[llvm] [DAG][X86] expandABD - add branchless abds/abdu expansion for 0/-1 comparison result cases (PR #92780)

[Jay Foad via llvm-commits]

================
@@ -9228,11 +9228,21 @@ SDValue TargetLowering::expandABD(SDNode *N, SelectionDAG &DAG) const {
                        DAG.getNode(ISD::USUBSAT, dl, VT, LHS, RHS),
                        DAG.getNode(ISD::USUBSAT, dl, VT, RHS, LHS));
 
-  // abds(lhs, rhs) -> select(sgt(lhs,rhs), sub(lhs,rhs), sub(rhs,lhs))
-  // abdu(lhs, rhs) -> select(ugt(lhs,rhs), sub(lhs,rhs), sub(rhs,lhs))
   EVT CCVT = getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(), VT);
   ISD::CondCode CC = IsSigned ? ISD::CondCode::SETGT : ISD::CondCode::SETUGT;
   SDValue Cmp = DAG.getSetCC(dl, CCVT, LHS, RHS, CC);
+
+  // Branchless expansion iif cmp result is allbits:
+  // abds(lhs, rhs) -> sub(sgt(lhs, rhs), xor(sgt(lhs, rhs), sub(lhs, rhs)))
+  // abdu(lhs, rhs) -> sub(ugt(lhs, rhs), xor(ugt(lhs, rhs), sub(lhs, rhs)))
----------------
jayfoad wrote:

Yeah, good point, sorry. For some reason I thought the operands of the final `sub` were the other way round.

https://github.com/llvm/llvm-project/pull/92780