Merge pull request #21531 from keszybz/log2-tables

Optimize log2 tables

src/basic/capability-util.c

@@ -336,7 +336,7 @@ int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 
         /* Now upgrade the permitted caps we still kept to effective caps */
         if (keep_capabilities != 0) {
-                cap_value_t bits[u64log2(keep_capabilities) + 1];
+                cap_value_t bits[log2u64(keep_capabilities) + 1];
                 _cleanup_cap_free_ cap_t d = NULL;
                 unsigned i, j = 0;
 

src/basic/util.h

@@ -22,11 +22,20 @@ void in_initrd_force(bool value);
 
 int on_ac_power(void);
 
-static inline unsigned u64log2(uint64_t n) {
+/* Note: log2(0) == log2(1) == 0 here and below. */
+
+#define CONST_LOG2ULL(x) ((x) > 1 ? (unsigned) __builtin_clzll(x) ^ 63U : 0)
+#define NONCONST_LOG2ULL(x) ({                                     \
+                unsigned long long _x = (x);                       \
+                _x > 1 ? (unsigned) __builtin_clzll(_x) ^ 63U : 0; \
+        })
+#define LOG2ULL(x) __builtin_choose_expr(__builtin_constant_p(x), CONST_LOG2ULL(x), NONCONST_LOG2ULL(x))
+
+static inline unsigned log2u64(uint64_t x) {
 #if __SIZEOF_LONG_LONG__ == 8
-        return (n > 1) ? (unsigned) __builtin_clzll(n) ^ 63U : 0;
+        return LOG2ULL(x);
 #else
-#error "Wut?"
+#  error "Wut?"
 #endif
 }
 
@@ -34,26 +43,27 @@ static inline unsigned u32ctz(uint32_t n) {
 #if __SIZEOF_INT__ == 4
         return n != 0 ? __builtin_ctz(n) : 32;
 #else
-#error "Wut?"
+#  error "Wut?"
 #endif
 }
 
-static inline unsigned log2i(int x) {
-        assert(x > 0);
+#define CONST_LOG2U(x) ((x) > 1 ? __SIZEOF_INT__ * 8 - __builtin_clz(x) - 1 : 0)
+#define NONCONST_LOG2U(x) ({                                             \
+                unsigned _x = (x);                                       \
+                _x > 1 ? __SIZEOF_INT__ * 8 - __builtin_clz(_x) - 1 : 0; \
+        })
+#define LOG2U(x) __builtin_choose_expr(__builtin_constant_p(x), CONST_LOG2U(x), NONCONST_LOG2U(x))
 
-        return __SIZEOF_INT__ * 8 - __builtin_clz(x) - 1;
+static inline unsigned log2i(int x) {
+        return LOG2U(x);
 }
 
 static inline unsigned log2u(unsigned x) {
-        assert(x > 0);
-
-        return sizeof(unsigned) * 8 - __builtin_clz(x) - 1;
+        return LOG2U(x);
 }
 
 static inline unsigned log2u_round_up(unsigned x) {
-        assert(x > 0);
-
-        if (x == 1)
+        if (x <= 1)
                 return 0;
 
         return log2u(x - 1) + 1;