[GISel][CombinerHelper] Add concat_vectors(build_vector, build_vector) => build_vector

Teach the combiner helper how to flatten concat_vectors of build_vectors
into a build_vector.

Add this combine as part of AArch64 pre-legalizer combiner.

Differential Revision: https://reviews.llvm.org/D69071

llvm-svn: 375066

1 files changed, 84 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
index da78327ebdb..e320fb32dc0 100644
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -89,6 +89,90 @@ void CombinerHelper::applyCombineCopy(MachineInstr &MI) {
   replaceRegWith(MRI, DstReg, SrcReg);
 }
 
+bool CombinerHelper::tryCombineConcatVectors(MachineInstr &MI) {
+  bool IsUndef = false;
+  SmallVector<Register, 4> Ops;
+  if (matchCombineConcatVectors(MI, IsUndef, Ops)) {
+    applyCombineConcatVectors(MI, IsUndef, Ops);
+    return true;
+  }
+  return false;
+}
+
+bool CombinerHelper::matchCombineConcatVectors(MachineInstr &MI, bool &IsUndef,
+                                               SmallVectorImpl<Register> &Ops) {
+  assert(MI.getOpcode() == TargetOpcode::G_CONCAT_VECTORS &&
+         "Invalid instruction");
+  IsUndef = true;
+  MachineInstr *Undef = nullptr;
+
+  // Walk over all the operands of concat vectors and check if they are
+  // build_vector themselves or undef.
+  // Then collect their operands in Ops.
+  for (const MachineOperand &MO : MI.operands()) {
+    // Skip the instruction definition.
+    if (MO.isDef())
+      continue;
+    Register Reg = MO.getReg();
+    MachineInstr *Def = MRI.getVRegDef(Reg);
+    assert(Def && "Operand not defined");
+    switch (Def->getOpcode()) {
+    case TargetOpcode::G_BUILD_VECTOR:
+      IsUndef = false;
+      // Remember the operands of the build_vector to fold
+      // them into the yet-to-build flattened concat vectors.
+      for (const MachineOperand &BuildVecMO : Def->operands()) {
+        // Skip the definition.
+        if (BuildVecMO.isDef())
+          continue;
+        Ops.push_back(BuildVecMO.getReg());
+      }
+      break;
+    case TargetOpcode::G_IMPLICIT_DEF: {
+      LLT OpType = MRI.getType(Reg);
+      // Keep one undef value for all the undef operands.
+      if (!Undef) {
+        Builder.setInsertPt(*MI.getParent(), MI);
+        Undef = Builder.buildUndef(OpType.getScalarType());
+      }
+      LLT UndefType = MRI.getType(Undef->getOperand(0).getReg());
+      assert(UndefType == OpType.getScalarType() &&
+             "All undefs should have the same type");
+      // Break the undef vector in as many scalar elements as needed
+      // for the flattening.
+      for (unsigned EltIdx = 0, EltEnd = OpType.getNumElements();
+           EltIdx != EltEnd; ++EltIdx)
+        Ops.push_back(Undef->getOperand(0).getReg());
+      break;
+    }
+    default:
+      return false;
+    }
+  }
+  return true;
+}
+void CombinerHelper::applyCombineConcatVectors(
+    MachineInstr &MI, bool IsUndef, const ArrayRef<Register> Ops) {
+  // We determined that the concat_vectors can be flatten.
+  // Generate the flattened build_vector.
+  Register DstReg = MI.getOperand(0).getReg();
+  Builder.setInsertPt(*MI.getParent(), MI);
+  Register NewDstReg = MRI.cloneVirtualRegister(DstReg);
+
+  // Note: IsUndef is sort of redundant. We could have determine it by
+  // checking that at all Ops are undef.  Alternatively, we could have
+  // generate a build_vector of undefs and rely on another combine to
+  // clean that up.  For now, given we already gather this information
+  // in tryCombineConcatVectors, just save compile time and issue the
+  // right thing.
+  if (IsUndef)
+    Builder.buildUndef(NewDstReg);
+  else
+    Builder.buildBuildVector(NewDstReg, Ops);
+  MI.eraseFromParent();
+  replaceRegWith(MRI, DstReg, NewDstReg);
+}
+
 namespace {
 
 /// Select a preference between two uses. CurrentUse is the current preference