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Re: LP64, unsigned int, vectorization, and PR 61247


On Thu, Oct 4, 2018 at 1:49 PM Steve Ellcey <sellcey@cavium.com> wrote:
>
> I was looking at PR tree-optimization/61247, where a loop with an unsigned
> int index on an LP64 platform was not getting vectorized and I noticed an
> odd thing.  In the function below, if I define N as 1000 or 10000, the
> loop does get vectorized, even in LP64 mode.  But if I define N as 100000,
> the loop does not get vectorized in LP64 mode.  I have not been able to
> figure out why this is or where the decision to vectorize (or not) is
> getting made.  Does anyone have an idea?  100000 is not a large enough value
> to hit the limit of a 32 bit int or unsigned int value so why can't it be
> vectorized like the other two cases?

i*N+j doesn't that wrap; e.g. when i is 100000-1, it is wrapping as
100000*(100000-1) needs 34 bits to be represented without wrapping ?

Thanks,
Andrew Pinski

>
> In the original test case that I added to this PR, N is an argument and
> we don't know what value it has.  It seems like this could be vectorized
> by including a test to make sure that the value is not larger than MAXINT
> and thus could not wrap when doing the array indexing.
>
> Steve Ellcey
> sellcey@cavium.com
>
>
>
> /* define N as 1000 - gets vectorized .... */
> /* define N as 10000 - gets vectorized .... */
> /* define N as 100000 - does not get vectorized .... */
>
> #define N 100000
>
> typedef unsigned int TYPE;
> void f(int *C, int *A, int val)
> {
>         TYPE i,j;
>         for (i=0; i<N; i++) {
>                 for (j=0; j<N; j++) {
>                         C[i*N+j]=A[i*N+j] * val;
>                 }
>         }
> }


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