@matheusbsbs / matheus-bagliano / commits / 0ec79358d0

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>

Nguyễn Thái Ngọc Duy committed Oct 27, 2018 at 19:29 UTC 0ec79358d07db38e5d9e9e8a7c0d78f455763844
3 files changed +94 -4
Makefile
+1 -1
@@ -991,6 +991,7 @@ LIB_OBJS += sub-process.o
991 LIB_OBJS += symlinks.o
992 LIB_OBJS += tag.o
993 LIB_OBJS += tempfile.o
994 +LIB_OBJS += thread-utils.o
995 LIB_OBJS += tmp-objdir.o
996 LIB_OBJS += trace.o
997 LIB_OBJS += trailer.o
@@ -1674,7 +1675,6 @@ ifdef NO_PTHREADS
1675 else
1676 BASIC_CFLAGS += $(PTHREAD_CFLAGS)
1677 EXTLIBS += $(PTHREAD_LIBS)
1677 - LIB_OBJS += thread-utils.o
1678 endif
1679
1680 ifdef HAVE_PATHS_H
thread-utils.c
+48
@@ -20,6 +20,9 @@
20
21 int online_cpus(void)
22 {
23 +#ifdef NO_PTHREADS
24 + return 1;
25 +#else
26 #ifdef _SC_NPROCESSORS_ONLN
27 long ncpus;
28 #endif
@@ -59,10 +62,12 @@ int online_cpus(void)
62 #endif
63
64 return 1;
65 +#endif
66 }
67
68 int init_recursive_mutex(pthread_mutex_t *m)
69 {
70 +#ifndef NO_PTHREADS
71 pthread_mutexattr_t a;
72 int ret;
73
@@ -74,4 +79,47 @@ int init_recursive_mutex(pthread_mutex_t *m)
79 pthread_mutexattr_destroy(&a);
80 }
81 return ret;
82 +#else
83 + return 0;
84 +#endif
85 +}
86 +
87 +#ifdef NO_PTHREADS
88 +int dummy_pthread_create(pthread_t *pthread, const void *attr,
89 + void *(*fn)(void *), void *data)
90 +{
91 + /*
92 + * Do nothing.
93 + *
94 + * The main purpose of this function is to break compiler's
95 + * flow analysis and avoid -Wunused-variable false warnings.
96 + */
97 + return ENOSYS;
98 +}
99 +
100 +int dummy_pthread_init(void *data)
101 +{
102 + /*
103 + * Do nothing.
104 + *
105 + * The main purpose of this function is to break compiler's
106 + * flow analysis or it may realize that functions like
107 + * pthread_mutex_init() is no-op, which means the (static)
108 + * variable is not used/initialized at all and trigger
109 + * -Wunused-variable
110 + */
111 + return ENOSYS;
112 }
113 +
114 +int dummy_pthread_join(pthread_t pthread, void **retval)
115 +{
116 + /*
117 + * Do nothing.
118 + *
119 + * The main purpose of this function is to break compiler's
120 + * flow analysis and avoid -Wunused-variable false warnings.
121 + */
122 + return ENOSYS;
123 +}
124 +
125 +#endif
thread-utils.h
+45 -3
@@ -4,12 +4,54 @@
4 #ifndef NO_PTHREADS
5 #include <pthread.h>
6
7 -extern int online_cpus(void);
8 -extern int init_recursive_mutex(pthread_mutex_t*);
7 +#define HAVE_THREADS 1
8
9 #else
10
12 -#define online_cpus() 1
11 +#define HAVE_THREADS 0
12 +
13 +/*
14 + * macros instead of typedefs because pthread definitions may have
15 + * been pulled in by some system dependencies even though the user
16 + * wants to disable pthread.
17 + */
18 +#define pthread_t int
19 +#define pthread_mutex_t int
20 +#define pthread_cond_t int
21 +#define pthread_key_t int
22 +
23 +#define pthread_mutex_init(mutex, attr) dummy_pthread_init(mutex)
24 +#define pthread_mutex_lock(mutex)
25 +#define pthread_mutex_unlock(mutex)
26 +#define pthread_mutex_destroy(mutex)
27 +
28 +#define pthread_cond_init(cond, attr) dummy_pthread_init(cond)
29 +#define pthread_cond_wait(cond, mutex)
30 +#define pthread_cond_signal(cond)
31 +#define pthread_cond_broadcast(cond)
32 +#define pthread_cond_destroy(cond)
33 +
34 +#define pthread_key_create(key, attr) dummy_pthread_init(key)
35 +#define pthread_key_delete(key)
36 +
37 +#define pthread_create(thread, attr, fn, data) \
38 + dummy_pthread_create(thread, attr, fn, data)
39 +#define pthread_join(thread, retval) \
40 + dummy_pthread_join(thread, retval)
41 +
42 +#define pthread_setspecific(key, data)
43 +#define pthread_getspecific(key) NULL
44 +
45 +int dummy_pthread_create(pthread_t *pthread, const void *attr,
46 + void *(*fn)(void *), void *data);
47 +int dummy_pthread_join(pthread_t pthread, void **retval);
48 +
49 +int dummy_pthread_init(void *);
50
51 #endif
52 +
53 +int online_cpus(void);
54 +int init_recursive_mutex(pthread_mutex_t*);
55 +
56 +
57 #endif /* THREAD_COMPAT_H */