[DAG][RISCV] Use vp_reduce_* when widening illegal types for reductions (llvm#105455)

This allows the use a single wider operation with a restricted EVL
instead of padding the vector with the neutral element.

For RISCV specifically, it's worth noting that an alternate padded
lowering is available when VL is one less than a power of two, and LMUL
<= m1. We could slide the vector operand up by one, and insert the
padding via a vslide1up. We don't currently pattern match this, but we
could. This form would arguably be better iff the surrounding code
wanted VL=4. This patch will force a VL toggle in that case instead.

Basically, it comes down to a question of whether we think odd sized
vectors are going to appear clustered with odd size vector operations,
or mixed in with larger power of two operations.

Note there is a potential downside of using vp nodes; we loose any
generic DAG combines which might have applied to the widened form.

Author: preames

llvm/include/llvm/IR/VPIntrinsics.def

@@ -651,63 +651,64 @@ END_REGISTER_VP(vp_gather, VP_GATHER)
 #error                                                                         \
     "The internal helper macro HELPER_REGISTER_REDUCTION_VP is already defined!"
 #endif
-#define HELPER_REGISTER_REDUCTION_VP(VPID, VPSD, INTRIN)                       \
+#define HELPER_REGISTER_REDUCTION_VP(VPID, VPSD, INTRIN, SDOPC)                \
   BEGIN_REGISTER_VP(VPID, 2, 3, VPSD, 1)                                       \
   VP_PROPERTY_FUNCTIONAL_INTRINSIC(INTRIN)                                     \
+  VP_PROPERTY_FUNCTIONAL_SDOPC(SDOPC)                                          \
   VP_PROPERTY_REDUCTION(0, 1)                                                  \
   END_REGISTER_VP(VPID, VPSD)
 
 // llvm.vp.reduce.add(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_add, VP_REDUCE_ADD,
-                             vector_reduce_add)
+                             vector_reduce_add, VECREDUCE_ADD)
 
 // llvm.vp.reduce.mul(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_mul, VP_REDUCE_MUL,
-                             vector_reduce_mul)
+                             vector_reduce_mul, VECREDUCE_MUL)
 
 // llvm.vp.reduce.and(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_and, VP_REDUCE_AND,
-                             vector_reduce_and)
+                             vector_reduce_and, VECREDUCE_AND)
 
 // llvm.vp.reduce.or(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_or, VP_REDUCE_OR,
-                             vector_reduce_or)
+                             vector_reduce_or, VECREDUCE_OR)
 
 // llvm.vp.reduce.xor(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_xor, VP_REDUCE_XOR,
-                             vector_reduce_xor)
+                             vector_reduce_xor, VECREDUCE_XOR)
 
 // llvm.vp.reduce.smax(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_smax, VP_REDUCE_SMAX,
-                             vector_reduce_smax)
+                             vector_reduce_smax, VECREDUCE_SMAX)
 
 // llvm.vp.reduce.smin(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_smin, VP_REDUCE_SMIN,
-                             vector_reduce_smin)
+                             vector_reduce_smin, VECREDUCE_SMIN)
 
 // llvm.vp.reduce.umax(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_umax, VP_REDUCE_UMAX,
-                             vector_reduce_umax)
+                             vector_reduce_umax, VECREDUCE_UMAX)
 
 // llvm.vp.reduce.umin(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_umin, VP_REDUCE_UMIN,
-                             vector_reduce_umin)
+                             vector_reduce_umin, VECREDUCE_UMIN)
 
 // llvm.vp.reduce.fmax(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_fmax, VP_REDUCE_FMAX,
-                             vector_reduce_fmax)
+                             vector_reduce_fmax, VECREDUCE_FMAX)
 
 // llvm.vp.reduce.fmin(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_fmin, VP_REDUCE_FMIN,
-                             vector_reduce_fmin)
+                             vector_reduce_fmin, VECREDUCE_FMIN)
 
 // llvm.vp.reduce.fmaximum(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_fmaximum, VP_REDUCE_FMAXIMUM,
-                             vector_reduce_fmaximum)
+                             vector_reduce_fmaximum, VECREDUCE_FMAXIMUM)
 
 // llvm.vp.reduce.fminimum(start,x,mask,vlen)
 HELPER_REGISTER_REDUCTION_VP(vp_reduce_fminimum, VP_REDUCE_FMINIMUM,
-                             vector_reduce_fminimum)
+                             vector_reduce_fminimum, VECREDUCE_FMINIMUM)
 
