[SelectionDAG] Improve codegen for udiv by constant if any divisors are 1.

[craig.topper]

Summary

If the divisor is 1, the magic algorithm does not return a correct
result and we end up using a select to pick the numerator for those
elements at the end.

Therefore we can use undef for that element of the earlier operations
when the divisor is 1. We sometimes get this through SimplifyDemandedVectorElts,
but not always. Definitely seems like we don't if the NPQ fixup is used.

Unfortunately, DAGCombiner is unable to fold srl X, <0, undef> to X so
I had to add flags to avoid emitting the srl unless one of the shift
amounts is non-zero.

[lebedev.ri]

LG, thanks.
There may be similar opportunity at least in the urem==C expansion.

DAGCombiner is unable to fold srl X, <0, undef> to X

This seems like a somewhat straight-forward missing optimization?
Although, then the second element becomes non-poison in the result.

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

6065

Not clang-formatted

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

if (!VT.isVector() && isa<ConstantSDNode>(N1)) {
     LeadingZeros = DAG.computeKnownBits(N0).countMinLeadingZeros();
     // UnsignedDivisionByConstantInfo doesn't work correctly if leading zeros in
     // the dividend exceeds the leading zeros for the divisor.
     LeadingZeros =
         std::min(LeadingZeros,
                  cast<ConstantSDNode>(N1)->getAPIntValue().countLeadingZeros());
   }
 
-  bool UseNPQ = false;
+  bool UseNPQ = false, UsePreShift = false, UsePostShift = false;
   SmallVector<SDValue, 16> PreShifts, PostShifts, MagicFactors, NPQFactors;
 
   auto BuildUDIVPattern = [&](ConstantSDNode *C) {
     if (C->isZero())
       return false;
     const APInt& Divisor = C->getAPIntValue();
 
-    bool SelNPQ = false;
-    APInt Magic(Divisor.getBitWidth(), 0);
-    unsigned PreShift = 0, PostShift = 0;
+    SDValue PreShift, MagicFactor, NPQFactor, PostShift;
 
     // Magic algorithm doesn't work for division by 1. We need to emit a select
     // at the end.
-    // TODO: Use undef values for divisor of 1.
-    if (!Divisor.isOne()) {
+    if (Divisor.isOne()) {
+      PreShift = PostShift = DAG.getUNDEF(ShSVT);
+      MagicFactor = NPQFactor = DAG.getUNDEF(SVT);
+    } else {
       UnsignedDivisionByConstantInfo magics =
           UnsignedDivisionByConstantInfo::get(Divisor, LeadingZeros);
 
-      Magic = std::move(magics.Magic);
+      MagicFactor = DAG.getConstant(magics.Magic, dl, SVT);
 
       assert(magics.PreShift < Divisor.getBitWidth() &&
              "We shouldn't generate an undefined shift!");
       assert(magics.PostShift < Divisor.getBitWidth() &&
              "We shouldn't generate an undefined shift!");
       assert((!magics.IsAdd || magics.PreShift == 0) &&
              "Unexpected pre-shift");
-      PreShift = magics.PreShift;
-      PostShift = magics.PostShift;
-      SelNPQ = magics.IsAdd;
+      PreShift = DAG.getConstant(magics.PreShift, dl, ShSVT);
+      PostShift = DAG.getConstant(magics.PostShift, dl, ShSVT);
+      NPQFactor = DAG.getConstant(
+          magics.IsAdd ? APInt::getOneBitSet(EltBits, EltBits - 1)
+                       : APInt::getZero(EltBits),
+          dl, SVT);
+      UseNPQ |= magics.IsAdd;
+      UsePreShift |= magics.PreShift != 0;
+      UsePostShift |= magics.PostShift != 0;
     }
 
