[PATCH] D133866: [SelectionDAG] Add helper function to check whether a SDValue is neutral element. NFC.

[Yeting Kuo via Phabricator via llvm-commits]

fakepaper56 created this revision.
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Using this helper makes work about neutral elements more easier. Although I only
find one case now, I think it will have more chance to be used since so many
combine works are related to neutral elements.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D133866

Files:
  llvm/include/llvm/CodeGen/SelectionDAGNodes.h
  llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp


Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -7968,17 +7968,9 @@
   if (!isOneConstant(ScalarV.getOperand(2)))
     return SDValue();
 
-  // TODO: Deal with value other than neutral element.
-  auto IsRVVNeutralElement = [Opc, &DAG](SDNode *N, SDValue V) {
-    if (Opc == ISD::FADD && N->getFlags().hasNoSignedZeros() &&
-        isNullFPConstant(V))
-      return true;
-    return DAG.getNeutralElement(Opc, SDLoc(V), V.getSimpleValueType(),
-                                 N->getFlags()) == V;
-  };
-
   // Check the scalar of ScalarV is neutral element
-  if (!IsRVVNeutralElement(N, ScalarV.getOperand(1)))
+  // TODO: Deal with value other than neutral element.
+  if (!isNeutralConstant(N->getOpcode(), N->getFlags(), ScalarV.getOperand(1)))
     return SDValue();
 
   if (!ScalarV.hasOneUse())
Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -10735,6 +10735,50 @@
   return Const != nullptr && Const->isMinSignedValue();
 }
 
+bool llvm::isNeutralConstant(unsigned Opcode, SDNodeFlags Flags, SDValue V) {
+  if (auto *Const = dyn_cast<ConstantSDNode>(V)) {
+    switch (Opcode) {
+    case ISD::ADD:
+    case ISD::OR:
+    case ISD::XOR:
+    case ISD::UMAX:
+      return Const->isZero();
+    case ISD::MUL:
+      return Const->isOne();
+    case ISD::AND:
+    case ISD::UMIN:
+      return Const->isAllOnes();
+    case ISD::SMAX:
+      return Const->isMinSignedValue();
+    case ISD::SMIN:
+      return Const->isMaxSignedValue();
+    }
+  } else if (auto *ConstFP = dyn_cast<ConstantFPSDNode>(V)) {
+    switch (Opcode) {
+    case ISD::FADD:
+      if (Flags.hasNoSignedZeros())
+        return ConstFP->isZero();
+      return ConstFP->isExactlyValue(-0.0);
+    case ISD::FMUL:
+      return ConstFP->isExactlyValue(1.0);
+    case ISD::FMINNUM:
+    case ISD::FMAXNUM: {
+      // Neutral element for fminnum is NaN, Inf or FLT_MAX, depending on FMF.
+      EVT VT = V.getValueType();
+      const fltSemantics &Semantics = SelectionDAG::EVTToAPFloatSemantics(VT);
+      APFloat NeutralAF = !Flags.hasNoNaNs()   ? APFloat::getQNaN(Semantics)
+                          : !Flags.hasNoInfs() ? APFloat::getInf(Semantics)
+                                               : APFloat::getLargest(Semantics);
+      if (Opcode == ISD::FMAXNUM)
+        NeutralAF.changeSign();
+
+      return ConstFP->isExactlyValue(NeutralAF);
+    }
+    }
+  }
+  return false;
+}
+
 SDValue llvm::peekThroughBitcasts(SDValue V) {
   while (V.getOpcode() == ISD::BITCAST)
     V = V.getOperand(0);
Index: llvm/include/llvm/CodeGen/SelectionDAGNodes.h
===================================================================
--- llvm/include/llvm/CodeGen/SelectionDAGNodes.h
+++ llvm/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -1689,6 +1689,9 @@
 /// Returns true if \p V is a constant min signed integer value.
 bool isMinSignedConstant(SDValue V);
 
+/// Returns true if \p V is a neutral element of Opc with Flags.
+bool isNeutralConstant(unsigned Opc, SDNodeFlags Flags, SDValue V);
+
 /// Return the non-bitcasted source operand of \p V if it exists.
 /// If \p V is not a bitcasted value, it is returned as-is.
 SDValue peekThroughBitcasts(SDValue V);


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