Introduction

This article discusses the tradeoff between the ease of debugging and the effectiveness of compiler optimization, while building with TI C/C++ compilers. It does not apply when building with other compilers.

The Tradeoff Described

Ease of debugging is characterized by things such as:

  • Does single stepping proceed through the code exactly as written? Or, does it skip around in seemingly random order?
  • Are all variables always available for inspection? Or do some go missing?
  • Do the values in the variables correspond to what is expected at that point in execution? Or something else?

Effectiveness of compiler optimization is characterized by things such as:

  • How fast does the code run?
  • Is the code size small enough?
  • Is power consumption low enough?

The compiler's main job is to generate instructions and data directives which implement the C/C++ source code. In addition, the compiler also emits information used by the debugger to keep track of things like where variables are located, or how to map an instruction address to a C source line.

The essential problem is that things the compiler does to make the code run faster may, at the same time, make the code harder to debug. Consider this simple loop.

for (i = 0; i < COUNT; i++)
   sum += array[i];

This loop is counting up, from 0 to COUNT. Suppose, to make it run faster, the compiler generates code which counts down, from COUNT to 0. What is the meaning of the variable i? If you inspect the value of i during execution of the loop, would it be confusing?

This is a simple example, contrived to illustrate the tradeoff. Actual instances in practice are usually more complex, sometimes much more complex.

What to Do

Control this tradeoff with the compiler switch --opt_level=value. The value can off, or 0-4. Though some compilers do not support --opt_level=4. Consult your compiler manual for the details. Here is how the value affects the tradeoff.

Which level is best varies for each system. For most users, the best tradeoff point is the lowest level of optimization which meets your system contraints. Your system constraints are usually some combination of size, speed, and power. Increase your optimization level until you meet your constraints. A higher level of optimization, while perhaps worthwile, incurs a risk to your debug experience.

What if the optimization level you need to meet your constraints is too hard to debug? Consider lowering the optimization level for just the one file you need to debug, and not the whole system. In CCS, use the File Specific Options feature. By lowering the optimization level for just one file, it is likely you will continue to meet your system constraints, while making it easier to debug that file.