-    PreShifts.push_back(DAG.getConstant(PreShift, dl, ShSVT));
-    MagicFactors.push_back(DAG.getConstant(Magic, dl, SVT));
-    NPQFactors.push_back(
-        DAG.getConstant(SelNPQ ? APInt::getOneBitSet(EltBits, EltBits - 1)
-                               : APInt::getZero(EltBits),
-                        dl, SVT));
-    PostShifts.push_back(DAG.getConstant(PostShift, dl, ShSVT));
-    UseNPQ |= SelNPQ;
+    PreShifts.push_back(PreShift);
+    MagicFactors.push_back(MagicFactor);
+    NPQFactors.push_back(NPQFactor);
+    PostShifts.push_back(PostShift);
     return true;
   };
 
   // Collect the shifts/magic values from each element.
   if (!ISD::matchUnaryPredicate(N1, BuildUDIVPattern))
     return SDValue();
 
   SDValue PreShift, PostShift, MagicFactor, NPQFactor;
SDValue TargetLowering::BuildUDIV(SDNode *N, SelectionDAG &DAG,
   } else {
     assert(isa<ConstantSDNode>(N1) && "Expected a constant");
     PreShift = PreShifts[0];
     MagicFactor = MagicFactors[0];
     PostShift = PostShifts[0];
   }
 
   SDValue Q = N0;
+  if (UsePreShift) {
     Q = DAG.getNode(ISD::SRL, dl, VT, Q, PreShift);
     Created.push_back(Q.getNode());
+  }
 
   // FIXME: We should support doing a MUL in a wider type.
   auto GetMULHU = [&](SDValue X, SDValue Y) {
     // If the type isn't legal, use a wider mul of the the type calculated
     // earlier.
     if (!isTypeLegal(VT)) {
       X = DAG.getNode(ISD::ZERO_EXTEND, dl, MulVT, X);
       Y = DAG.getNode(ISD::ZERO_EXTEND, dl, MulVT, Y);
else
       NPQ = DAG.getNode(ISD::SRL, dl, VT, NPQ, DAG.getConstant(1, dl, ShVT));
 
     Created.push_back(NPQ.getNode());
 
     Q = DAG.getNode(ISD::ADD, dl, VT, NPQ, Q);
     Created.push_back(Q.getNode());
   }
 
+  if (UsePostShift) {
     Q = DAG.getNode(ISD::SRL, dl, VT, Q, PostShift);
     Created.push_back(Q.getNode());
+  }
 
   EVT SetCCVT = getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(), VT);
 
   SDValue One = DAG.getConstant(1, dl, VT);
   SDValue IsOne = DAG.getSetCC(dl, SetCCVT, N1, One, ISD::SETEQ);
   return DAG.getSelect(dl, VT, IsOne, N0, Q);
 }
 

llvm/test/CodeGen/AArch64/GlobalISel/combine-udiv.ll

 ; GISEL-NEXT:    ret
   %1 = udiv <8 x i16> %x, <i16 7, i16 23, i16 25, i16 27, i16 31, i16 47, i16 63, i16 127>
   ret <8 x i16> %1
 }
 
 define <16 x i8> @combine_vec_udiv_nonuniform4(<16 x i8> %x) {
 ; SDAG-LABEL: combine_vec_udiv_nonuniform4:
 ; SDAG:       // %bb.0:
+; SDAG-NEXT:    movi v1.16b, #171
 ; SDAG-NEXT:    adrp x8, .LCPI4_0
-; SDAG-NEXT:    adrp x9, .LCPI4_2
-; SDAG-NEXT:    ldr q1, [x8, :lo12:.LCPI4_0]
-; SDAG-NEXT:    adrp x8, .LCPI4_1
-; SDAG-NEXT:    ldr q3, [x9, :lo12:.LCPI4_2]
+; SDAG-NEXT:    adrp x9, .LCPI4_1
 ; SDAG-NEXT:    umull2 v2.8h, v0.16b, v1.16b
+; SDAG-NEXT:    ldr q3, [x9, :lo12:.LCPI4_1]
 ; SDAG-NEXT:    umull v1.8h, v0.8b, v1.8b
+; SDAG-NEXT:    and v0.16b, v0.16b, v3.16b
 ; SDAG-NEXT:    uzp2 v1.16b, v1.16b, v2.16b
-; SDAG-NEXT:    ldr q2, [x8, :lo12:.LCPI4_1]
-; SDAG-NEXT:    adrp x8, .LCPI4_3
-; SDAG-NEXT:    ushl v1.16b, v1.16b, v2.16b
-; SDAG-NEXT:    ldr q2, [x8, :lo12:.LCPI4_3]
-; SDAG-NEXT:    and v1.16b, v1.16b, v3.16b
-; SDAG-NEXT:    and v0.16b, v0.16b, v2.16b
+; SDAG-NEXT:    ldr q2, [x8, :lo12:.LCPI4_0]
+; SDAG-NEXT:    ushr v1.16b, v1.16b, #7
+; SDAG-NEXT:    and v1.16b, v1.16b, v2.16b
 ; SDAG-NEXT:    orr v0.16b, v0.16b, v1.16b
 ; SDAG-NEXT:    ret
 ;
 ; GISEL-LABEL: combine_vec_udiv_nonuniform4:
 ; GISEL:       // %bb.0:
 ; GISEL-NEXT:    adrp x8, .LCPI4_2
 ; GISEL-NEXT:    adrp x9, .LCPI4_1
 ; GISEL-NEXT:    ldr q1, [x8, :lo12:.LCPI4_2]

