4.3 Basic block profilers

There are several good reasons to desire a finer level of detail than you can see with a subroutine profiler. For humans trying to understand how a subroutine or function is used, a profiler that tells which lines of source code were actually executed, and how often, is invaluable; a few clues about where to focus your tuning efforts can save you time. Also, such a profiler saves you from discovering that a particularly clever optimization makes no difference because you put it in a section of code that never gets executed.

As part of an overall strategy, a subroutine profile can direct you to a handful of routines that account for most of the runtime, but it takes a basic block profiler A basic block is a section of code with only one entrance and one exit. If you know how many times the block was entered, you know how many times each of the statements in the block was executed, which gives you a line-by-line profile. The concept of a basic block is explained in detail in [link] to get you to the associated source code lines.

Basic block profilers can also provide compilers with information they need to perform their own optimizations. Most compilers work in the dark. They can restructure and unroll loops, but they cannot tell when it will pay off. Worse yet, misplaced optimizations often have an adverse effect of slowing down the code! This can be the result of added instruction cache burden, wasted tests introduced by the compiler, or incorrect assumptions about which way a branch would go at runtime. If the compiler can automatically interpret the results of a basic block profile, or if you can supply the compiler with hints, it often means a reduced run- time with little effort on your part.

There are several basic block profilers in the world. The closest thing to a standard, tcov , is shipped with Sun workstations; it’s standard because the installed base is so big. On MIPS-based workstations, such as those from Silicon Graphics and DEC, the profiler (packaged as an extension to prof ) is called pixie . We explain briefly how to run each profiler using a reasonable set of switches. You can consult your manual pages for other options.

Tcov

tcov , available on Sun workstations and other SPARC machines that run SunOS, gives execution statistics that describe the number of times each source statement was executed. It is very easy to use. Assume for illustration that we have a source program called foo.c . The following steps create a basic block profile:

The -a option tells the compiler to include the necessary support for tcov . On Sun Solaris systems, the –xa option is used. Several files are created in the process. One called foo.d accumulates a history of the exe- cution frequencies within the program foo . That is, old data is updated with new data each time foo is run, so you can get an overall picture of what happens inside foo , given a variety of data sets. Just remember to clean out the old data if you want to start over. The profile itself goes into a file called foo.tcov .