osdep: add MP_UNREACHABLE

This seems to work on gcc, clang and mingw as-is, but I made it
conditional on __GNUC__ just in case, even though I can't figure out
which compilers we care about that don't export this define.

Also replace all instances of assert(0) in the code by MP_UNREACHABLE(),
which is a strict improvement.

demux/demux.c

@@ -1506,7 +1506,7 @@ static void find_backward_restart_pos(struct demux_stream *ds)
     // Or if preroll is involved, the first preroll packet.
     while (ds->reader_head != target) {
         if (!advance_reader_head(ds))
-            assert(0); // target must be in list
+            MP_UNREACHABLE(); // target must be in list
     }
 
     double seek_pts;

demux/demux_mkv.c

@@ -2872,7 +2872,7 @@ static int read_next_block_into_queue(demuxer_t *demuxer)
         if (mkv_d->cluster_end != EBML_UINT_INVALID)
             mkv_d->cluster_end += stream_tell(s);
     }
-    assert(0); // unreachable
+    MP_UNREACHABLE();
 
 add_block:
     index_block(demuxer, &block);

filters/f_lavfi.c

@@ -394,7 +394,7 @@ static bool init_pads(struct lavfi *c)
         } else if (pad->type == MP_FRAME_VIDEO) {
             dst_filter = avfilter_get_by_name("buffersink");
         } else {
-            assert(0);
+            MP_UNREACHABLE();
         }
 
         if (!dst_filter)
@@ -484,7 +484,7 @@ static bool init_pads(struct lavfi *c)
             params->frame_rate = av_d2q(fmt->nominal_fps, 1000000);
             filter_name = "buffer";
         } else {
-            assert(0);
+            MP_UNREACHABLE();
         }
 
         params->time_base = pad->timebase;

filters/f_utils.c

@@ -250,7 +250,7 @@ static void fixed_aframe_size_process(struct mp_filter *f)
         int in_samples = mp_aframe_get_size(p->in);
         int copy = MPMIN(in_samples, p->samples - p->out_written);
         if (!mp_aframe_copy_samples(p->out, p->out_written, p->in, 0, copy))
-            assert(0);
+            MP_UNREACHABLE();
         mp_aframe_skip_samples(p->in, copy);
         p->out_written += copy;
     }

filters/filter.c

@@ -372,7 +372,7 @@ static struct mp_pin *find_connected_end(struct mp_pin *p)
             return other;
         p = other->user_conn;
     }
-    assert(0);
+    MP_UNREACHABLE();
 }
 
 // With p being part of a connection, create the pin_connection and set all

misc/thread_pool.c

@@ -105,7 +105,7 @@ static void *worker_thread(void *arg)
                 return NULL;
             }
         }
-        assert(0);
+        MP_UNREACHABLE();
     }
 
     pthread_mutex_unlock(&pool->lock);

options/m_config_core.c

@@ -879,7 +879,7 @@ void mp_read_option_raw(struct mpv_global *global, const char *name,
         }
     }
 
-    assert(0); // not found
+    MP_UNREACHABLE(); // not found
 }
 
 static const struct m_config_group *find_group(struct mpv_global *global,

options/parse_configfile.c

@@ -176,7 +176,7 @@ static bstr read_file(struct mp_log *log, const char *filename)
         }
         size += s;
     }
-    assert(0);
+    MP_UNREACHABLE();
 }
 
 // Load options and profiles from from a config file.

osdep/compiler.h

@@ -23,4 +23,10 @@
 #define alignof(x) (offsetof(struct {char unalign_; x u;}, u))
 #endif
 
+#ifdef __GNUC__
+#define MP_UNREACHABLE() (assert(!"unreachable"), __builtin_unreachable())
+#else
+#define MP_UNREACHABLE() (assert(!"unreachable"), abort())
+#endif
+
 #endif

player/command.c

@@ -155,7 +155,7 @@ static void hook_remove(struct MPContext *mpctx, struct hook_handler *h)
             return;
         }
     }
-    assert(0);
+    MP_UNREACHABLE();
 }
 
 bool mp_hook_test_completion(struct MPContext *mpctx, char *type)

video/mp_image.c

@@ -593,7 +593,7 @@ static bool endian_swap_bytes(void *d, size_t bytes, size_t word_size)
             AV_WL32(ud + x * 2, AV_RB32(ud + x * 2));
         break;
     default:
-        assert(0);
+        MP_UNREACHABLE();
     }
 
     return true;

video/out/gpu/video.c

@@ -4237,7 +4237,7 @@ static void gl_video_dr_free_buffer(void *opaque, uint8_t *data)
         }
     }
     // not found - must not happen
-    assert(0);
+    MP_UNREACHABLE();
 }
 
 struct mp_image *gl_video_get_image(struct gl_video *p, int imgfmt, int w, int h,

video/repack.c

@@ -169,7 +169,7 @@ static void swap_endian(struct mp_image *dst, int dst_x, int dst_y,
                     ((uint32_t *)d)[x] = av_bswap32(((uint32_t *)s)[x]);
                 break;
             default:
-                assert(0);
+                MP_UNREACHABLE();
             }
         }
     }