Check gcc or g++ Compiler Options Enabled by -march=native

When you're optimizing code for performance, especially in C or C++, using compiler options like -march=native can significantly boost speed by enabling all the SIMD instruction sets and CPU-specific optimizations available on the machine. This option tells the compiler to auto-detect the processor's architecture and use every feature it supports - like AVX2, and other instruction-level enhancements. However, this automation can feel like a black box. This tutorial shows how to check gcc or g++ compiler options enabled by -march=native.

Here's how we can find out which options -march=native expands into:

gcc -march=native -v -E - < /dev/null 2>&1 | grep cc1

g++ -march=native -v -E -x c++ - < /dev/null 2>&1 | grep cc1plus

• -v - verbose output (shows the internal commands).
• -E - run only the preprocessor stage (no compilation or linking).
• - - the hyphen here tells the compiler to read from standard input (stdin) instead of a file, and it works in tandem with the following part of the command.
• < /dev/null - this redirects an empty input stream to the compiler, so it processes no actual code - just a blank input.
• 2>&1 - this redirects standard error (stderr) to standard output (stdout). By doing this, both error messages and regular output are combined into one stream.
• | grep cc1 and | grep cc1plus - filters out lines containing cc1 or cc1plus.
• -x c++ - this tells the compiler to treat the input as C++ source code, even though we're not providing an actual file. This is used because we're redirecting an empty stream, and the option ensures that the compiler knows it's dealing with C++ code.