User guide

Variable Tracking at Assignments

Note

Not all documented characteristics will apply to all instances of GDB because it is still in active

development, and as such the capabilities vary (to some extent) depending on the platform and

target for which it is built.

5.3. 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.

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.

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. Refer to man gcc for details about this option.

For more information about the infrastructure and development of VTA, refer to 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.

5.4. Python Pretty-Printers

The GDB command print outputs comprehensive debugging information for a target application.

GDB aims to provide as much debugging data as it can to users; however, this means that for highly

complex programs the amount of data can become very cryptic.

In addition, GDB does not provide any tools that help decipher GDB print output. GDB does not

even empower users to easily create tools that can help decipher program data. This makes the

practice of reading and understanding debugging data quite arcane, particularly for large, complex

projects.

For most developers, the only way to customize GDB print output (and make it more meaningful)

is to revise and recompile GDB. However, very few developers can actually do this. Further, this

practice will not scale well, particularly if the developer needs to also debug other programs that are

heterogenous and contain equally complex debugging data.