diff --git a/Makefile b/Makefile
index a6bba79ba1f46a..6a0a1c958fa2a1 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-CFLAGS=-g
+CFLAGS=-g -O3 -Wall
 CC=gcc
 
 PROG=update-cache show-diff init-db write-tree read-tree commit-tree cat-file
diff --git a/cat-file.c b/cat-file.c
index 3071672c6ce2ff..3829fb6097ad98 100644
--- a/cat-file.c
+++ b/cat-file.c
@@ -11,7 +11,6 @@ int main(int argc, char **argv)
 	char type[20];
 	void *buf;
 	unsigned long size;
-	int fd;
 
 	if (argc != 3 || get_sha1_hex(argv[2], sha1))
 		usage("cat-file: cat-file [-t | tagname] <sha1>");
diff --git a/init-db.c b/init-db.c
index bc65086368b44f..1fcef0b373ccec 100644
--- a/init-db.c
+++ b/init-db.c
@@ -8,7 +8,7 @@
 int main(int argc, char **argv)
 {
 	char *sha1_dir = getenv(DB_ENVIRONMENT), *path;
-	int len, i, fd;
+	int len, i;
 
 	if (mkdir(".dircache", 0700) < 0) {
 		perror("unable to create .dircache");
@@ -25,7 +25,7 @@ int main(int argc, char **argv)
 	if (sha1_dir) {
 		struct stat st;
 		if (!stat(sha1_dir, &st) < 0 && S_ISDIR(st.st_mode))
-			return;
+			return 0;
 		fprintf(stderr, "DB_ENVIRONMENT set to bad directory %s: ", sha1_dir);
 	}
 
diff --git a/read-cache.c b/read-cache.c
index b151981c1b3d15..60a0b5b2e78f40 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -89,7 +89,7 @@ void * read_sha1_file(unsigned char *sha1, char *type, unsigned long *size)
 	z_stream stream;
 	char buffer[8192];
 	struct stat st;
-	int i, fd, ret, bytes;
+	int fd, ret, bytes;
 	void *map, *buf;
 	char *filename = sha1_file_name(sha1);
 
@@ -173,7 +173,7 @@ int write_sha1_file(char *buf, unsigned len)
 int write_sha1_buffer(unsigned char *sha1, void *buf, unsigned int size)
 {
 	char *filename = sha1_file_name(sha1);
-	int i, fd;
+	int fd;
 
 	fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0666);
 	if (fd < 0)
@@ -228,6 +228,7 @@ int read_cache(void)
 	if (fd < 0)
 		return (errno == ENOENT) ? 0 : error("open failed");
 
+	size = 0; // avoid gcc warning
 	map = (void *)-1;
 	if (!fstat(fd, &st)) {
 		map = NULL;
diff --git a/read-tree.c b/read-tree.c
index 3785e259756b4b..af486ff33f6f66 100644
--- a/read-tree.c
+++ b/read-tree.c
@@ -72,7 +72,6 @@ static int unpack(unsigned char *sha1)
 
 int main(int argc, char **argv)
 {
-	int fd;
 	unsigned char sha1[20];
 
 	if (argc != 2)
diff --git a/show-diff.c b/show-diff.c
index e7aa60b4ab92ab..3c6d4f98b48634 100644
--- a/show-diff.c
+++ b/show-diff.c
@@ -60,7 +60,6 @@ int main(int argc, char **argv)
 		struct stat st;
 		struct cache_entry *ce = active_cache[i];
 		int n, changed;
-		unsigned int mode;
 		unsigned long size;
 		char type[20];
 		void *new;
diff --git a/update-cache.c b/update-cache.c
index 0bcd5d40819133..413e09d48fa0aa 100644
--- a/update-cache.c
+++ b/update-cache.c
@@ -50,6 +50,7 @@ static int remove_file_from_cache(char *path)
 		if (pos < active_nr)
 			memmove(active_cache + pos, active_cache + pos + 1, (active_nr - pos - 1) * sizeof(struct cache_entry *));
 	}
+	return 0;
 }
 
 static int add_cache_entry(struct cache_entry *ce)
@@ -250,4 +251,5 @@ int main(int argc, char **argv)
 		return 0;
 out:
 	unlink(".dircache/index.lock");
+	return 0;
 }
diff --git a/write-tree.c b/write-tree.c
index 012eeeff1b5dd9..6e531b40a84914 100644
--- a/write-tree.c
+++ b/write-tree.c
@@ -31,7 +31,7 @@ static int prepend_integer(char *buffer, unsigned val, int i)
 
 int main(int argc, char **argv)
 {
-	unsigned long size, offset, val;
+	unsigned long size, offset;
 	int i, entries = read_cache();
 	char *buffer;