[MIPS] For vectors, select `add %x, C` as `sub %x, -C` if it results in inline immediate

Summary:
As discussed in https://reviews.llvm.org/D62341#1515637,
for MIPS `add %x, -1` isn't optimal. Unlike X86 there
are no fastpaths to matearialize such `-1`/`1` vector constants,
and `sub %x, 1` results in better codegen,
so undo canonicalization

Reviewers: atanasyan, Petar.Avramovic, RKSimon

Reviewed By: atanasyan

Subscribers: sdardis, arichardson, hiraditya, jrtc27, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66805

llvm-svn: 372254

2 files changed, 56 insertions, 0 deletions

diff --git a/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp b/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp
index 42d158f44f3..e5997af3bcc 100644
--- a/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp
+++ b/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp
@@ -217,6 +217,51 @@ bool MipsDAGToDAGISel::selectVSplatMaskR(SDValue N, SDValue &Imm) const {
   return false;
 }
 
+/// Convert vector addition with vector subtraction if that allows to encode
+/// constant as an immediate and thus avoid extra 'ldi' instruction.
+/// add X, <-1, -1...> --> sub X, <1, 1...>
+bool MipsDAGToDAGISel::selectVecAddAsVecSubIfProfitable(SDNode *Node) {
+  assert(Node->getOpcode() == ISD::ADD && "Should only get 'add' here.");
+
+  EVT VT = Node->getValueType(0);
+  assert(VT.isVector() && "Should only be called for vectors.");
+
+  SDValue X = Node->getOperand(0);
+  SDValue C = Node->getOperand(1);
+
+  auto *BVN = dyn_cast<BuildVectorSDNode>(C);
+  if (!BVN)
+    return false;
+
+  APInt SplatValue, SplatUndef;
+  unsigned SplatBitSize;
+  bool HasAnyUndefs;
+
+  if (!BVN->isConstantSplat(SplatValue, SplatUndef, SplatBitSize, HasAnyUndefs,
+                            8, !Subtarget->isLittle()))
+    return false;
+
+  auto IsInlineConstant = [](const APInt &Imm) { return Imm.isIntN(5); };
+
+  if (IsInlineConstant(SplatValue))
+    return false; // Can already be encoded as an immediate.
+
+  APInt NegSplatValue = 0 - SplatValue;
+  if (!IsInlineConstant(NegSplatValue))
+    return false; // Even if we negate it it won't help.
+
+  SDLoc DL(Node);
+
+  SDValue NegC = CurDAG->FoldConstantArithmetic(
+      ISD::SUB, DL, VT, CurDAG->getConstant(0, DL, VT).getNode(), C.getNode());
+  assert(NegC && "Constant-folding failed!");
+  SDValue NewNode = CurDAG->getNode(ISD::SUB, DL, VT, X, NegC);
+
+  ReplaceNode(Node, NewNode.getNode());
+  SelectCode(NewNode.getNode());
+  return true;
+}
+
 /// Select instructions not customized! Used for
 /// expanded, promoted and normal instructions
 void MipsDAGToDAGISel::Select(SDNode *Node) {
@@ -236,6 +281,12 @@ void MipsDAGToDAGISel::Select(SDNode *Node) {
   switch(Opcode) {
   default: break;
 
+  case ISD::ADD:
+    if (Node->getSimpleValueType(0).isVector() &&
+        selectVecAddAsVecSubIfProfitable(Node))
+      return;
+    break;
+
   // Get target GOT address.
   case ISD::GLOBAL_OFFSET_TABLE:
     ReplaceNode(Node, getGlobalBaseReg());
diff --git a/llvm/lib/Target/Mips/MipsISelDAGToDAG.h b/llvm/lib/Target/Mips/MipsISelDAGToDAG.h
index bae3bbf71f3..a768589b374 100644
--- a/llvm/lib/Target/Mips/MipsISelDAGToDAG.h
+++ b/llvm/lib/Target/Mips/MipsISelDAGToDAG.h
@@ -125,6 +125,11 @@ private:
   /// starting at bit zero.
   virtual bool selectVSplatMaskR(SDValue N, SDValue &Imm) const;
 
+  /// Convert vector addition with vector subtraction if that allows to encode
+  /// constant as an immediate and thus avoid extra 'ldi' instruction.
+  /// add X, <-1, -1...> --> sub X, <1, 1...>
+  bool selectVecAddAsVecSubIfProfitable(SDNode *Node);
+
   void Select(SDNode *N) override;
 
   virtual bool trySelect(SDNode *Node) = 0;