Use __builtin_popcount if available to replace bit_count()

If the compiler knows of a better method for counting the number of
bits set in a word for the target CPU, let it use that, instead of the
classic algorithm optimized for PDP-6.

Based on xorg/lib/libxext@490a25e6f8a4d2482af4364c700b68ad11a4d10b

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/test/rendercheck/-/merge_requests/4>

2 files changed, 24 insertions, 15 deletions

diff --git a/main.c b/main.c
index 5ec8090..c18c2d1 100644
--- a/main.c
+++ b/main.c
@@ -42,18 +42,6 @@ int pixmap_move_iter = 1;
 int win_width = 40;
 int win_height = 200;
 
-int
-bit_count(int i)
-{
-	int count;
-
-	count = (i >> 1) & 033333333333;
-	count = i - count - ((count >> 1) & 033333333333);
-	count = (((count + (count >> 3)) & 030707070707) % 077);
-	/* HAKMEM 169 */
-	return count;
-}
-
 /* This is not complete, but decent enough for now.*/
 void
 describe_format(char **desc, const char *prefix, XRenderPictFormat *format)
diff --git a/rendercheck.h b/rendercheck.h
index 30ced40..d8c5e41 100644
--- a/rendercheck.h
+++ b/rendercheck.h
@@ -49,6 +49,30 @@ static inline void errx(int eval, const char *fmt, ...)
 #define max(a, b) (a > b ? a : b)
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
 
+#ifndef __has_builtin
+# define __has_builtin(x) 0     /* Compatibility with older compilers */
+#endif
+
+#if __has_builtin(__builtin_popcount)
+# define bit_count __builtin_popcount
+#else
+/*
+ * Count the number of bits set to 1 in a 32-bit word.
+ * Algorithm from MIT AI Lab Memo 239: "HAKMEM", ITEM 169.
+ * https://dspace.mit.edu/handle/1721.1/6086
+ */
+static inline int
+bit_count(int i)
+{
+	int count;
+
+	count = (i >> 1) & 033333333333;
+	count = i - count - ((count >> 1) & 033333333333);
+	count = (((count + (count >> 3)) & 030707070707) % 077);
+	return count;
+}
+#endif
+
 typedef struct _color4d
 {
 	double r, g, b, a;
@@ -165,9 +189,6 @@ extern int num_colors;
 void
 describe_format(char **desc, const char *prefix, XRenderPictFormat *format);
 
-int
-bit_count(int i);
-
 void
 print_tests(FILE *file, int tests);