How can I disable vectorization while using GCC?

Question

I am compiling my code using following command:

gcc -O3 -ftree-vectorizer-verbose=6 -msse4.1 -ffast-math 

With this all the optimizations are enabled.

But I want to disable vectorization while keeping the other optimizations.

Answer 1

Most of the GCC switches can be used with a no prefix to disable their behavior. Try with -fno-tree-vectorize (after -O3 on the command line).

Answer 2

you can also selectively enable and disable vectorization with the optimize function attributes or pragmas

http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

http://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html

e.g.

__attribute__((optimize("no-tree-vectorize")))
void f(double * restrict a, double * restrict b)
{
    for (int i = 0; i < 256; i++)
        a[i] += b[i];
}