How to see compiler reformulation of C++ code with optimizations

Question

I'm wondering if/how it's possible to see how the compiler reformulates a piece of code with clang++/g++ when optimizations are turned on. I know that the Intel compiler has a flag to produce relevant output, but I can't seem to find the equivalent in the other compilers.

Answer 1

So, thanks to your directions I was able to discover something really cool, so I thought I'd share:

With Clang++-4.0, one can compile the executable as follows:

clang++-4.0 -std=c++14 -O3  -fsave-optimization-record -foptimization-record-file=myOptfile.yaml sourceFile.cpp

This saves a record of successful and unsuccessful optimizations in myOptfile.yaml. This can be viewed using llvm-opt-report-4.0, but its true power is if viewed using llvm/utils/opt-viewer.py.

In order to do that, clone the llvm repository, navigate to your source directory, and run the following after you generate myOptFile.yaml:

python ~/myInstallDir/llvm/utils/opt-viewer/opt-viewer.py myOptFile.yaml reportsDirectory/

This will create a lot of html files that you can navigate using index.html (in the reportsDirectory folder).

The result is awesome, and looks like this:

Most things are clickable, so you can navigate using the html hyperlinks to other parts of the source code, including c++ libraries and see what happened!