Modern C/C++ compilers support many automatic low-level optimisations which can improve the performance of the compiled code. The actual performance gain depends on your specific code, but it is not uncommon to see 5–10x speedups, and in some cases, 50x or more, especially for compute-intensive tasks.

Compiler optimisations are not enabled by default because they make debugging more difficult. Optimised builds can:

  • Reorder or eliminate code, making it harder to set breakpoints or inspect variables
  • Inline functions, obscuring the call stack
  • Remove or coalesce variables that appear unused

As a result, developers typically use unoptimised builds (debug) during development and optimised builds (release) for production.

Enabling Optimisations

If you wish to enable compiler optimisations (or check whether they are enabled) the guidance differs slightly between compilers.

GCC / Clang

The main optimisation flags provided by GCC and Clang are:

  • -O0: No optimisation (default)
  • -O1, -O2, -O3: Increasing levels of optimisation
  • -Ofast: Aggressive optimisations, may break strict standards compliance
  • -Os: Optimize for size

You should typically ensure that -O2 is passed to the compiler (either on the command-line or within the makefile/project) when compiling for release, to take advantage of compiler optimisations. If you’re not sure whether they’re being used, look out for them in the compile log.

-O3 and -Ofast can provide mixed results, as they use the most aggressive optimisations which can have inadvertent effects such as greatly increasing the binary size for limited to no further impact to performance.

CMake

CMake supports Debug and Release build configurations via the CMAKE_BUILD_TYPE argument at configure time.

cmake -DCMAKE_BUILD_TYPE=Release .
cmake --build .

If the CMake project is correctly structured (it may be worth reviewing the compilation log to make sure you can see an optimisation flag), passing Release will enable compiler optimisations . It’s up to the project maintainer in this scenario, whether the default configuration corresponds to Debug, Release or something else entirely.

Visual Studio (MSVC)

The default project structure within visual studio will contain both a Debug and a Release build configuration, the latter will have compiler optimisations enabled and should be used for production builds.

To enable compiler optimisations manually, navigate to Project Properties > C/C++ > Optimization and set this value to /O2 (Maxmimise speed) or /Ox (Full optimisation).

The Technical Detail

There are many different optimisations which can be performed by a compiler, GCC lists most of their compiler optimisations and allows them to be individually toggled.