 #undef HELPER_REGISTER_REDUCTION_VP
 

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

@@ -7271,9 +7271,29 @@ SDValue DAGTypeLegalizer::WidenVecOp_STRICT_FSETCC(SDNode *N) {
   return DAG.getBuildVector(VT, dl, Scalars);
 }
 
+static unsigned getExtendForIntVecReduction(unsigned Opc) {
+  switch (Opc) {
+  default:
+    llvm_unreachable("Expected integer vector reduction");
+  case ISD::VECREDUCE_ADD:
+  case ISD::VECREDUCE_MUL:
+  case ISD::VECREDUCE_AND:
+  case ISD::VECREDUCE_OR:
+  case ISD::VECREDUCE_XOR:
+    return ISD::ANY_EXTEND;
+  case ISD::VECREDUCE_SMAX:
+  case ISD::VECREDUCE_SMIN:
+    return ISD::SIGN_EXTEND;
+  case ISD::VECREDUCE_UMAX:
+  case ISD::VECREDUCE_UMIN:
+    return ISD::ZERO_EXTEND;
+  }
+}
+
 SDValue DAGTypeLegalizer::WidenVecOp_VECREDUCE(SDNode *N) {
   SDLoc dl(N);
   SDValue Op = GetWidenedVector(N->getOperand(0));
+  EVT VT = N->getValueType(0);
   EVT OrigVT = N->getOperand(0).getValueType();
   EVT WideVT = Op.getValueType();
   EVT ElemVT = OrigVT.getVectorElementType();
@@ -7288,6 +7308,25 @@ SDValue DAGTypeLegalizer::WidenVecOp_VECREDUCE(SDNode *N) {
   unsigned OrigElts = OrigVT.getVectorMinNumElements();
   unsigned WideElts = WideVT.getVectorMinNumElements();
 
+  // Generate a vp.reduce_op if it is custom/legal for the target.  This avoids
+  // needing to pad the source vector, because the inactive lanes can simply be
+  // disabled and not contribute to the result.
+  // TODO: VECREDUCE_FADD, VECREDUCE_FMUL aren't currently mapped correctly,
+  // and thus don't take this path.
+  if (auto VPOpcode = ISD::getVPForBaseOpcode(Opc);
+      VPOpcode && TLI.isOperationLegalOrCustom(*VPOpcode, WideVT)) {
+    SDValue Start = NeutralElem;
+    if (VT.isInteger())
+      Start = DAG.getNode(getExtendForIntVecReduction(Opc), dl, VT, Start);
+    assert(Start.getValueType() == VT);
+    EVT WideMaskVT = EVT::getVectorVT(*DAG.getContext(), MVT::i1,
+                                      WideVT.getVectorElementCount());
+    SDValue Mask = DAG.getAllOnesConstant(dl, WideMaskVT);
+    SDValue EVL = DAG.getElementCount(dl, TLI.getVPExplicitVectorLengthTy(),
+                                      OrigVT.getVectorElementCount());
+    return DAG.getNode(*VPOpcode, dl, VT, {Start, Op, Mask, EVL}, Flags);
+  }
+
   if (WideVT.isScalableVector()) {
     unsigned GCD = std::gcd(OrigElts, WideElts);
     EVT SplatVT = EVT::getVectorVT(*DAG.getContext(), ElemVT,
@@ -7296,14 +7335,14 @@ SDValue DAGTypeLegalizer::WidenVecOp_VECREDUCE(SDNode *N) {
     for (unsigned Idx = OrigElts; Idx < WideElts; Idx = Idx + GCD)
       Op = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, WideVT, Op, SplatNeutral,
                        DAG.getVectorIdxConstant(Idx, dl));
-    return DAG.getNode(Opc, dl, N->getValueType(0), Op, Flags);
+    return DAG.getNode(Opc, dl, VT, Op, Flags);
   }
 
   for (unsigned Idx = OrigElts; Idx < WideElts; Idx++)
     Op = DAG.getNode(ISD::INSERT_VECTOR_ELT, dl, WideVT, Op, NeutralElem,
                      DAG.getVectorIdxConstant(Idx, dl));
 
-  return DAG.getNode(Opc, dl, N->getValueType(0), Op, Flags);
+  return DAG.getNode(Opc, dl, VT, Op, Flags);
 }
 
 SDValue DAGTypeLegalizer::WidenVecOp_VECREDUCE_SEQ(SDNode *N) {