This pattern occurs when lowering for overflow operations
introduce an xor after select_cc has already been formed.
I had to rework another combine that looked for select_cc of an xor
with 1. That xor will now get combined away so we just need to
look for the RHS of the select_cc being 1.
Reviewed By: luismarques
Differential Revision: https://reviews.llvm.org/D98130
Rather than converting 3 signbits to bools and comparing them,
we can do bitwise logic on the whole vector and convert the
resulting sign bit to a bool at the end.
This is still a different algorithm than what we do in LegalizeDAG
through expandSADDOSSUBO. That algorithm needs to know that the
RHS of SSUBO is > 0, but that's costly when the type is split.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D97325
This code creates 3 setccs that need to be expanded. It was
creating a sign bit test as setge X, 0 which is non-canonical.
Canonical would be setgt X, -1. This misses the special case in
IntegerExpandSetCCOperands for sign bit tests that assumes
canonical form. If we don't hit this special case we end up
with a multipart setcc instead of just checking the sign of
the high part.
To fix this I've reversed the polarity of all of the setccs to
setlt X, 0 which is canonical. The rest of the logic should
still work. This seems to produce better code on RISCV which
lacks a setgt instruction.
This probably still isn't the best code sequence we could use here.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D97181
Craig Topper