[gold commit] PR 19842: Fix problem where gold fails to issue an undefined symbol error during LTO

[Cary Coutant <ccoutant at gmail dot com>]

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.

-cary


2016-03-21  Cary Coutant  <ccoutant@gmail.com>

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.

Attachment: pr19842.patch
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