assert.h: Try to evaluate assertions at compile time

Many places in coreboot seem to like to do things like

 assert(CONFIG(SOME_KCONFIG));

This is somewhat suboptimal since assert() is a runtime check, so you
don't see that this fails until someone actually tries to boot it even
though the compiler is totally aware of it already. We already have the
dead_code() macro to do this better:

 if (CONFIG(SOME_KCONFIG))
   dead_code();

Rather than fixing all these and trying to carefully educate people
about which type of check is more appropriate in what situation, we can
just employ the magic of __builtin_constant_p() to automatically make
the former statement behave like the latter.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I06691b732598eb2a847a17167a1cb92149710916
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44044
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/include/assert.h

@@ -22,9 +22,16 @@
 #define __ASSERT_LINE__ __LINE__
 #endif
 
+#ifndef _PORTING_H_	/* TODO: Isolate AGESA properly. */
+#define __build_time_assert(x) \
+	(__builtin_constant_p(x) ? ((x) ? 1 : dead_code_t(int)) : 0)
+#else
+#define __build_time_assert(x) 0
+#endif
+
 /* GCC and CAR versions */
 #define ASSERT(x) {							\
-	if (!(x)) {							\
+	if (!__build_time_assert(x) && !(x)) {				\
 		printk(BIOS_EMERG,					\
 			"ASSERTION ERROR: file '%s', line %d\n",	\
 			__ASSERT_FILE__, __ASSERT_LINE__);		\
@@ -33,7 +40,7 @@
 	}								\
 }
 #define ASSERT_MSG(x, msg) {						\
-	if (!(x)) {							\
+	if (!__build_time_assert(x) && !(x)) {				\
 		printk(BIOS_EMERG,					\
 			"ASSERTION ERROR: file '%s', line %d\n",	\
 			__ASSERT_FILE__, __ASSERT_LINE__);		\