https://bugs.llvm.org/show_bug.cgi?id=35299
Bug ID: 35299
Summary: Potential improvement for Loop Vectorization: XMM vs
YMM vs ZMM
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: serguei.kat...@azul.com
CC: llvm-bugs@lists.llvm.org
What is motivation to choose the lowest Vectorization Factor for equal vector
cost?
Specifically in LoopVectorizationCostModel::selectVectorizationFactor(unsigned
MaxVF):
for (unsigned i = 2; i <= MaxVF; i *= 2) {
...
if (VectorCost < Cost) {
Cost = VectorCost;
Width = i;
}
}
...
VectorizationFactor Factor = {Width, (unsigned)(Width * Cost)};
return Factor;
So we update vectorization factor to higher value only if the cost is strongly
less than previous computed cost.
So if the computed vectorization cost is equal for XMM, YMM and ZMM we always
will use XMM.
So my question is why it is done on this way?
why not to choose the maximum VF for equal cost?
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