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To deduce whether the optimization is legal we need to compare the target
features between caller and callee versions. The criteria for bypassing
the resolver are the following:
* If the callee's feature set is a subset of the caller's feature set,
then the callee is a candidate for direct call.
* Among such candidates the one of highest priority is the best match
and it shall be picked, unless there is a version of the callee with
higher priority than the best match which cannot be picked from a
higher priority caller (directly or through the resolver).
* For every higher priority callee version than the best match, there
is a higher priority caller version whose feature set availability
is implied by the callee's feature set.
Example:
Callers and Callees are ordered in decreasing priority.
The arrows indicate successful call redirections.
Caller Callee Explanation
=========================================================================
mops+sve2 --+--> mops all the callee versions are subsets of the
| caller but mops has the highest priority
|
mops --+ sve2 between mops and default callees, mops wins
sve sve between sve and default callees, sve wins
but sve2 does not have a high priority caller
default -----> default sve (callee) implies sve (caller),
sve2(callee) implies sve (caller),
mops(callee) implies mops(caller)
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