From: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Date: Fri, 26 Nov 2021 10:46:54 +0000 (+0100)
Subject: basic/util: add macro variants of log2 functions
X-Git-Tag: v250-rc1~125^2~6
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f4ada1b42ffd14a12bbad48b40d65332673bc869;p=thirdparty%2Fsystemd.git

basic/util: add macro variants of log2 functions

The macro variants can be used in static initializers.

The same guard against calling __builtin_clz(0) is added as for
__builtin_clzll(0), since that's undefined behaviour too. Our code
wouldn't call it, but this avoids a potential pitfall with the macro.
All variants map 0→0. Otherwise we'd often have to handle 0 specially
in callers.

__builtin_clz takes unsigned as the argument, so there's no LOG2I macro.
---

diff --git a/src/basic/util.h b/src/basic/util.h
index 20bda9bbf3e..f5434c96418 100644
--- a/src/basic/util.h
+++ b/src/basic/util.h
@@ -22,11 +22,20 @@ void in_initrd_force(bool value);
 
 int on_ac_power(void);
 
-static inline unsigned log2u64(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;
diff --git a/src/test/test-util.c b/src/test/test-util.c
index 8e8ad05205f..21ab016c228 100644
--- a/src/test/test-util.c
+++ b/src/test/test-util.c
@@ -17,8 +17,42 @@
 #include "tests.h"
 #include "util.h"
 
+TEST(LOG2ULL) {
+        assert_se(LOG2ULL(0) == 0);
+        assert_se(LOG2ULL(1) == 0);
+        assert_se(LOG2ULL(8) == 3);
+        assert_se(LOG2ULL(9) == 3);
+        assert_se(LOG2ULL(15) == 3);
+        assert_se(LOG2ULL(16) == 4);
+        assert_se(LOG2ULL(1024*1024) == 20);
+        assert_se(LOG2ULL(1024*1024+5) == 20);
+}
+
+TEST(CONST_LOG2ULL) {
+        assert_se(CONST_LOG2ULL(0) == 0);
+        assert_se(CONST_LOG2ULL(1) == 0);
+        assert_se(CONST_LOG2ULL(8) == 3);
+        assert_se(CONST_LOG2ULL(9) == 3);
+        assert_se(CONST_LOG2ULL(15) == 3);
+        assert_se(CONST_LOG2ULL(16) == 4);
+        assert_se(CONST_LOG2ULL(1024*1024) == 20);
+        assert_se(CONST_LOG2ULL(1024*1024+5) == 20);
+}
+
+TEST(NONCONST_LOG2ULL) {
+        assert_se(NONCONST_LOG2ULL(0) == 0);
+        assert_se(NONCONST_LOG2ULL(1) == 0);
+        assert_se(NONCONST_LOG2ULL(8) == 3);
+        assert_se(NONCONST_LOG2ULL(9) == 3);
+        assert_se(NONCONST_LOG2ULL(15) == 3);
+        assert_se(NONCONST_LOG2ULL(16) == 4);
+        assert_se(NONCONST_LOG2ULL(1024*1024) == 20);
+        assert_se(NONCONST_LOG2ULL(1024*1024+5) == 20);
+}
+
 TEST(log2u64) {
         assert_se(log2u64(0) == 0);
+        assert_se(log2u64(1) == 0);
         assert_se(log2u64(8) == 3);
         assert_se(log2u64(9) == 3);
         assert_se(log2u64(15) == 3);
@@ -27,6 +61,30 @@ TEST(log2u64) {
         assert_se(log2u64(1024*1024+5) == 20);
 }
 
+TEST(log2u) {
+        assert_se(log2u(0) == 0);
+        assert_se(log2u(1) == 0);
+        assert_se(log2u(2) == 1);
+        assert_se(log2u(3) == 1);
+        assert_se(log2u(4) == 2);
+        assert_se(log2u(32) == 5);
+        assert_se(log2u(33) == 5);
+        assert_se(log2u(63) == 5);
+        assert_se(log2u(INT_MAX) == sizeof(int)*8-2);
+}
+
+TEST(log2i) {
+        assert_se(log2i(0) == 0);
+        assert_se(log2i(1) == 0);
+        assert_se(log2i(2) == 1);
+        assert_se(log2i(3) == 1);
+        assert_se(log2i(4) == 2);
+        assert_se(log2i(32) == 5);
+        assert_se(log2i(33) == 5);
+        assert_se(log2i(63) == 5);
+        assert_se(log2i(INT_MAX) == sizeof(int)*8-2);
+}
+
 TEST(protect_errno) {
         errno = 12;
         {
@@ -58,17 +116,6 @@ TEST(unprotect_errno) {
         assert_se(errno == 4711);
 }
 
-TEST(log2i) {
-        assert_se(log2i(1) == 0);
-        assert_se(log2i(2) == 1);
-        assert_se(log2i(3) == 1);
-        assert_se(log2i(4) == 2);
-        assert_se(log2i(32) == 5);
-        assert_se(log2i(33) == 5);
-        assert_se(log2i(63) == 5);
-        assert_se(log2i(INT_MAX) == sizeof(int)*8-2);
-}
-
 TEST(eqzero) {
         const uint32_t zeros[] = {0, 0, 0};
         const uint32_t ones[] = {1, 1};