How much overhead can the -fPIC flag add?

Question

Question

I am testing a simple code which calculates Mandelbrot fractal. I have been checking its performance depending on the number of iterations in the function

Answer 1

As other people already pointed out -fPIC forces GCC to disable many optimizations e.g. inlining and cloning. I'd like to point out several ways to overcome this:

• replace -fPIC with -fPIE if you are compiling main executable (not libraries) as this allows compiler to assume that interposition is not possible;
• use -fvisibility=hidden and __attribute__((visibility("default"))) to export only necessary functions from the library and hide the rest; this would allow GCC to optimize hidden functions more aggressively;
• use private symbol aliases (__attribute__((alias ("__f")));) to refer to library functions from within the library; this would again untie GCC's hands
• previous suggestion can be automated with -fno-semantic-interposition flag that was added in recent GCC versions

It's interesting to note that Clang is different from GCC as it allows all optimizations by default regardless of -fPIC (can be overridden with -fsemantic-interposition to obtain GCC-like behavior).