Cilk array notation prevents gcc auto-vectorization?

Christopher_F_ — Tue, 17 Sep 2013 00:57:22 GMT

Hi all,

I am having a problem getting Cilk array notation to play well with Gcc auto-vectorization (i.e., -ftree-vectorize). For example,

(Let's refer to the code snippet at http://www.cilkplus.org/tutorial-add-arrays as copy.c).

If I run

> gcc -fcilkplus -lcilkrts -Wall -O3 copy.c

using "gcc version 4.8.1 20130520 (prerelease) (GCC)" downloaded from https://www.cilkplus.org/download, I get a functionally correct code, but only the reference loop (i.e.,

for (int i = 0; i < array_size; i++) c = a + b;

)

will vectorize (according to both -ftree-vectorizer-verbose=1 and manual assembly inspection). I am having similar luck when explicitely using -ftree-vectorize without -O3. I have verified that -O3 is not pruning the loops and the problem is still the same with other vector operators.

The simplest problem is of course a missing flag, but I don't see documentation for such a flag (beyond -fcilkplus which is needed for array notation to compile). Further, it seems that the simplest loops described in array notation do vectorize; e.g., 'c[:] = 5;'

Thanks for any tips!

Chris Fletcher

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Cilk array notation prevents gcc auto-vectorization?