[gdb/symtab] Prefer def over decl (inter-CU case)

When running test-case gdb.threads/tls.exp with target board -readnow, we
have:
...
(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 with native 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
...

The difference in behaviour can be explained as follows.  Without -readnow, we
have two a_thread_locals, the def and the decl, each in a different CU:
...
$ 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
...
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
...

Fix the FAIL by preferring the def over the decl (something we already do
intra-CU since the fix for PR24971, commit 93e55f0 "[gdb/symtab] Prefer var
def over decl").

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-04-23  Tom de Vries  <[email protected]>

	PR symtab/25807
	* block.c (best_symbol, better_symbol): Promote to external.
	* block.h (best_symbol, better_symbol): Declare.
	* symtab.c (lookup_symbol_in_objfile_symtabs): Prefer def over
	decl.

gdb/testsuite/ChangeLog:

2020-04-23  Tom de Vries  <[email protected]>

	* gdb.base/decl-before-def-decl.c: New test.
	* gdb.base/decl-before-def-def.c: New test.
	* gdb.base/decl-before-def.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2020-04-23  Tom de Vries  <[email protected]>
+
+	PR symtab/25807
+	* block.c (best_symbol, better_symbol): Promote to external.
+	* block.h (best_symbol, better_symbol): Declare.
+	* symtab.c (lookup_symbol_in_objfile_symtabs): Prefer def over
+	decl.
+
 2020-04-23  Tom Tromey  <[email protected]>
 
 	PR ada/25837:

gdb/block.c

@@ -657,19 +657,18 @@ block_iter_match_next (const lookup_name_info &name,
   return block_iter_match_step (iterator, name, 0);
 }
 
-/* Return true if symbol A is the best match possible for DOMAIN.  */
+/* See block.h.  */
 
-static bool
+bool
 best_symbol (struct symbol *a, const domain_enum domain)
 {
   return (SYMBOL_DOMAIN (a) == domain
 	  && SYMBOL_CLASS (a) != LOC_UNRESOLVED);
 }
 
-/* Return symbol B if it is a better match than symbol A for DOMAIN.
-   Otherwise return A.  */
+/* See block.h.  */
 
-static struct symbol *
+struct symbol *
 better_symbol (struct symbol *a, struct symbol *b, const domain_enum domain)
 {
   if (a == NULL)

gdb/block.h

@@ -342,6 +342,16 @@ extern struct symbol *block_iter_match_first (const struct block *block,
 extern struct symbol *block_iter_match_next
   (const lookup_name_info &name, struct block_iterator *iterator);
 
+/* Return true if symbol A is the best match possible for DOMAIN.  */
+
+extern bool best_symbol (struct symbol *a, const domain_enum domain);
+
+/* Return symbol B if it is a better match than symbol A for DOMAIN.
+   Otherwise return A.  */
+
+extern struct symbol *better_symbol (struct symbol *a, struct symbol *b,
+				     const domain_enum domain);
+
 /* Search BLOCK for symbol NAME in DOMAIN.  */
 
 extern struct symbol *block_lookup_symbol (const struct block *block,

gdb/symtab.c

@@ -2285,6 +2285,8 @@ lookup_symbol_in_objfile_symtabs (struct objfile *objfile,
 			  name, domain_name (domain));
     }
 
+  struct block_symbol other;
+  other.symbol = NULL;
   for (compunit_symtab *cust : objfile->compunits ())
     {
       const struct blockvector *bv;
@@ -2295,18 +2297,36 @@ lookup_symbol_in_objfile_symtabs (struct objfile *objfile,
       block = BLOCKVECTOR_BLOCK (bv, block_index);
       result.symbol = block_lookup_symbol_primary (block, name, domain);
       result.block = block;
-      if (result.symbol != NULL)
+      if (result.symbol == NULL)
+	continue;
+      if (best_symbol (result.symbol, domain))
 	{
-	  if (symbol_lookup_debug > 1)
+	  other = result;
+	  break;
+	}
+      if (symbol_matches_domain (result.symbol->language (),
+				 SYMBOL_DOMAIN (result.symbol), domain))
+	{
+	  struct symbol *better
+	    = better_symbol (other.symbol, result.symbol, domain);
+	  if (better != other.symbol)
 	    {
-	      fprintf_unfiltered (gdb_stdlog, " = %s (block %s)\n",
-				  host_address_to_string (result.symbol),
-				  host_address_to_string (block));
+	      other.symbol = better;
+	      other.block = block;
 	    }
-	  result.symbol = fixup_symbol_section (result.symbol, objfile);
-	  return result;
+	}
+    }
 
+  if (other.symbol != NULL)
+    {
+      if (symbol_lookup_debug > 1)
+	{
+	  fprintf_unfiltered (gdb_stdlog, " = %s (block %s)\n",
+			      host_address_to_string (other.symbol),
+			      host_address_to_string (other.block));
 	}
+      other.symbol = fixup_symbol_section (other.symbol, objfile);
+      return other;
     }
 
   if (symbol_lookup_debug > 1)

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-23  Tom de Vries  <[email protected]>
+
+	* gdb.base/decl-before-def-decl.c: New test.
+	* gdb.base/decl-before-def-def.c: New test.
+	* gdb.base/decl-before-def.exp: New file.
+
 2020-04-23  Tom de Vries  <[email protected]>
 
 	* gdb.base/readnever.exp: Skip if GDBFLAGS contain -readnow/--readnow.

gdb/testsuite/gdb.base/decl-before-def-decl.c

@@ -0,0 +1,25 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+extern int a[];
+
+int
+main (void)
+{
+  a[0] = 1;
+  return 0;
+}

gdb/testsuite/gdb.base/decl-before-def-def.c

@@ -0,0 +1,18 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int a[2] = { 1, 2 };

gdb/testsuite/gdb.base/decl-before-def.exp

@@ -0,0 +1,26 @@
+# Copyright 2020 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+standard_testfile decl-before-def-decl.c decl-before-def-def.c
+set sources [list $srcfile $srcfile2]
+
+if {[prepare_for_testing "failed to prepare" $testfile $sources]} {
+    return -1
+}
+
+# This is required due to PR25764.
+gdb_test "maint expand-symtabs"
+
+gdb_test "p a" { = \{1, 2\}}