[ARM] Make PerformSHLSimplify add nodes to the DAG worklist correctly.

Intentionally excluding nodes from the DAGCombine worklist is likely to
lead to weird optimizations and infinite loops, so it's generally a bad
idea.

To avoid the infinite loops, fix DAGCombine to use the
isDesirableToCommuteWithShift target hook before performing the
transforms in question, and implement the target hook in the ARM backend
disable the transforms in question.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38530 . (I don't have a
reduced testcase for that bug. But we should have sufficient test
coverage for PerformSHLSimplify given that we're not playing weird
tricks with the worklist. I can try to bugpoint it if necessary,
though.)

Differential Revision: https://reviews.llvm.org/D50667

llvm-svn: 339734

diff --git a/llvm/include/llvm/CodeGen/TargetLowering.h b/llvm/include/llvm/CodeGen/TargetLowering.h
index dce4168..6e2e102 100644 (file)
--- a/llvm/include/llvm/CodeGen/TargetLowering.h
+++ b/llvm/include/llvm/CodeGen/TargetLowering.h
@@ -2942,12 +2942,16 @@ public:
   ///
   virtual SDValue PerformDAGCombine(SDNode *N, DAGCombinerInfo &DCI) const;
 
-  /// Return true if it is profitable to move a following shift through this
-  //  node, adjusting any immediate operands as necessary to preserve semantics.
-  //  This transformation may not be desirable if it disrupts a particularly
-  //  auspicious target-specific tree (e.g. bitfield extraction in AArch64).
-  //  By default, it returns true.
-  virtual bool isDesirableToCommuteWithShift(const SDNode *N) const {
+  /// Return true if it is profitable to move this shift by a constant amount
+  /// though its operand, adjusting any immediate operands as necessary to
+  /// preserve semantics. This transformation may not be desirable if it
+  /// disrupts a particularly auspicious target-specific tree (e.g. bitfield
+  /// extraction in AArch64). By default, it returns true.
+  ///
+  /// @param N the shift node
+  /// @param Level the current DAGCombine legalization level.
+  virtual bool isDesirableToCommuteWithShift(const SDNode *N,
+                                             CombineLevel Level) const {
     return true;
   }
 

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 5405f79..12a177a 100644 (file)
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6206,7 +6206,7 @@ SDValue DAGCombiner::visitShiftByConstant(SDNode *N, ConstantSDNode *Amt) {
       return SDValue();
   }
 
-  if (!TLI.isDesirableToCommuteWithShift(LHS))
+  if (!TLI.isDesirableToCommuteWithShift(N, Level))
     return SDValue();
 
   // Fold the constants, shifting the binop RHS by the shift amount.
@@ -6510,7 +6510,8 @@ SDValue DAGCombiner::visitSHL(SDNode *N) {
   if ((N0.getOpcode() == ISD::ADD || N0.getOpcode() == ISD::OR) &&
       N0.getNode()->hasOneUse() &&
       isConstantOrConstantVector(N1, /* No Opaques */ true) &&
-      isConstantOrConstantVector(N0.getOperand(1), /* No Opaques */ true)) {
+      isConstantOrConstantVector(N0.getOperand(1), /* No Opaques */ true) &&
+      TLI.isDesirableToCommuteWithShift(N, Level)) {
     SDValue Shl0 = DAG.getNode(ISD::SHL, SDLoc(N0), VT, N0.getOperand(0), N1);
     SDValue Shl1 = DAG.getNode(ISD::SHL, SDLoc(N1), VT, N0.getOperand(1), N1);
     AddToWorklist(Shl0.getNode());

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index ec1143f..cbe48a5 100644 (file)
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -8473,7 +8473,9 @@ AArch64TargetLowering::getScratchRegisters(CallingConv::ID) const {
 }
 
 bool
-AArch64TargetLowering::isDesirableToCommuteWithShift(const SDNode *N) const {
+AArch64TargetLowering::isDesirableToCommuteWithShift(const SDNode *N,
+                                                     CombineLevel Level) const {
+  N = N->getOperand(0).getNode();
   EVT VT = N->getValueType(0);
     // If N is unsigned bit extraction: ((x >> C) & mask), then do not combine
     // it with shift to let it be lowered to UBFX.

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.h b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
index d783c8a..a6d66ae 100644 (file)
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.h
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
@@ -363,7 +363,8 @@ public:
   const MCPhysReg *getScratchRegisters(CallingConv::ID CC) const override;
 
   /// Returns false if N is a bit extraction pattern of (X >> C) & Mask.
-  bool isDesirableToCommuteWithShift(const SDNode *N) const override;
+  bool isDesirableToCommuteWithShift(const SDNode *N,
+                                     CombineLevel Level) const override;
 
   /// Returns true if it is beneficial to convert a load of a constant
   /// to just the constant itself.

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index f4aeac1..533ac02 100644 (file)
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -10447,6 +10447,25 @@ static SDValue PerformADDCombineWithOperands(SDNode *N, SDValue N0, SDValue N1,
   return SDValue();
 }
 
+bool
+ARMTargetLowering::isDesirableToCommuteWithShift(const SDNode *N,
+                                                 CombineLevel Level) const {
+  if (Level == BeforeLegalizeTypes)
+    return true;
+
+  if (Subtarget->isThumb() && Subtarget->isThumb1Only())
+    return true;
+
+  if (N->getOpcode() != ISD::SHL)
+    return true;
+
+  // Turn off commute-with-shift transform after legalization, so it doesn't
+  // conflict with PerformSHLSimplify.  (We could try to detect when
+  // PerformSHLSimplify would trigger more precisely, but it isn't
+  // really necessary.)
+  return false;
+}
+
 static SDValue PerformSHLSimplify(SDNode *N,
                                 TargetLowering::DAGCombinerInfo &DCI,
                                 const ARMSubtarget *ST) {
@@ -10546,9 +10565,7 @@ static SDValue PerformSHLSimplify(SDNode *N,
   LLVM_DEBUG(dbgs() << "Simplify shl use:\n"; SHL.getOperand(0).dump();
              SHL.dump(); N->dump());
   LLVM_DEBUG(dbgs() << "Into:\n"; X.dump(); BinOp.dump(); Res.dump());
-
-  DAG.ReplaceAllUsesWith(SDValue(N, 0), Res);
-  return SDValue(N, 0);
+  return Res;
 }
 
 

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.h b/llvm/lib/Target/ARM/ARMISelLowering.h
index 47b20aa..c2a6dfb 100644 (file)
--- a/llvm/lib/Target/ARM/ARMISelLowering.h
+++ b/llvm/lib/Target/ARM/ARMISelLowering.h
@@ -586,6 +586,9 @@ class VectorType;
     unsigned getABIAlignmentForCallingConv(Type *ArgTy,
                                            DataLayout DL) const override;
 
+    bool isDesirableToCommuteWithShift(const SDNode *N,
+                                       CombineLevel Level) const override;
+
   protected:
     std::pair<const TargetRegisterClass *, uint8_t>
     findRepresentativeClass(const TargetRegisterInfo *TRI,