Warn if a global function has no previous declaration

If a global function has no previous declaration, either the declaration
is missing in the corresponding header file or the function should be
declared static. Static functions are local to the translation unit,
which allows the compiler to apply some optimizations earlier (when
compiling the translation unit rather than during link-time
optimization).

The commit enables the warning for gcc, clang, and mingw. It also fixes
the reported warnings by declaring the functions static or by adding a
header file (benchmark.h).

closes https://github.com/official-stockfish/Stockfish/pull/4325

No functional change

src/Makefile

@@ -366,7 +366,7 @@ endif
 ifeq ($(COMP),gcc)
 	comp=gcc
 	CXX=g++
-	CXXFLAGS += -pedantic -Wextra -Wshadow
+	CXXFLAGS += -pedantic -Wextra -Wshadow -Wmissing-declarations
 
 	ifeq ($(arch),$(filter $(arch),armv7 armv8 riscv64))
 		ifeq ($(OS),Android)
@@ -410,7 +410,7 @@ ifeq ($(COMP),mingw)
 			CXX=i686-w64-mingw32-c++-posix
 		endif
 	endif
-	CXXFLAGS += -pedantic -Wextra -Wshadow
+	CXXFLAGS += -pedantic -Wextra -Wshadow -Wmissing-declarations
 endif
 
 ifeq ($(COMP),icc)
@@ -426,7 +426,7 @@ ifeq ($(COMP),clang)
 		CXX=x86_64-w64-mingw32-clang++
 	endif
 
-	CXXFLAGS += -pedantic -Wextra -Wshadow
+	CXXFLAGS += -pedantic -Wextra -Wshadow -Wmissing-prototypes
 
 	ifeq ($(filter $(KERNEL),Darwin OpenBSD FreeBSD),)
 	ifeq ($(target_windows),)

src/benchmark.cpp

@@ -16,6 +16,8 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include "benchmark.h"
+
 #include <fstream>
 #include <iostream>
 #include <istream>

src/benchmark.h

@@ -0,0 +1,34 @@
+/*
+  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
+  Copyright (C) 2004-2023 The Stockfish developers (see AUTHORS file)
+
+  Stockfish 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.
+
+  Stockfish 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/>.
+*/
+
+#ifndef BENCHMARK_H_INCLUDED
+#define BENCHMARK_H_INCLUDED
+
+#include <iosfwd>
+#include <string>
+#include <vector>
+
+namespace Stockfish {
+
+class Position;
+
+std::vector<std::string> setup_bench(const Position&, std::istream&);
+
+} // namespace Stockfish
+
+#endif // #ifndef BENCHMARK_H_INCLUDED

src/evaluate.cpp

@@ -159,24 +159,24 @@ namespace Trace {
 
   Score scores[TERM_NB][COLOR_NB];
 
-  double to_cp(Value v) { return double(v) / UCI::NormalizeToPawnValue; }
+  static double to_cp(Value v) { return double(v) / UCI::NormalizeToPawnValue; }
 
-  void add(int idx, Color c, Score s) {
+  static void add(int idx, Color c, Score s) {
     scores[idx][c] = s;
   }
 
-  void add(int idx, Score w, Score b = SCORE_ZERO) {
+  static void add(int idx, Score w, Score b = SCORE_ZERO) {
     scores[idx][WHITE] = w;
     scores[idx][BLACK] = b;
   }
 
-  std::ostream& operator<<(std::ostream& os, Score s) {
+  static std::ostream& operator<<(std::ostream& os, Score s) {
     os << std::setw(5) << to_cp(mg_value(s)) << " "
        << std::setw(5) << to_cp(eg_value(s));
     return os;
   }
 
-  std::ostream& operator<<(std::ostream& os, Term t) {
+  static std::ostream& operator<<(std::ostream& os, Term t) {
 
     if (t == MATERIAL || t == IMBALANCE || t == WINNABLE || t == TOTAL)
         os << " ----  ----"    << " | " << " ----  ----";

src/misc.cpp

@@ -517,7 +517,7 @@ void bindThisThread(size_t) {}
 /// API and returns the best node id for the thread with index idx. Original
 /// code from Texel by Peter Österlund.
 
