbol error during LTO.

During LTO, if (1) an IR file contains a COMDAT group that is kept,
(2) a later non-claimed file contains the same group, which we discard,
and (3) the plugin fails to provide a definition of the symbols in that
COMDAT group, gold silently resolves any references to those symbols
to 0.

This patch adds a check for a placeholder symbol when deciding
whether to issue an undefined symbol error. It also adds an extra
note after any undefined placeholder symbol error that explains
that a definition was expected from the plugin.

gold/
	PR gold/19842
	* errors.cc (Errors::undefined_symbol): Add info message when
	symbol should have been provided by a plugin.
	* target-reloc.h (issue_undefined_symbol_error): Check for
	placeholder symbols defined in discarded sections.
	* testsuite/Makefile.am (plugin_test_9b): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/plugin_test_9b_elf.cc: New test source file.
	* testsuite/plugin_test_9b_ir.cc: New test source file.

1 files changed, 4 insertions, 2 deletions

diff --git a/gold/target-reloc.h b/gold/target-reloc.h
index bdf673d..e275d33 100644
--- a/gold/target-reloc.h
+++ b/gold/target-reloc.h
@@ -186,8 +186,10 @@ issue_undefined_symbol_error(const Symbol* sym)
   if (sym->is_weak_undefined())
     return false;
 
-  // We don't report symbols defined in discarded sections.
-  if (sym->is_defined_in_discarded_section())
+  // We don't report symbols defined in discarded sections,
+  // unless they're placeholder symbols that should have been
+  // provided by a plugin.
+  if (sym->is_defined_in_discarded_section() && !sym->is_placeholder())
     return false;
 
   // If the target defines this symbol, don't report it here.