macro: add CONST_MAX() macro

The CONST_MAX() macro is similar to MAX(), but verifies that both
arguments have the same type and are constant expressions. Furthermore,
the result of CONST_MAX() is again a constant-expression.

CONST_MAX() avoids any statement-expressions and other non-trivial
expression-types. This avoids rather arbitrary restrictions in both GCC
and LLVM, which both either fail with statement-expressions inside
type-declarations or statement-expressions inside static-const
initializations.

If anybody knows how to circumvent this, please feel free to unify
CONST_MAX() and MAX().

2 files changed, 32 insertions, 0 deletions

diff --git a/src/shared/macro.h b/src/shared/macro.h
index 11bd8b3a9..179b24c98 100644
--- a/src/shared/macro.h
+++ b/src/shared/macro.h
@@ -140,6 +140,15 @@ static inline unsigned long ALIGN_POWER2(unsigned long u) {
                         _a > _b ? _a : _b;              \
                 })
 
+/* evaluates to (void) if _A or _B are not constant or of different types */
+#define CONST_MAX(_A, _B) \
+        __extension__ (__builtin_choose_expr(                           \
+                __builtin_constant_p(_A) &&                             \
+                __builtin_constant_p(_B) &&                             \
+                __builtin_types_compatible_p(typeof(_A), typeof(_B)),   \
+                ((_A) > (_B)) ? (_A) : (_B),                            \
+                (void)0))
+
 #define MAX3(x,y,z)                                     \
         __extension__ ({                                \
                         const typeof(x) _c = MAX(x,y);  \
diff --git a/src/test/test-util.c b/src/test/test-util.c
index 7d81b0b7b..a8fa48aed 100644
--- a/src/test/test-util.c
+++ b/src/test/test-util.c
@@ -70,6 +70,28 @@ static void test_align_power2(void) {
         }
 }
 
+static void test_max(void) {
+        static const struct {
+                int a;
+                int b[CONST_MAX(10, 100)];
+        } val1 = {
+                .a = CONST_MAX(10, 100),
+        };
+        int d = 0;
+
+        assert_cc(sizeof(val1.b) == sizeof(int) * 100);
+
+        /* CONST_MAX returns (void) instead of a value if the passed arguments
+         * are not of the same type or not constant expressions. */
+        assert_cc(__builtin_types_compatible_p(typeof(CONST_MAX(1, 10)), int));
+        assert_cc(__builtin_types_compatible_p(typeof(CONST_MAX(d, 10)), void));
+        assert_cc(__builtin_types_compatible_p(typeof(CONST_MAX(1, 1U)), void));
+
+        assert_se(val1.a == 100);
+        assert_se(MAX(++d, 0) == 1);
+        assert_se(d == 1);
+}
+
 static void test_first_word(void) {
         assert_se(first_word("Hello", ""));
         assert_se(first_word("Hello", "Hello"));
@@ -927,6 +949,7 @@ int main(int argc, char *argv[]) {
 
         test_streq_ptr();
         test_align_power2();
+        test_max();
         test_first_word();
         test_close_many();
         test_parse_boolean();