glib-aux: let NM_MIN()/NM_MAX() return a compile time constant

Glib's MIN()/MAX() should not be used, in favor of NM_MIN()/NM_MAX().
That's because the NM variants

- evaluate arguments only once
- have a static assertion that the signedness of the arguments matches

However, previously those macros never evaluated to a compile time
constant. Unlike the glib variants, which do so when the arguments are
compile time constants. That is sometimes important when using the
macros in a context that requires a constant.

Extend NM_MIN()/NM_MAX() to be a compile time constant, when possible.

Note that there are still a few places where NM_MIN()/NM_MAX() cannot be
used due to the expression statement. For those cases, there is
NM_MIN_CONST()/NM_MAX_CONST().

2 files changed, 55 insertions, 23 deletions

diff --git a/src/libnm-glib-aux/tests/test-shared-general.c b/src/libnm-glib-aux/tests/test-shared-general.c
index 416d26384f..55f8cbb1d1 100644
--- a/src/libnm-glib-aux/tests/test-shared-general.c
+++ b/src/libnm-glib-aux/tests/test-shared-general.c
@@ -67,6 +67,20 @@ test_nm_static_assert(void)
 /*****************************************************************************/
 
 static void
+test_max(void)
+{
+    /* Check that NM_MAX() of constant expressions is itself a constant. We
+     * build with -Wvla, so this is a constant! */
+    char buf1[NM_MAX(55, 40)];
+    char buf2[NM_MAX(1, NM_MAX(40, 55))];
+
+    G_STATIC_ASSERT(sizeof(buf1) == 55);
+    G_STATIC_ASSERT(sizeof(buf2) == 55);
+}
+
+/*****************************************************************************/
+
+static void
 test_gpid(void)
 {
     const int *int_ptr;
@@ -2624,6 +2638,7 @@ main(int argc, char **argv)
     nmtst_init(&argc, &argv, TRUE);
 
     g_test_add_func("/general/test_nm_static_assert", test_nm_static_assert);
+    g_test_add_func("/general/test_max", test_max);
     g_test_add_func("/general/test_gpid", test_gpid);
     g_test_add_func("/general/test_monotonic_timestamp", test_monotonic_timestamp);
     g_test_add_func("/general/test_timespect_to", test_timespect_to);
diff --git a/src/libnm-std-aux/nm-std-aux.h b/src/libnm-std-aux/nm-std-aux.h
index ebc43dd0ab..819f3284cf 100644
--- a/src/libnm-std-aux/nm-std-aux.h
+++ b/src/libnm-std-aux/nm-std-aux.h
@@ -392,33 +392,50 @@ nm_mult_clamped_u(unsigned a, unsigned b)
     return c;
 }
 
-/* glib's MIN()/MAX() macros don't have function-like behavior, in that they evaluate
- * the argument possibly twice.
- *
- * Taken from systemd's MIN()/MAX() macros. */
-
-#define NM_MIN(a, b) __NM_MIN(NM_UNIQ, a, NM_UNIQ, b)
-#define __NM_MIN(aq, a, bq, b)                                                         \
-    ({                                                                                 \
-        typeof(a) NM_UNIQ_T(A, aq) = (a);                                              \
-        typeof(b) NM_UNIQ_T(B, bq) = (b);                                              \
-                                                                                       \
-        NM_STATIC_ASSERT(_NM_INT_SAME_SIGNEDNESS(NM_UNIQ_T(A, aq), NM_UNIQ_T(B, bq))); \
-                                                                                       \
-        ((NM_UNIQ_T(A, aq) < NM_UNIQ_T(B, bq)) ? NM_UNIQ_T(A, aq) : NM_UNIQ_T(B, bq)); \
+/* In a few places where a constant expression is required, NM_MIN() doesn't work.
+ * In that case, use NM_MIN_CONST(). */
+#define NM_MIN_CONST(a, b)                                                                   \
+    ((NM_STATIC_ASSERT_EXPR_1(_NM_INT_SAME_SIGNEDNESS((a), (b)) && __builtin_constant_p((a)) \
+                              && __builtin_constant_p((b)))                                  \
+      && ((a) <= (b)))                                                                       \
+         ? (a)                                                                               \
+         : (b))
+
+#define _NM_MIN_V(aq, a, bq, b)                                                         \
+    ({                                                                                  \
+        typeof(a) NM_UNIQ_T(A, aq) = (a);                                               \
+        typeof(b) NM_UNIQ_T(B, bq) = (b);                                               \
+                                                                                        \
+        ((NM_UNIQ_T(A, aq) <= NM_UNIQ_T(B, bq)) ? NM_UNIQ_T(A, aq) : NM_UNIQ_T(B, bq)); \
     })
 
-#define NM_MAX(a, b) __NM_MAX(NM_UNIQ, a, NM_UNIQ, b)
-#define __NM_MAX(aq, a, bq, b)                                                         \
-    ({                                                                                 \
-        typeof(a) NM_UNIQ_T(A, aq) = (a);                                              \
-        typeof(b) NM_UNIQ_T(B, bq) = (b);                                              \
-                                                                                       \
-        NM_STATIC_ASSERT(_NM_INT_SAME_SIGNEDNESS(NM_UNIQ_T(A, aq), NM_UNIQ_T(B, bq))); \
-                                                                                       \
-        ((NM_UNIQ_T(A, aq) > NM_UNIQ_T(B, bq)) ? NM_UNIQ_T(A, aq) : NM_UNIQ_T(B, bq)); \
+#define NM_MIN(a, b)                                                                         \
+    __builtin_choose_expr(__builtin_constant_p((a)) && __builtin_constant_p((b))             \
+                              && NM_STATIC_ASSERT_EXPR_1(_NM_INT_SAME_SIGNEDNESS((a), (b))), \
+                          (((a) <= (b)) ? (a) : (b)),                                        \
+                          _NM_MIN_V(NM_UNIQ, a, NM_UNIQ, b))
+
+#define NM_MAX_CONST(a, b)                                                                   \
+    ((NM_STATIC_ASSERT_EXPR_1(_NM_INT_SAME_SIGNEDNESS((a), (b)) && __builtin_constant_p((a)) \
+                              && __builtin_constant_p((b)))                                  \
+      && ((a) >= (b)))                                                                       \
+         ? (a)                                                                               \
+         : (b))
+
+#define _NM_MAX_V(aq, a, bq, b)                                                         \
+    ({                                                                                  \
+        typeof(a) NM_UNIQ_T(A, aq) = (a);                                               \
+        typeof(b) NM_UNIQ_T(B, bq) = (b);                                               \
+                                                                                        \
+        ((NM_UNIQ_T(A, aq) >= NM_UNIQ_T(B, bq)) ? NM_UNIQ_T(A, aq) : NM_UNIQ_T(B, bq)); \
     })
 
+#define NM_MAX(a, b)                                                                         \
+    __builtin_choose_expr(__builtin_constant_p((a)) && __builtin_constant_p((b))             \
+                              && NM_STATIC_ASSERT_EXPR_1(_NM_INT_SAME_SIGNEDNESS((a), (b))), \
+                          (((a) >= (b)) ? (a) : (b)),                                        \
+                          _NM_MAX_V(NM_UNIQ, a, NM_UNIQ, b))
+
 #define NM_CLAMP(x, low, high) __NM_CLAMP(NM_UNIQ, x, NM_UNIQ, low, NM_UNIQ, high)
 #define __NM_CLAMP(xq, x, lowq, low, highq, high)                                            \
     ({                                                                                       \