Add misc example of shift warning

misc/shift_warning.c

@@ -0,0 +1,40 @@
+/*
+ *  With 32-bit integers the expression:
+ *
+ *      uint32_t x = 26 << 27;
+ *
+ *  causes a warning on some compilers because the signed integer 26 (0x1a) is
+ *  first shifted 27 positions to the left which remains within unsigned 32-bit
+ *  range but overflows the signed 32-bit range.  This has no practical impact
+ *  (at least on common compilers) because the result is ultimately an unsigned
+ *  32-bit integer, 0xd0000000.
+ *
+ *  In GCC 4.x there doesn't seem to be a warning option to detect this.
+ *  In clang this works (also -Weverything):
+ *
+ *      $ clang -Wshift-sign-overflow -otest misc/shift_warning.c
+ *      misc/shift_warning.c:19:9: warning: signed shift result (0xD0000000)
+ *           sets the sign bit of the shift expression's type ('int') and
+ *           becomes negative [-Wshift-sign-overflow]
+ *             t = 26 << 27;
+ *                 ~~ ^  ~~
+ *      1 warning generated.
+ *
+ *      $ ./test
+ *      d0000000
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+
+int main(int argc, char *argv[]) {
+	uint32_t t;
+
+	t = 26 << 27;
+	printf("%08lx\n", (unsigned long) t);
+
+	(void) argc;
+	(void) argv;
+
+	return 0;
+}