Maximizing Software Quality Through Effective Debugging Measures

1. Microprocessors, AdvancedProfessional Debugging II February 3, 2012Jack Ganssle

2. Two Important Measures Defect potential= number of bugs found in development + those found within 90 days of delivery Defect removal efficiency = percent removed in development

3. Measuring Defects “When these measures were introduced into large corporations such as IBM and ITT, in less than four years the volumes of delivered defects had declined by more than 50%; maintenance costs were reduced by more than 40%; development schedules were shortened by more than 15%. There are no other measurements that can yield such positive benefits in such a short time span.” - Capers Jones Percent of bugs shipped

4. Benchmarking Your Group

6. // Find the positive solution of a quadratic // equation. float solve_pos(float a, float b, float c) { float32_t result; result = (-b + sqrt(b*b - 4*a*c))/(2*a); return result; }

7. // Find the positive solution of a quadratic // equation. float solve_pos(float32_t a, b, c) { float32_t result; result = (-b + sqrt(b*b - 4*a*c))/(2*a); return result; }

8. // Find the positive solution of a quadratic // equation. float solve_pos(float32_t a, b, c) { float32_t result; assert (a!=0); assert ((b*b-4*a*c) >= 0); result = (-b + sqrt(b*b - 4*a*c))/(2*a); assert (isnormal(result)); return result; }

9. What’s wrong with assert()? An assertion is true for one point in time Assertions can have side effects assert(y > 0 && x = MAX) assert(pop(x) == 0)

10. #define precondition(a) ((void)(a), assert(a)) #define postcondition(a)((void)(a), assert(a)) //lint -emacro(730,require) Better Than Assert() precondition(n>=0); precondition(m=n); warning 666 - expression with side effects precondition(g(m)); warning 666 - expression with side effects

11. // Find the positive solution of a quadratic // equation. float solve_pos(float32_t a, b, c) { float32_result; precondition (a!=0); precondition ((b*b-4*a*c) >= 0); result = (-b + sqrt(b*b - 4*a*c))/(2*a); postcondition (isnormal(result)); return result; }

12. // Push a number on a stack void push(int data) { *count = data; ++count; }

13. // Push a number on a stack void push(int data) { precondition (count != NULL); *count = data; ++count; postcondition (*(count-1) == data); postcondition (count <= BUFFER_END); }

14. What to Check Fail fast! Don’t check for correct inputs from error-prone devices… like people. Do check for “impossible” conditions No side effects: “=“ is not allowed Do check for “obvious” situations

15. Assertions vs Bugs

16. Faster Debugging Cost to Fix Bug Number

18. A Hot Day

19. $500m… or 3 Preconditions? This: Or this: Require horizontal bias < 215

20. Production Code? A philosophical issue. NASA’s mantra: “test what you fly, fly what you test.” Resource issues may mean assertions should be removed.

21. malloc() - the Curse of Embedded Systems • Problems: • Heap fragmentation! • May be slow • No one knows how to compute heap size

22. malloc() - the Curse of Embedded Systems • Solutions: • Use static allocation • Always, always check malloc()’s return value • And the argument passed to it! • Use multiple heaps: • http://members.chello.nl/h.robbers/ • http://www.fourmilab.ch/bget/

23. malloc() - the Curse of Embedded Systems • mem - a malloc() diagnostic tool: • Detects extra free()s • Detects point over- and under-runs • Finds out of memory conditions • Plus… it will find memory leaks. • http://www8.cs.umu.se/~isak/snippets/

24. Debugging Speed Problems

25. ISR Min and Max times

26. Scoping Cores

27. Measure Idle Time while(1) { set_bit(): unset_bit(); handle_other_stuff; }

28. uC/OS-II Idle Task Hook /************************************************* IDLE TASK HOOK **************************************************/ void OSTaskIdleHook (void) { outportb(test_port, 1); // Assert pin outportb(test_port, 0); // … but just for a moment } On a 33 MHz 186 this adds just 480 nsec of overhead.

29. A $15 VOM == $10k Performance Analyzer 8.6% duty cycle Change 29.2K resistor to: R=(duty cycle/100) * (Max volts) * 2kΩ

30. Measuring Task Activity /******************************** ASK SWITCH HOOK ********************************/ void OSTaskSwHook (void) { outportb(test_port, OSTCBCur->OSTCBId); } R-2R Ladder

31. Executing Tasks

32. uC/Probe (www.micrium.com)

33. Seed Vectors

34. Seeding Memory • Always program unused flash to logical states: • SWI on ARM CPUs • Illegal instruction on 68k CPUs • INT3 on x86 CPUs • Any software interrupt

35. Questions?