One of bazel's most attractive features is the ability for it to remotely cache artifacts to reduce unnecessary work for large builds. Unfortunately users quickly discover this comes with non-trivial financial and bandwidth implications.

There are many ways, of varying difficulty, to try and improve your cache usage. From breaking unnecessary dependencies, to adding larger local storage for CI workers, builds without the bytes, build avoidance, etc.

For codebases with lots of C or C++ one of the potentially easiest wins is to enable compressed debug information¹. Let's look at an example from our codebase.

Looking at the size of a non-trivial C++ binary built with -g -O2 (similar to cmake's RelWithDebInfo configuration), or binary clocks in at ~530mbs:

% du -sh bin
536M    bin

To get a sense of what percentage of this binary is debug info, we can use llvm-objcopy to strip the debug info entirely:

% llvm-objcopy --strip-debug bin strippedbin
% du -sh strippedbin
159M    strippedbin

This shows us that almost 70%(!!) of the binary size is taken up with debug info. In release configurations we can eliminate this entirely with bazel's --strip argument, but for developer builds, or other use cases where you need debug info, we can still improve this.

If we use llvm-objcopy again, this time to compress the debug info, we can immediately see our potential gains:

% llvm-objcopy --compress-debug-sections bin compressedbin
% du -sh compressedbin
290M    compressedbin

This shows us we can get an almost 50%(!!) improvement in binary size in this example.

To enable this in bazel, assuming you're using a relatively recent version of gcc or clang, you can add something like this to your .bazelrc²:

build --enable_platform_specific_config
build:linux --copt=-gz --host_copt=-gz
build:linux --linkopt=-gz --host_linkopt=-gz

In practice we saw cache reads drop by nearly 60% when we rolled out this change.

Reducing binary size with this approach has a lot of benefits, but it's even more pronounced when using bazel and closely monitoring your cache download sizes.