Calling ifunc functions when target has no debug info but resolver has

After the previous patch, on Fedora 27 (glibc 2.26), if you try
calling strlen in the inferior, you now get:

  (top-gdb) p strlen ("hello")
  '__strlen_avx2' has unknown return type; cast the call to its declared return type

This is correct, because __strlen_avx2 is written in assembly.

We can improve on this though -- if the final ifunc resolved/target
function has no debug info, but the ifunc _resolver_ does have debug
info, we can try extracting the final function's type from the type
that the resolver returns.  E.g.,:

  typedef size_t (*strlen_t) (const char*);

  size_t my_strlen (const char *) { /* some implementation */ }
  strlen_t strlen_resolver (unsigned long hwcap) { return my_strlen; }

  extern size_t strlen (const char *s);
  __typeof (strlen) strlen __attribute__ ((ifunc ("strlen_resolver")));

In the strlen example above, the resolver returns strlen_t, which is a
typedef for pointer to a function that returns size_t.  "strlen_t" is
the type of both the user-visible "strlen", and of the the target
function that implements it.

This patch teaches GDB to extract that type.

This is done for actual inferior function calls (in infcall.c), and
for ptype (in eval_call).  By the time we get to either of these
places, we've already lost the original symbol/minsym, and only have
values and types to work with.  Hence the changes to c-exp.y and
evaluate_var_msym_value, to ensure that we propagate the ifunc
minsymbol's info.

The change to make ifunc symbols have no/unknown return type exposes a
latent problem -- gdb.compile/compile-ifunc.exp calls a no-debug-info
function, but we did not warn about it.  The test is fixed by this
commit too.

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

	* blockframe.c (find_gnu_ifunc_target_type): New function.
	(find_function_type): New.
	* eval.c (evaluate_var_msym_value): For GNU ifunc types, always
	return a value with a memory address.
	(eval_call): For calls to GNU ifunc functions, try to find the
	type of the target function from the type that the resolver
	returns.
	* gdbtypes.c (objfile_type): Don't install a return type for ifunc
	symbols.
	* infcall.c (find_function_return_type): Delete.
	(find_function_addr): Add 'function_type' parameter.  For calls to
	GNU ifunc functions, try to find the type of the target function
	from the type that the resolver returns, and return it via
	FUNCTION_TYPE.
	(call_function_by_hand_dummy): Adjust to use the function type
	returned by find_function_addr.
	(find_function_addr): Add 'function_type' parameter and move
	description here.
	* symtab.h (find_function_type, find_gnu_ifunc_target_type): New
	declarations.

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

	* gdb.compile/compile-ifunc.exp: Also expect "function has unknown
	return type" warnings.

gdb/infcall.h

@@ -25,8 +25,15 @@
 struct value;
 struct type;
 
+/* Determine a function's address and its return type from its value.
+   If the function is a GNU ifunc, then return the address of the
+   target function, and set *FUNCTION_TYPE to the target function's
+   type, and *RETVAL_TYPE to the target function's return type.
+   Calls error() if the function is not valid for calling.  */
+
 extern CORE_ADDR find_function_addr (struct value *function, 
-				     struct type **retval_type);
+				     struct type **retval_type,
+				     struct type **function_type = NULL);
 
 /* Perform a function call in the inferior.