build: Add generic compiler builtins support

Wrap compiler builtins into shared functions with proper generic
implementations. __builtin_clz() isn't wrapped for now because its use
by screenshooter is pretty specific. It will be properly wrapped in
the next commit which needs a round up to the next power of 2
function.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>

clients/simple-dmabuf-v4l.c

@@ -88,7 +88,7 @@ static inline const char *
 dump_format(uint32_t format, char out[4])
 {
 #if BYTE_ORDER == BIG_ENDIAN
-	format = __builtin_bswap32(format);
+	format = bswap32(format);
 #endif
 	memcpy(out, &format, 4);
 	return out;

libweston/data-device.c

@@ -201,7 +201,7 @@ data_offer_set_actions(struct wl_client *client,
 
 	if (preferred_action &&
 	    (!(preferred_action & dnd_actions) ||
-	     __builtin_popcount(preferred_action) > 1)) {
+	     bitcount32(preferred_action) > 1)) {
 		wl_resource_post_error(offer->resource,
 				       WL_DATA_OFFER_ERROR_INVALID_ACTION,
 				       "invalid action %x", preferred_action);

libweston/renderer-gl/gl-renderer.c

@@ -242,7 +242,7 @@ static inline const char *
 dump_format(uint32_t format, char out[4])
 {
 #if BYTE_ORDER == BIG_ENDIAN
-	format = __builtin_bswap32(format);
+	format = bswap32(format);
 #endif
 	memcpy(out, &format, 4);
 	return out;

libweston/screenshooter.c

@@ -252,6 +252,9 @@ static uint32_t *
 output_run(uint32_t *p, uint32_t delta, int run)
 {
 	int i;
+#if !defined(HAVE_BUILTIN_CLZ)
+	int tmp;
+#endif
 
 	while (run > 0) {
 		if (run <= 0xe0) {
@@ -259,7 +262,11 @@ output_run(uint32_t *p, uint32_t delta, int run)
 			break;
 		}
 
+#if defined(HAVE_BUILTIN_CLZ)
 		i = 24 - __builtin_clz(run);
+#else
+		for (i = 0, tmp = u >> 8; tmp; i++, tmp >>= 1);
+#endif
 		*p++ = delta | ((i + 0xe0) << 24);
 		run -= 1 << (7 + i);
 	}

meson.build

@@ -94,6 +94,17 @@ foreach hdr : optional_system_headers
 	endif
 endforeach
 
+optional_builtins = {
+	'builtin_clz': 'return __builtin_clz(1);',
+	'builtin_bswap32': 'return __builtin_bswap32(0);',
+	'builtin_popcount': 'return __builtin_popcount(0);',
+}
+foreach name, check : optional_builtins
+	if cc.links('int main(void) { @0@ }'.format(check), name: name)
+		config_h.set('HAVE_' + name.to_upper(), 1)
+	endif
+endforeach
+
 env_modmap = ''
 
 config_h.set('_GNU_SOURCE', '1')

shared/helpers.h

@@ -22,6 +22,8 @@
 #ifndef WESTON_HELPERS_H
 #define WESTON_HELPERS_H
 
+#include "config.h"
+
 #include <stdint.h>
 
 #ifdef  __cplusplus
@@ -233,6 +235,46 @@ do {                        \
 #define unreachable(str) assert(!str)
 #endif
 
+/**
+ * Returns number of bits set in 32-bit value x.
+ *
+ * @param x a 32-bit value.
+ * @return the number of bits set.
+ */
+static inline int
+bitcount32(uint32_t x)
+{
+#if defined(HAVE_BUILTIN_POPCOUNT)
+	return __builtin_popcount(x);
+#else
+	int n;
+
+	for (n = 0; x; n++)
+		x &= x - 1;
+
+	return n;
+#endif
+}
+
+/**
+ * Returns 32-bit value x in reversed byte order.
+ *
+ * @param x a 32-bit value.
+ * @return the reversed 32-bit value.
+ */
+static inline uint32_t
+bswap32(uint32_t x)
+{
+#if defined(HAVE_BUILTIN_BSWAP32)
+	return __builtin_bswap32(x);
+#else
+	return (x >> 24) |
+		((x >> 8) & 0x0000ff00) |
+		((x << 8) & 0x00ff0000) |
+		(x << 24);
+#endif
+}
+
 #ifdef  __cplusplus
 }
 #endif