[gdb/symtab] Store external var decls in psymtab

[gdb/symtab] Store external var decls in psymtab

Consider a test-case consisting of source file test.c:
...
extern int aaa;
int
main (void)
{
  return 0;
}
...
and test-2.c:
...
int aaa = 33;
...
compiled with debug info only for test.c:
...
$ gcc -c test.c -g; gcc -c test2.c; gcc test.o test2.o -g
...

When trying to print aaa, we get:
...
$ gdb -batch a.out -ex "print aaa"
'aaa' has unknown type; cast it to its declared type
...
but with -readnow we have:
...
$ gdb -readnow -batch a.out -ex "print aaa"
$1 = 33
...

In the -readnow case, the symbol for aaa in the full symtab has
LOC_UNRESOLVED, and the symbol type is combined with the minimal symbol
address, to read the value and print it without cast.

Without the -readnow, we create partial symbols, but the aaa decl is missing
from the partial symtabs, so we find it only in the minimal symbols, resulting
in the cast request.  If the aaa decl would have been in the partial symtabs,
it would have been found, and the full symtab would have been expanded, after
which things would be as with -readnow.

The function add_partial_symbol has a comment on the LOC_UNRESOLVED +
minimal symbol addres construct at DW_TAG_variable handling:
...
      else if (pdi->is_external)
	{
	  /* Global Variable.
	     Don't enter into the minimal symbol tables as there is
	     a minimal symbol table entry from the ELF symbols already.
	     Enter into partial symbol table if it has a location
	     descriptor or a type.
	     If the location descriptor is missing, new_symbol will create
	     a LOC_UNRESOLVED symbol, the address of the variable will then
	     be determined from the minimal symbol table whenever the variable
	     is referenced.
...
but it's not triggered due to this test in scan_partial_symbols:
...
            case DW_TAG_variable:
	    ...
              if (!pdi->is_declaration)
                {
                  add_partial_symbol (pdi, cu);
                }
...

Fix this in scan_partial_symbols by allowing external variable decls to be
added to the partial symtabs.

Build and reg-tested on x86_64-linux.

The patch caused this regression:
...
(gdb) print a_thread_local^M
Cannot find thread-local storage for process 0, executable file tls/tls:^M
Cannot find thread-local variables on this target^M
(gdb) FAIL: gdb.threads/tls.exp: print a_thread_local
...
while without the patch we have:
...
(gdb) print a_thread_local^M
Cannot read `a_thread_local' without registers^M
(gdb) PASS: gdb.threads/tls.exp: print a_thread_local
...

However, without the patch but with -readnow we have the same FAIL as with the
patch (filed as PR25807).  In other words, the patch has the effect that we
get the same result with and without -readnow.

This can be explained as follows.  Without the patch, and without -readnow, we
have two a_thread_locals, the def and the decl:
...
$ gdb -batch outputs/gdb.threads/tls/tls \
    -ex "maint expand-symtabs" \
    -ex "print a_thread_local" \
    -ex "maint print symbols" \
    | grep "a_thread_local;"
Cannot read `a_thread_local' without registers
 int a_thread_local; computed at runtime
 int a_thread_local; unresolved
...
while without the patch and with -readnow, we have the opposite order:
...
$ gdb -readnow -batch outputs/gdb.threads/tls/tls  \
    -ex "maint expand-symtabs" \
    -ex "print a_thread_local" \
    -ex "maint print symbols" \
    | grep "a_thread_local;"
Cannot find thread-local storage for process 0, executable file tls/tls:
Cannot find thread-local variables on this target
 int a_thread_local; unresolved
 int a_thread_local; computed at runtime
...

With the patch we have the same order with and without -readnow, but just a
different one than before without -readnow.

Mark the "Cannot find thread-local variables on this target" variant a PR25807
kfail.

gdb/ChangeLog:

2020-04-22  Tom de Vries  <tdevries@suse.de>

	PR symtab/25764
	* dwarf2/read.c (scan_partial_symbols): Allow external variable decls
	in psymtabs.

gdb/testsuite/ChangeLog:

2020-04-22  Tom de Vries  <tdevries@suse.de>

	PR symtab/25764
	* gdb.base/psym-external-decl-2.c: New test.
	* gdb.base/psym-external-decl.c: New test.
	* gdb.base/psym-external-decl.exp: New file.
	* gdb.threads/tls.exp: Add PR25807 kfail.