llvm/test/CodeGen/X86/combine-udiv.ll

 ; XOP-NEXT:    retq
   %div = udiv <16 x i8> %x, <i8 -64, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1, i8 1>
   ret <16 x i8> %div
 }
 
 define <8 x i16> @pr38477(<8 x i16> %a0) {
 ; SSE2-LABEL: pr38477:
 ; SSE2:       # %bb.0:
-; SSE2-NEXT:    movdqa {{.*#+}} xmm1 = [0,4957,57457,4103,16385,35545,2048,2115]
+; SSE2-NEXT:    movdqa {{.*#+}} xmm1 = <u,4957,57457,4103,16385,35545,2048,2115>
 ; SSE2-NEXT:    pmulhuw %xmm0, %xmm1
 ; SSE2-NEXT:    movdqa %xmm0, %xmm2
 ; SSE2-NEXT:    psubw %xmm1, %xmm2
 ; SSE2-NEXT:    pmulhuw {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm2
 ; SSE2-NEXT:    paddw %xmm1, %xmm2
 ; SSE2-NEXT:    movdqa {{.*#+}} xmm1 = [65535,65535,65535,65535,65535,65535,0,65535]
 ; SSE2-NEXT:    pandn %xmm2, %xmm1
 ; SSE2-NEXT:    pmulhuw {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm2
 ; SSE2-NEXT:    por %xmm1, %xmm2
 ; SSE2-NEXT:    movdqa {{.*#+}} xmm1 = [0,65535,65535,65535,65535,65535,65535,65535]
 ; SSE2-NEXT:    pand %xmm1, %xmm2
 ; SSE2-NEXT:    pandn %xmm0, %xmm1
 ; SSE2-NEXT:    por %xmm2, %xmm1
 ; SSE2-NEXT:    movdqa %xmm1, %xmm0
 ; SSE2-NEXT:    retq
 ;
 ; SSE41-LABEL: pr38477:
 ; SSE41:       # %bb.0:
-; SSE41-NEXT:    movdqa {{.*#+}} xmm1 = [0,4957,57457,4103,16385,35545,2048,2115]
+; SSE41-NEXT:    movdqa {{.*#+}} xmm1 = <u,4957,57457,4103,16385,35545,2048,2115>
 ; SSE41-NEXT:    pmulhuw %xmm0, %xmm1
 ; SSE41-NEXT:    movdqa %xmm0, %xmm2
 ; SSE41-NEXT:    psubw %xmm1, %xmm2
 ; SSE41-NEXT:    pmulhuw {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm2
 ; SSE41-NEXT:    paddw %xmm1, %xmm2
 ; SSE41-NEXT:    movdqa {{.*#+}} xmm1 = <u,1024,1024,16,4,1024,u,4096>
 ; SSE41-NEXT:    pmulhuw %xmm2, %xmm1
 ; SSE41-NEXT:    pblendw {{.*#+}} xmm1 = xmm1[0,1,2,3,4,5],xmm2[6],xmm1[7]