GCC: vectorization difference between two similar loops

Question

When compiling with gcc -O3, why does the following loop not vectorize (automatically):

#define SIZE (65536)

int a[SIZE], b[SIZE], c[SIZE];

in         


        

Answer 1

First one just trivially changing a[] many times(temporary). Second one uses the last value of a[] each time(not temporary).

Until a version of patch, you could use "volatile" variable to vectorize.

Use

int * c=malloc(sizeof(int));

to make it aligned;

v.c:9: note: Unknown alignment for access: c

Shows "c" having different storage area than b and a.

I assume "movaps"-like instructions for being "vectorized" (from SSE-AVX instruction list)

Here: http://gcc.gnu.org/projects/tree-ssa/vectorization.html#using

the 6th and 7th examples show similar difficulties.