-int best_node(size_t idx) {
+static int best_node(size_t idx) {
 
   int threads = 0;
   int nodes = 0;

src/nnue/evaluate_nnue.cpp

@@ -83,7 +83,7 @@ namespace Stockfish::Eval::NNUE {
   }  // namespace Detail
 
   // Initialize the evaluation function parameters
-  void initialize() {
+  static void initialize() {
 
     Detail::initialize(featureTransformer);
     for (std::size_t i = 0; i < LayerStacks; ++i)
@@ -91,7 +91,7 @@ namespace Stockfish::Eval::NNUE {
   }
 
   // Read network header
-  bool read_header(std::istream& stream, std::uint32_t* hashValue, std::string* desc)
+  static bool read_header(std::istream& stream, std::uint32_t* hashValue, std::string* desc)
   {
     std::uint32_t version, size;
 
@@ -105,7 +105,7 @@ namespace Stockfish::Eval::NNUE {
   }
 
   // Write network header
-  bool write_header(std::ostream& stream, std::uint32_t hashValue, const std::string& desc)
+  static bool write_header(std::ostream& stream, std::uint32_t hashValue, const std::string& desc)
   {
     write_little_endian<std::uint32_t>(stream, Version);
     write_little_endian<std::uint32_t>(stream, hashValue);
@@ -115,7 +115,7 @@ namespace Stockfish::Eval::NNUE {
   }
 
   // Read network parameters
-  bool read_parameters(std::istream& stream) {
+  static bool read_parameters(std::istream& stream) {
 
     std::uint32_t hashValue;
     if (!read_header(stream, &hashValue, &netDescription)) return false;
@@ -127,7 +127,7 @@ namespace Stockfish::Eval::NNUE {
   }
 
   // Write network parameters
-  bool write_parameters(std::ostream& stream) {
+  static bool write_parameters(std::ostream& stream) {
 
     if (!write_header(stream, HashValue, netDescription)) return false;
     if (!Detail::write_parameters(stream, *featureTransformer)) return false;

src/uci.cpp

@@ -22,6 +22,7 @@
 #include <sstream>
 #include <string>
 
+#include "benchmark.h"
 #include "evaluate.h"
 #include "movegen.h"
 #include "position.h"
@@ -36,8 +37,6 @@ using namespace std;
 
 namespace Stockfish {
 
-vector<string> setup_bench(const Position&, istream&);
-
 namespace {
 
   // FEN string for the initial position in standard chess

src/ucioption.cpp

@@ -38,13 +38,13 @@ UCI::OptionsMap Options; // Global object
 namespace UCI {
 
 /// 'On change' actions, triggered by an option's value change
-void on_clear_hash(const Option&) { Search::clear(); }
-void on_hash_size(const Option& o) { TT.resize(size_t(o)); }
-void on_logger(const Option& o) { start_logger(o); }
-void on_threads(const Option& o) { Threads.set(size_t(o)); }
-void on_tb_path(const Option& o) { Tablebases::init(o); }
-void on_use_NNUE(const Option& ) { Eval::NNUE::init(); }
-void on_eval_file(const Option& ) { Eval::NNUE::init(); }
+static void on_clear_hash(const Option&) { Search::clear(); }
+static void on_hash_size(const Option& o) { TT.resize(size_t(o)); }
+static void on_logger(const Option& o) { start_logger(o); }
+static void on_threads(const Option& o) { Threads.set(size_t(o)); }
+static void on_tb_path(const Option& o) { Tablebases::init(o); }
+static void on_use_NNUE(const Option&) { Eval::NNUE::init(); }
+static void on_eval_file(const Option&) { Eval::NNUE::init(); }
 
 /// Our case insensitive less() function as required by UCI protocol
 bool CaseInsensitiveLess::operator() (const string& s1, const string& s2) const {