The current approach to lower a vsetult is to flip the sign bit of the
operands, swap the operands and then use a (signed) pcmpgt. psubus (unsigned
saturating subtract) can be used to emulate a vsetult more efficiently:
+ case ISD::SETULT: {
+ // If the comparison is against a constant we can turn this into a
+ // setule. With psubus, setule does not require a swap. This is
+ // beneficial because the constant in the register is no longer
+ // destructed as the destination so it can be hoisted out of a loop.
I also enable lowering via psubus in a few other cases where it's clearly
beneficial: setule and setuge if minu/maxu cannot be used.
rdar://problem/14338765
Patch by Adam Nemet <anemet@apple.com>.
llvm-svn: 202301
Occasionally DAGCombiner can spot that a SETCC operation is completely
redundant and reduce it to "all true" or "all false". If this happens to a
vector, the value produced has to take account of what a normal comparison
would have produced, which may be an all-1s bitmask.
The fix in SelectionDAG.cpp is tested, however, as far as I can see the code in
TargetLowering.cpp is possibly unreachable and almost certainly irrelevant when
triggered so there are no tests. However, I believe it's still clearly the
right change and may save someone else some hassle if it suddenly becomes
reachable. So I'm doing it anyway.
llvm-svn: 190147
Use PMIN/PMAX for UGE/ULE vector comparions to reduce the number of required
instructions. This trick also works for UGT/ULT, but there is no advantage in
doing so. It wouldn't reduce the number of instructions and it would actually
reduce performance.
Reviewer: Ben
radar:5972691
llvm-svn: 186432
Simon Pilgrim