Calling ifunc functions when resolver has debug info, user symbol same name

Calling ifunc functions when resolver has debug info, user symbol same name

If the GNU ifunc resolver has the same name as the user visible
symbol, and the resolver has debug info, then the DWARF info for the
resolver masks the ifunc minsym.  In that scenario, if you try calling
the ifunc from GDB, you call the resolver instead.  With the
gnu-ifunc.exp testcase added in a following patch, you'd see:

  (gdb) p gnu_ifunc (3)
  $1 = (int (*)(int)) 0x400753 <final>
  (gdb) FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=0: p gnu_ifunc (3)
                                                       ^^^^^^^^^^^^^^^^

That is, we called the ifunc resolver manually, which returned a
pointer to the ifunc target function ("final").  The "final" symbol is
the function that GDB should have called automatically,

  ~~~~~~~~~~~~
  int
  final (int arg)
  {
    return arg + 1;
  }
  ~~~~~~~~~

which is what happens if you don't have debug info for the resolver:

  (gdb) p gnu_ifunc (3)
  $1 = 4
  (gdb) PASS: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=0: resolved_debug=1: p gnu_ifunc (3)
                                                       ^^^^^^^^^^^^^^^^

or if the resolver's symbol has a different name from the ifunc (as is
the case with modern uses of ifunc via __attribute__ ifunc, such as
glibc uses):

  (gdb) p gnu_ifunc (3)
  $1 = 4
  (gdb) PASS: gdb.base/gnu-ifunc.exp: resolver_attr=1: resolver_debug=1: resolved_debug=0: p gnu_ifunc (3)
                                      ^^^^^^^^^^^^^^^

in which case after this patch, you can still call the resolver
directly if you want:

  (gdb) p gnu_ifunc_resolver (3)
  $1 = (int (*)(int)) 0x400753 <final>

gdb/ChangeLog:
2018-04-26  Pedro Alves  <palves@redhat.com>

	* c-exp.y (variable production): Prefer ifunc minsyms over
	regular function symbols.
	* symtab.c (find_gnu_ifunc): New function.
	* minsyms.h (lookup_msym_prefer): New enum.
	(lookup_minimal_symbol_by_pc_section): Replace 'want_trampoline'
	parameter by a lookup_msym_prefer parameter.
	* symtab.h (find_gnu_ifunc): New declaration.