toolchain/gcc: Add 'compiler_barrier' in CODE_UNREACHABLE

GCC 14.3 will happily delete any code that appears before
__builtin_unreachable that isn't separated with an obvious branch. That
includes __asm__ statements, even those which generate traps.

The failure case that I debugged was on x86 in
z_check_stack_sentinel. There is a store to restore the sentinel to the
correct value just before the ARCH_EXCEPT, and that macro emits 'int $32'
followed by CODE_UNREACHABLE. Because the compiler didn't understand that
ARCH_EXCEPT was changing execution flow, it decided that the sentinel
restoring store 'couldn't' be reached and elided it.

I added the compiler barrier to CODE_UNREACHABLE as that construct is
required for this problem to occur, but a more surgical fix might be to add
:"memory" to the relevant __asm__ statements. Finding and fixing all of
those now and forever seems like it would be too burdensome to be worth any
possible code improvement.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

include/zephyr/toolchain/gcc.h

@@ -118,7 +118,10 @@
 	__builtin_unreachable(); \
 }
 #else
-#define CODE_UNREACHABLE __builtin_unreachable()
+#define CODE_UNREACHABLE do {			\
+	compiler_barrier();			\
+	__builtin_unreachable();		\
+	} while(0)
 #endif
 #define FUNC_NORETURN    __attribute__((__noreturn__))