thread-utils: macros to unconditionally compile pthreads API

When built with NO_PTHREADS, the macros are used make the code build
even though pthreads header and library may be missing. The code can
still have different code paths for no threads support with
HAVE_THREADS variable.

There are of course impacts on no-pthreads builds:

- data structure may get slightly bigger because all the mutexes and
  pthread_t are present (as an int)

- code execution is not impacted much. Locking (in hot path) is
  no-op. Other wrapper function calls really should not matter much.

- the binary size grows bigger because of threaded code. But at least
  on Linux this does not matter, if some code is not executed, it's
  not mapped in memory.

This is a preparation step to remove "#ifdef NO_PTHREADS" in the code
mostly because of maintainability. As Jeff put it

> it's probably OK to stop thinking of it as "non-threaded platforms
> are the default and must pay zero cost" and more as "threaded
> platforms are the default, and non-threaded ones are OK to pay a
> small cost as long as they still work".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Makefile b/Makefile
index b08d5ea258c69a78745dfa73fe698c11d021858a..321540a736f1253ed2f469e3ca8caea356cd0d7b 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -991,6 +991,7 @@ LIB_OBJS += sub-process.o
 LIB_OBJS += symlinks.o
 LIB_OBJS += tag.o
 LIB_OBJS += tempfile.o
+LIB_OBJS += thread-utils.o
 LIB_OBJS += tmp-objdir.o
 LIB_OBJS += trace.o
 LIB_OBJS += trailer.o
@@ -1674,7 +1675,6 @@ ifdef NO_PTHREADS
 else
        BASIC_CFLAGS += $(PTHREAD_CFLAGS)
        EXTLIBS += $(PTHREAD_LIBS)
-       LIB_OBJS += thread-utils.o
 endif
 
 ifdef HAVE_PATHS_H

diff --git a/thread-utils.c b/thread-utils.c
index a2135e0743ac60d8d4512210a81bac2102948fa1..532984569132de11f6f05b561f817817523a3468 100644 (file)
--- a/thread-utils.c
+++ b/thread-utils.c
@@ -20,6 +20,9 @@
 
 int online_cpus(void)
 {
+#ifdef NO_PTHREADS
+       return 1;
+#else
 #ifdef _SC_NPROCESSORS_ONLN
        long ncpus;
 #endif
@@ -59,10 +62,12 @@ int online_cpus(void)
 #endif
 
        return 1;
+#endif
 }
 
 int init_recursive_mutex(pthread_mutex_t *m)
 {
+#ifndef NO_PTHREADS
        pthread_mutexattr_t a;
        int ret;
 
@@ -74,4 +79,47 @@ int init_recursive_mutex(pthread_mutex_t *m)
                pthread_mutexattr_destroy(&a);
        }
        return ret;
+#else
+       return 0;
+#endif
+}
+
+#ifdef NO_PTHREADS
+int dummy_pthread_create(pthread_t *pthread, const void *attr,
+                        void *(*fn)(void *), void *data)
+{
+       /*
+        * Do nothing.
+        *
+        * The main purpose of this function is to break compiler's
+        * flow analysis and avoid -Wunused-variable false warnings.
+        */
+       return ENOSYS;
+}
+
+int dummy_pthread_init(void *data)
+{
+       /*
+        * Do nothing.
+        *
+        * The main purpose of this function is to break compiler's
+        * flow analysis or it may realize that functions like
+        * pthread_mutex_init() is no-op, which means the (static)
+        * variable is not used/initialized at all and trigger
+        * -Wunused-variable
+        */
+       return ENOSYS;
 }
+
+int dummy_pthread_join(pthread_t pthread, void **retval)
+{
+       /*
+        * Do nothing.
+        *
+        * The main purpose of this function is to break compiler's
+        * flow analysis and avoid -Wunused-variable false warnings.
+        */
+       return ENOSYS;
+}
+
+#endif

diff --git a/thread-utils.h b/thread-utils.h
index d9a769d19084587b397c50e9420843be38e89903..4961487ed914f4fea817df26904109bb05f2a2c5 100644 (file)
--- a/thread-utils.h
+++ b/thread-utils.h
@@ -4,12 +4,54 @@
 #ifndef NO_PTHREADS
 #include <pthread.h>
 
-extern int online_cpus(void);
-extern int init_recursive_mutex(pthread_mutex_t*);
+#define HAVE_THREADS 1
 
 #else
 
-#define online_cpus() 1
+#define HAVE_THREADS 0
+
+/*
+ * macros instead of typedefs because pthread definitions may have
+ * been pulled in by some system dependencies even though the user
+ * wants to disable pthread.
+ */
+#define pthread_t int
+#define pthread_mutex_t int
+#define pthread_cond_t int
+#define pthread_key_t int
+
+#define pthread_mutex_init(mutex, attr) dummy_pthread_init(mutex)
+#define pthread_mutex_lock(mutex)
+#define pthread_mutex_unlock(mutex)
+#define pthread_mutex_destroy(mutex)
+
+#define pthread_cond_init(cond, attr) dummy_pthread_init(cond)
+#define pthread_cond_wait(cond, mutex)
+#define pthread_cond_signal(cond)
+#define pthread_cond_broadcast(cond)
+#define pthread_cond_destroy(cond)
+
+#define pthread_key_create(key, attr) dummy_pthread_init(key)
+#define pthread_key_delete(key)
+
+#define pthread_create(thread, attr, fn, data) \
+       dummy_pthread_create(thread, attr, fn, data)
+#define pthread_join(thread, retval) \
+       dummy_pthread_join(thread, retval)
+
+#define pthread_setspecific(key, data)
+#define pthread_getspecific(key) NULL
+
+int dummy_pthread_create(pthread_t *pthread, const void *attr,
+                        void *(*fn)(void *), void *data);
+int dummy_pthread_join(pthread_t pthread, void **retval);
+
+int dummy_pthread_init(void *);
 
 #endif
+
+int online_cpus(void);
+int init_recursive_mutex(pthread_mutex_t*);
+
+
 #endif /* THREAD_COMPAT_H */