[GlobalISel] Remove references to rhs of shufflevector if rhs is undef

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

@@ -867,6 +867,10 @@ class CombinerHelper {
   /// register and different indices.
   bool matchExtractVectorElementWithDifferentIndices(const MachineOperand &MO,
                                                      BuildFnTy &MatchInfo);
+
+  /// Remove references to rhs if it is undef
+  bool matchShuffleUndefRHS(MachineInstr &MI, BuildFnTy &MatchInfo);
+
   /// Use a function which takes in a MachineIRBuilder to perform a combine.
   /// By default, it erases the instruction def'd on \p MO from the function.
   void applyBuildFnMO(const MachineOperand &MO, BuildFnTy &MatchInfo);

llvm/include/llvm/Target/GlobalISel/Combine.td

@@ -1568,6 +1568,14 @@ def expand_const_fpowi : GICombineRule<
           [{ return Helper.matchFPowIExpansion(*${root}, ${imm}.getCImm()->getSExtValue()); }]),
    (apply [{ Helper.applyExpandFPowI(*${root}, ${imm}.getCImm()->getSExtValue()); }])>;
 
+def combine_shuffle_undef_rhs : GICombineRule<
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (G_IMPLICIT_DEF $undef),
+         (G_SHUFFLE_VECTOR $root, $src1, $undef, $mask):$root,
+        [{ return Helper.matchShuffleUndefRHS(*${root}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])
+>;
+
 // match_extract_of_element and insert_vector_elt_oob must be the first!
 def vector_ops_combines: GICombineGroup<[
 match_extract_of_element_undef_vector,
@@ -1917,6 +1925,9 @@ def constant_fold_binops : GICombineGroup<[constant_fold_binop,
 
 def prefer_sign_combines : GICombineGroup<[nneg_zext]>;
 
+def shuffle_combines : GICombineGroup<[combine_shuffle_concat,
+                                       combine_shuffle_undef_rhs]>;
+
 def all_combines : GICombineGroup<[integer_reassoc_combines, trivial_combines,
     vector_ops_combines, freeze_combines, cast_combines,
     insert_vec_elt_combines, extract_vec_elt_combines, combines_for_extload,
@@ -1938,7 +1949,7 @@ def all_combines : GICombineGroup<[integer_reassoc_combines, trivial_combines,
     sub_add_reg, select_to_minmax,
     fsub_to_fneg, commute_constant_to_rhs, match_ands, match_ors,
     combine_concat_vector, match_addos,
-    sext_trunc, zext_trunc, prefer_sign_combines, combine_shuffle_concat,
+    sext_trunc, zext_trunc, prefer_sign_combines, shuffle_combines,
     combine_use_vector_truncate, merge_combines]>;
 
 // A combine group used to for prelegalizer combiners at -O0. The combines in

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -7696,3 +7696,33 @@ bool CombinerHelper::matchUnmergeValuesAnyExtBuildVector(const MachineInstr &MI,
 
   return false;
 }
+
+bool CombinerHelper::matchShuffleUndefRHS(MachineInstr &MI,
+                                          BuildFnTy &MatchInfo) {
+
+  bool Changed = false;
+  auto &Shuffle = cast<GShuffleVector>(MI);
+  ArrayRef<int> OrigMask = Shuffle.getMask();
+  SmallVector<int, 16> NewMask;
+  const LLT SrcTy = MRI.getType(Shuffle.getSrc1Reg());
+  const unsigned NumSrcElems = SrcTy.isVector() ? SrcTy.getNumElements() : 1;
+  const unsigned NumDstElts = OrigMask.size();
+  for (unsigned i = 0; i != NumDstElts; ++i) {
+    int Idx = OrigMask[i];
+    if (Idx >= (int)NumSrcElems) {
+      Idx = -1;
+      Changed = true;
+    }
+    NewMask.push_back(Idx);
+  }
+
+  if (!Changed)
+    return false;
+
+  MatchInfo = [&, NewMask](MachineIRBuilder &B) {
+    B.buildShuffleVector(MI.getOperand(0), MI.getOperand(1), MI.getOperand(2),
+                         NewMask);
+  };
+
+  return true;
+}

llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp

@@ -772,13 +772,13 @@ MachineInstrBuilder MachineIRBuilder::buildShuffleVector(const DstOp &Res,
   LLT DstTy = Res.getLLTTy(*getMRI());
   LLT Src1Ty = Src1.getLLTTy(*getMRI());
   LLT Src2Ty = Src2.getLLTTy(*getMRI());
-  assert((size_t)(Src1Ty.getNumElements() + Src2Ty.getNumElements()) >=
-         Mask.size());
-  assert(DstTy.getElementType() == Src1Ty.getElementType() &&
-         DstTy.getElementType() == Src2Ty.getElementType());
-  (void)DstTy;
-  (void)Src1Ty;
-  (void)Src2Ty;
+  const LLT DstElemTy = DstTy.isVector() ? DstTy.getElementType() : DstTy;
+  const LLT ElemTy1 = Src1Ty.isVector() ? Src1Ty.getElementType() : Src1Ty;
+  const LLT ElemTy2 = Src2Ty.isVector() ? Src2Ty.getElementType() : Src2Ty;
+  assert(DstElemTy == ElemTy1 && DstElemTy == ElemTy2);
+  (void)DstElemTy;
+  (void)ElemTy1;
+  (void)ElemTy2;
   ArrayRef<int> MaskAlloc = getMF().allocateShuffleMask(Mask);
   return buildInstr(TargetOpcode::G_SHUFFLE_VECTOR, {Res}, {Src1, Src2})
       .addShuffleMask(MaskAlloc);

llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-shuffle-vector-undef-rhs.mir

@@ -0,0 +1,42 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64  -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
+
+---
+name: shuffle_vector_undef_rhs
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $d0
+
+    ; CHECK-LABEL: name: shuffle_vector_undef_rhs
+    ; CHECK: liveins: $d0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:_(<2 x s32>) = COPY $d0
+    ; CHECK-NEXT: [[DEF:%[0-9]+]]:_(<2 x s32>) = G_IMPLICIT_DEF
+    ; CHECK-NEXT: [[SHUF:%[0-9]+]]:_(<4 x s32>) = G_SHUFFLE_VECTOR [[COPY]](<2 x s32>), [[DEF]], shufflemask(0, undef, 1, undef)
+    ; CHECK-NEXT: RET_ReallyLR implicit [[SHUF]](<4 x s32>)
+    %0:_(<2 x s32>) = COPY $d0
+    %1:_(<2 x s32>) = G_IMPLICIT_DEF
+    %2:_(<4 x s32>) = G_SHUFFLE_VECTOR %0(<2 x s32>), %1(<2 x s32>), shufflemask(0, 2, 1, 3)
+    RET_ReallyLR implicit %2
+...
+
+---
+name: shuffle_vector_undef_rhs_scalar
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $x0
+
+    ; CHECK-LABEL: name: shuffle_vector_undef_rhs_scalar
+    ; CHECK: liveins: $x0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:_(s64) = COPY $x0
+    ; CHECK-NEXT: [[DEF:%[0-9]+]]:_(s64) = G_IMPLICIT_DEF
+    ; CHECK-NEXT: [[BUILD_VECTOR:%[0-9]+]]:_(<2 x s64>) = G_BUILD_VECTOR [[COPY]](s64), [[DEF]](s64)
+    ; CHECK-NEXT: RET_ReallyLR implicit [[BUILD_VECTOR]](<2 x s64>)
+    %0:_(s64) = COPY $x0
+    %1:_(s64) = G_IMPLICIT_DEF
+    %2:_(<2 x s64>) = G_SHUFFLE_VECTOR %0(s64), %1(s64), shufflemask(0, 1)
+    RET_ReallyLR implicit %2
+...