Profiling the C++ compilation process

Question

I tend to write rather large templated header-only C++ libraries and my users commonly complain about compilation times. After thinking about the matter, it occurred to me that I have no idea where the time is going. Is there some simple way to profile the C++ compilation process with common compilers, such as g++, icc, and xlC? For instance, is it possible to get an idea of how much time is spent within each of the phases of C++ compilation?

Answer 1

You can separate them out to some extent (I'm assuming make)

• add a build rule that only preprocesses files (using the -E switch), and a .PHONY target that depends on the preprocessor output files just like the normal binary target depends on .o files. Measure how long it takes to build this target
• add a 'PHONY target that depends on all the .o files but doesn't link them. Measure how long it takes to build this target (from clean)
• measure how long it takes to do a clean build of the usual binary

Now you have some idea how long it takes to pre-process, compile, and link. You can also compare optimized and non-optimized (-O0) versions of the second and third target, to see how long is spent in the optimizer.

Answer 2

There's a tool from the Boost project, which could be useful for pretty much any compiler and build system.

The tool requires source code instrumentation with TEMPLATE_PROFILE_ENTER() and TEMPLATE_PROFILE_EXIT() macro calls. These macros then generate specific diagnostics (warnings) at compile-time, which are timed and collected along with instantiation callstacks (which consequently allow building and visualizing callgraphs) by a script. Not bad, IMO.

I didn't use it yet though.

Answer 3

I have not yet tried it, but templight looks VERY promising: https://github.com/mikael-s-persson/templight

Answer 4

You might be able to get some traction with some variant on strace -e trace=process -f -r -ttt -T, at least for compilers like g++ that are broken up into many processes.

Answer 5

For GCC there are debugging options to find how much time is spent within each of the phases of C++ compilation?

-Q Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-ftime-report Makes the compiler print some statistics about the time consumed by each pass when it finishes.

Passes are described in GCCINT 9: Passes and Files of the Compiler.

You can post output of g++ compilation of single source file with -v -ftime-report here to discuss it. There could be some help on the GCC mailing list.

---

For compilers other than GCC (or GCC more ancient than 3.3.6) see the other options in this thread.