4.4. Variable Tracking at Assignments

Variable Tracking at Assignments (VTA) is a new infrastructure included in GCC used to improve variable tracking during optimizations. This allows GCC to produce more precise, meaningful, and useful debugging information for GDB, SystemTap, and other debugging tools.

When GCC compiles code with optimizations enabled, variables are renamed, moved around, or even removed altogether. As such, optimized compiling can cause a debugger to report that some variables have been <optimized out>. With VTA enabled, optimized code is internally annotated to ensure that optimization passes to transparently keep track of each variable's value, regardless of whether the variable is moved or removed. The effect of this is more parameter and variable values available, even for the optimized (gcc -O2 -g built) code. It also displays the <optimized out> message less.

VTA's benefits are more pronounced when debugging applications with inlined functions. Without VTA, optimization could completely remove some arguments of an inlined function, preventing the debugger from inspecting its value. With VTA, optimization will still happen, and appropriate debugging information will be generated for any missing arguments.

VTA is enabled by default when compiling code with optimizations and debugging information enabled (that is, gcc -O -g or, more commonly, gcc -O2 -g). To disable VTA during such builds, add the -fno-var-tracking-assignments. In addition, the VTA infrastructure includes the new gcc option -fcompare-debug. This option tests code compiled by GCC with debug information and without debug information: the test passes if the two binaries are identical. This test ensures that executable code is not affected by any debugging options, which further ensures that there are no hidden bugs in the debug code. Note that -fcompare-debug adds significant cost in compilation time. See man gcc for details about this option.

For more information about the infrastructure and development of VTA, see A Plan to Fix Local Variable Debug Information in GCC, available at the following link:

http://gcc.gnu.org/wiki/Var_Tracking_Assignments

A slide deck version of this whitepaper is also available at http://people.redhat.com/aoliva/papers/vta/slides.pdf.