What is the difference between "-c opt" and "--copt=-O3" in Bazel build (or GCC)

Question

I'm learning GCC and Bazel. I want to enable all the optimization for Bazel to build a project which requires the best performance.

Then I found -c opt which means to set the compilation mode to optimized without debug information.

And --copt=-O3 means set the optimization level to the third one. There are -O2, -Os, etc.

I'm confused with these two options.

1. What is the difference between -c opt and --copt=-O3?
2. Will they trigger each other. So I only need to write one of them with bazel build?

Answer 1

--copt is for passing args to to the compiler.

-c is a short form of --compilation-mode. Its effect is described in the user-manual:

• It sets compiler options (e.g. -c opt implies -O2 -DNDEBUG)
• There are different output directories per compilation mode, so you can switch between debug and optimized builds without full recompilation.

So usually, -c optis enough. If you want the behaviour of -c opt but with a different optimization level, you combine the two options like in -c opt --copt=-O3 and the compiler will get both options -O2 and -O3, but the last one will win.

And watch out, there is a third similar option:

--config=configname is for selecting a configuration. You can have a .bazelrc which defines default options. Some of them are not always active, but some only if you activate them by the --config=configname command line option. Now opt is a popular configname, so if you have a .bazelrc that contains

build:opt --copt=-O3

then bazel build --config=opt has the same effect as bazel build --copt=-O3