http://llvm.org/bugs/show_bug.cgi?id=17673
Bug ID: 17673
Summary: [Vectorization] Recognize strided access for
vectorization
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Stride Access is not supported in the LLVM vectorizer. In order to recognize
loops like:
for (i..N/3) {
a[3*i] = b[3*i] + I;
a[3*i+1] = b[3*i+1] + J;
a[3*i+2] = b[3*i+2] + K;
}
We can't re-roll the loop, nor we can use sequential access, since I, J and K
might have different values (assuming I, J, K, N constant). Detecting the
stride in this case is essential to be able to use interleaved loads/stores
(such as VLDN/VSTN on ARM).
The case where I==J==K can, sometimes, be rerolled, and on such cases, the
current vectorizer can already deal with the transformed loop, but we might
still want the vectorizer to deal with such cases, when loop re-roller is
disabled or it can't detect specific semantics.
Another case, when "a" is a reduction variable, like:
for (i..N/3) {
a += b[3*i] + I;
a += b[3*i+1] + J;
a += b[3*i+2] + K;
}
Should be dealt with the same way, so we need to make sure that the reduction
logic can also cope with non-unit strides.
Finally, non-unit memory induction should be transformed/mapped into the stride
access above, so that loops such as this:
for (i..N/3) {
a++ = b++ + I;
a++ = b++ + J;
a++ = b++ + K;
}
can also be vectorized.
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