[CGP] Add support for sinking operands to their users, if they are free.

This patch improves code generation for some AArch64 ACLE intrinsics. It adds
support to CGP to duplicate and sink operands to their user, if they can be
folded into a target instruction, like zexts and sub into usubl. It adds a
TargetLowering hook shouldSinkOperands, which looks at the operands of
instructions to see if sinking is profitable.

I decided to add a new target hook, as for the sinking to be profitable,
at least on AArch64, we have to look at multiple operands of an
instruction, instead of looking at the users of a zext for example.

The sinking is done in CGP, because it works around an instruction
selection limitation. If instruction selection is not limited to a
single basic block, this patch should not be needed any longer.

Alternatively this could be done in the LoopSink pass, which tries to
undo LICM for instructions in blocks that are not executed frequently.

Note that we do not force the operands to sink to have a single user,
because we duplicate them before sinking. Therefore this is only
desirable if they really can be done for free. Additionally we could
consider the impact on live ranges later on.

This should fix https://bugs.llvm.org/show_bug.cgi?id=40025.

As for performance, we have internal code that uses intrinsics and can
be speed up by 10% by this change.

Reviewers: SjoerdMeijer, t.p.northover, samparker, efriedma, RKSimon, spatel

Reviewed By: samparker

Differential Revision: https://reviews.llvm.org/D57377

llvm-svn: 353152

Author: fhahn

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -2280,6 +2280,16 @@ class TargetLoweringBase {
     return false;
   }
 
+  /// Return true if sinking I's operands to the same basic block as I is
+  /// profitable, e.g. because the operands can be folded into a target
+  /// instruction during instruction selection. After calling the function
+  /// \p Ops contains the Uses to sink ordered by dominance (dominating users
+  /// come first).
+  virtual bool shouldSinkOperands(Instruction *I,
+                                  SmallVectorImpl<Use *> &Ops) const {
+    return false;
+  }
+
   /// Return true if the target supplies and combines to a paired load
   /// two loaded values of type LoadedType next to each other in memory.
   /// RequiredAlignment gives the minimal alignment constraints that must be met

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -375,6 +375,8 @@ class TypePromotionTransaction;
         SmallVectorImpl<Instruction *> &SpeculativelyMovedExts);
     bool splitBranchCondition(Function &F);
     bool simplifyOffsetableRelocate(Instruction &I);
+
+    bool tryToSinkFreeOperands(Instruction *I);
   };
 
 } // end anonymous namespace
@@ -1752,6 +1754,7 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, bool &ModifiedDT) {
       InsertedInsts.insert(ExtVal);
       return true;
     }
+
     case Intrinsic::launder_invariant_group:
     case Intrinsic::strip_invariant_group: {
       Value *ArgVal = II->getArgOperand(0);
@@ -5973,6 +5976,48 @@ bool CodeGenPrepare::optimizeShuffleVectorInst(ShuffleVectorInst *SVI) {
   return MadeChange;
 }
 
+bool CodeGenPrepare::tryToSinkFreeOperands(Instruction *I) {
+  // If the operands of I can be folded into a target instruction together with
+  // I, duplicate and sink them.
+  SmallVector<Use *, 4> OpsToSink;
+  if (!TLI || !TLI->shouldSinkOperands(I, OpsToSink))
+    return false;
+
+  // OpsToSink can contain multiple uses in a use chain (e.g.
+  // (%u1 with %u1 = shufflevector), (%u2 with %u2 = zext %u1)). The dominating
+  // uses must come first, which means they are sunk first, temporarily creating
+  // invalid IR. This will be fixed once their dominated users are sunk and
+  // updated.
+  BasicBlock *TargetBB = I->getParent();
+  bool Changed = false;
+  SmallVector<Use *, 4> ToReplace;
+  for (Use *U : OpsToSink) {
+    auto *UI = cast<Instruction>(U->get());
+    if (UI->getParent() == TargetBB || isa<PHINode>(UI))
+      continue;
+    ToReplace.push_back(U);
+  }
+
+  SmallPtrSet<Instruction *, 4> MaybeDead;
+  for (Use *U : ToReplace) {
+    auto *UI = cast<Instruction>(U->get());
+    Instruction *NI = UI->clone();
+    MaybeDead.insert(UI);
+    LLVM_DEBUG(dbgs() << "Sinking " << *UI << " to user " << *I << "\n");
+    NI->insertBefore(I);
+    InsertedInsts.insert(NI);
+    U->set(NI);
+    Changed = true;
+  }
+
+  // Remove instructions that are dead after sinking.
+  for (auto *I : MaybeDead)
+    if (!I->hasNUsesOrMore(1))
+      I->eraseFromParent();
+
+  return Changed;
+}
+
 bool CodeGenPrepare::optimizeSwitchInst(SwitchInst *SI) {
   if (!TLI || !DL)
     return false;
@@ -6787,6 +6832,9 @@ bool CodeGenPrepare::optimizeInst(Instruction *I, bool &ModifiedDT) {
     return false;
   }
 
