Finished cyccnt

cargo_klee_examples/examples/cyccnt.rs

@@ -19,7 +19,7 @@ fn main() {
 
     let end = core.DWT.cyccnt.read();
 
-    let _time = end - start;
+    let _time = end.wrapping_sub(start);
 }
 
 // A) Running KLEE on embedded code:
@@ -167,6 +167,9 @@ fn main() {
 // What do you get, and why?
 //
 // [your answer here]
+// start/end = <optimized out>
+// start and end are only used once and that is when calculating _time, _time is never used. Thus
+// the compiler probably removed start, end and _time  because they are never really used.
 //
 // As you should have seen, this was not particularly informative, right?
 //
@@ -198,14 +201,23 @@ fn main() {
 // Value of `start`.
 //
 // [your answer here]
+// (gdb) print start
+// $7 = 512
+// 
 //
 // Value of `end`
 //
 // [your answer here]
+// (gdb) print end
+// $8 = 0
+// 
 //
 // Why does these values cause an error debug/dev build but not in a release build?
 //
 // [your answer here]
+// The result of 0 - 512 = -512 which a u32 can't represent because it is unsigned. Thus the
+// compiler panics because the value of _time is wrong. The release build removes this calculation
+// because _time is never used, thus it never runs into this problem.
 //
 // C) Fix the problem!
 //
@@ -218,6 +230,9 @@ fn main() {
 // Argue for your solution in your own words.
 //
 // [your answer here]
+// I switch the subtraction to a wrapping one this will remove the error. I choose this solution
+// because it is short and easy to implement. It also calculates the correct diff value as long as
+// end has not wrapped around more then once.
 //
 // D) Learning outcomes and major takeaways.
 //
@@ -237,6 +252,17 @@ fn main() {
 // How long time would lines 16/17 take to run to trigger the error?
 //
 // [your answer here]
+// * I think that these kind of errors are hard to find because I often think of the counter as a number
+// when I program and mathematically there is no problem. But variable needs to have a fixed size
+// and thus the programmer has to remember to think of the number as having a fixed size.
+//
+// * The condition that triggers this error is that start > end because cyccnt has wrapped around.
+//
+// * Cyccnt is a 32 bit registry and counts for ever. When the value becomes more then the 32 bit
+// registry can hold it starts over again(wraps around).
+//
+// * 2^32 / 8MHz = 2^32 / (8*10^6) = 536.870912 seconds (~537 sec)
+// 
 //
 // Of course this is a contrived example, and may not occur in practice.
 // But, it represents a class of problems/errors/bugs that is