Auto-vectorizer and (mis-)alignment support assumptions

Frederic Riss frederic.riss@gmail.com
Thu Sep 12 08:40:00 GMT 2013


Hello,

I have coded SIMD support for my target, but I'm hitting some issues
relative to alignment. I can't find any documentation or comment
describing the assumptions that the vectorizer makes about target
support, if it exists please just point me at it (I'm based on GCC 4.7
for now).

On my target, vectors need to be aligned on their full size. For
example V2SI vectors need to be aligned on 8 byte boundaries.

Now take this simple code:

unsigned long foo (unsigned long *input)
{
        unsigned long i, res;
        unsigned long data[80];

        memcpy (data, input, sizeof(data));
        for (i=16; i<80; i++)
                data[i] = data[i-3];

        return data[80];
}

In this incarnation, whatever target hooks I implement to tell the
auto-vectorizer that my V2SI vectors need 8 byte alignment (and that
it doesn't support misalignment), it decides that the loop is
vectorizable and generates:

  vector(2) long unsigned int * vect_pdata.20;
  vector(2) long unsigned int vect_var_.19;
  vector(2) long unsigned int * vect_pdata.18;
  vector(2) long unsigned int * vect_pdata.15;
[...]
  vect_var_.19_35 = MEM[(long unsigned int[80] *)vect_pdata.15_33];
  MEM[(long unsigned int[80] *)vect_pdata.20_37] = vect_var_.19_35;
  vect_pdata.15_34 = vect_pdata.15_33 + 8;
  vect_pdata.20_38 = vect_pdata.20_37 + 8;

Where the array references start with the following definition:

  vect_pdata.18_32 = &MEM[(void *)&data + 52B];
  vect_pdata.23_36 = &MEM[(void *)&data + 64B];

As you see, both vector pointers can't possibly be aligned on an 8
byte boundary.

Interestingly, If I put the data array as a global variable, then it
refuses to vectorize because the alignment of the accesses is unknown.
I suppose that when the array is local it can be forced to whatever
alignment fits. This doesn't however guarantee that all accesses can
be forced to the right alignment, because their relative offsets are
fixed.

Thus I'm wondering if the vectorizer makes the assumption that a
target must be able to load/store vectors aligned on any element size
boundary. If it's the case, how can a target determine that a specific
access might not be fully aligned (and handle it specifically)?

If this assumption is not made, please point me at the code that
checks the relative alignment of vectorized accesses, so that I can
find out what my targeting lacks.

Many thanks,
Fred



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