[LLVMdev] Trip count and Loop Vectorizer

Murali, Sriram sriram.murali at intel.com
Fri Sep 27 09:41:05 PDT 2013

I am trying to get a small loop to *not vectorize* for cases where it doesn't make sense. For instance, this loop:
void foo(int a[4][8], int n)
    int b[4][8];
    for(int i = 0; i < 4; i++) {
        for(int j = 0; j < n; j++) {
            a[i][j] = b[i][j];
* Has maximum of 8ints copy. LLVM tries to use Memcpy for the inner loop. It is not helpful to perform memcpy for such small moves, especially when the outer loop is unrolled since the trip count is constant (4). The 4 calls to memcpy is not efficient.
* Therefore, I disabled the memcpy optimization for such cases, and found that LLVM  LoopVectorizer successfully vectorizes and unrolls the inner loop. However, in order to take the fast path (vmovups) it must copy at least 32 ints, where as in this case we only do an 8int copy.
** Upon closer look, LoopVectorizer obtains the TripCount for the innerloop using getSmallConstantTripCount(Loop,...). This value is 0 for the loop with unknown trip count. Loop unrolling is disabled when TC > 0. Should this be changed to TC >= 0 (which does the job for this testcase)? Or is there a better way to disable loop unrolling for such trivial loops, at least the ones with known array size?

Thanks for your feedback


Sriram Murali
+1 (519) 772 - 2579

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