On Tue, 25 Aug 2009, Ira Rosen wrote:
My loop doesn't vectorize. The message says:
./TestAutoVect.cpp:31: note: not vectorized: unhandled data-ref
22
//-----------------------------------------------------------------------------
23 void MaskRGBA(
24 const int *in,
25 const int * const inEnd,
26 int *out,
27 const int * const m)
28 {
29 for (;in<inEnd; ++in, ++out)
30 {
31 *out=*in&*m;
32 }
33 }
The problem is that '*m' cannot be taken out of the loop because the alias
analysis cannot prove that it doesn't alias other memory accesses in the
loop. This causes data references analysis to fail since the above access
is loop invariant. Therefore, this loop is not vectorized.
I found that when I change it to this
23
//-----------------------------------------------------------------------------
24 void MaskRGBA(
25 const int *in,
26 const int * const inEnd,
27 int *out,
28 const int * const mask)
29 {
30 const int m=*mask;
31 for (;in<inEnd; ++in, ++out)
32 {
33 *out=*in&m;
34 }
35 }
Here there is no such problem.
Actually there still is possible aliasing here, if (out-in) is positive
and small enough. However, gcc is clever enough to test if this is the
case and use the vectorized code only when it is safe. Supposedly, it
could as well check if m is somewhere after out, although it increases the
overhead again, and the appropriate logic is possibly missing in that
version of the compiler.
You can also use __restrict__ to help vectorization:
Indeed. Too bad restrict is not standard in C++ :-(
In the example shown here, the mask looks like it should be passed by
value instead of by pointer, but I guess the real code modifies the mask
afterwards.
--
Marc Glisse
