Mac 上的未知 GCC 编译指示

Question

warning: unknown pragma ignored [-Wunknown-pragmas]
#pragma GCC optimize("O3")

I want to enable GCC optimization flags, but they are ignored when i'm compiling code, although they are present in documentation . Why does this happen?

Answer 1

The problem was that g++ is a clang alias in MacOS by default So i did this

cd /usr/local/bin
ln -s g++-11 g++

this fixes the problem and g++ command will run actual gcc compuler instead of clang

Answer 2

In summary, the most commonly used optimization flags are these:

-O0 for no optimization; -O1 for a few optimizations; -O2 for lots of optimizations; -O3 for all of the O2 optimizations plus some other expensive ones; -Os the same as O2 plus a few optimizations for reducing the size of the binary; -flto for usually expensive link-time optimizations.

In GCC, you might write something like g++ -std=c++20 -Wall -O3 sourcefilename.cpp -o myprogram .

Check these links too:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89038

Suppress -Wunknown-pragmas warning in GCC