+  if (tryToSinkFreeOperands(I))
+    return true;
+
   if (CallInst *CI = dyn_cast<CallInst>(I))
     return optimizeCallInst(CI, ModifiedDT);
 

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -54,9 +54,11 @@
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/OperandTraits.h"
+#include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Use.h"
 #include "llvm/IR/Value.h"
@@ -86,6 +88,7 @@
 #include <vector>
 
 using namespace llvm;
+using namespace llvm::PatternMatch;
 
 #define DEBUG_TYPE "aarch64-lower"
 
@@ -8270,6 +8273,110 @@ bool AArch64TargetLowering::isExtFreeImpl(const Instruction *Ext) const {
   return true;
 }
 
+/// Check if both Op1 and Op2 are shufflevector extracts of either the lower
+/// or upper half of the vector elements.
+static bool areExtractShuffleVectors(Value *Op1, Value *Op2) {
+  auto areTypesHalfed = [](Value *FullV, Value *HalfV) {
+    auto *FullVT = cast<VectorType>(FullV->getType());
+    auto *HalfVT = cast<VectorType>(HalfV->getType());
+    return FullVT->getBitWidth() == 2 * HalfVT->getBitWidth();
+  };
+
+  auto extractHalf = [](Value *FullV, Value *HalfV) {
+    auto *FullVT = cast<VectorType>(FullV->getType());
+    auto *HalfVT = cast<VectorType>(HalfV->getType());
+    return FullVT->getNumElements() == 2 * HalfVT->getNumElements();
+  };
+
+  Constant *M1, *M2;
+  Value *S1Op1, *S2Op1;
+  if (!match(Op1, m_ShuffleVector(m_Value(S1Op1), m_Undef(), m_Constant(M1))) ||
+      !match(Op2, m_ShuffleVector(m_Value(S2Op1), m_Undef(), m_Constant(M2))))
+    return false;
+
+  // Check that the operands are half as wide as the result and we extract
+  // half of the elements of the input vectors.
+  if (!areTypesHalfed(S1Op1, Op1) || !areTypesHalfed(S2Op1, Op2) ||
+      !extractHalf(S1Op1, Op1) || !extractHalf(S2Op1, Op2))
+    return false;
+
+  // Check the mask extracts either the lower or upper half of vector
+  // elements.
+  int M1Start = -1;
+  int M2Start = -1;
+  int NumElements = cast<VectorType>(Op1->getType())->getNumElements() * 2;
+  if (!ShuffleVectorInst::isExtractSubvectorMask(M1, NumElements, M1Start) ||
+      !ShuffleVectorInst::isExtractSubvectorMask(M2, NumElements, M2Start) ||
+      M1Start != M2Start || (M1Start != 0 && M2Start != (NumElements / 2)))
+    return false;
+
+  return true;
+}
+
+/// Check if Ext1 and Ext2 are extends of the same type, doubling the bitwidth
+/// of the vector elements.
+static bool areExtractExts(Value *Ext1, Value *Ext2) {
+  auto areExtDoubled = [](Instruction *Ext) {
+    return Ext->getType()->getScalarSizeInBits() ==
+           2 * Ext->getOperand(0)->getType()->getScalarSizeInBits();
+  };
+
+  if (!match(Ext1, m_ZExtOrSExt(m_Value())) ||
+      !match(Ext2, m_ZExtOrSExt(m_Value())) ||
+      !areExtDoubled(cast<Instruction>(Ext1)) ||
+      !areExtDoubled(cast<Instruction>(Ext2)))
+    return false;
+
+  return true;
+}
+
+/// Check if sinking \p I's operands to I's basic block is profitable, because
+/// the operands can be folded into a target instruction, e.g.
+/// shufflevectors extracts and/or sext/zext can be folded into (u,s)subl(2).
+bool AArch64TargetLowering::shouldSinkOperands(
+    Instruction *I, SmallVectorImpl<Use *> &Ops) const {
+  if (!I->getType()->isVectorTy())
+    return false;
+
+  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
+    switch (II->getIntrinsicID()) {
+    case Intrinsic::aarch64_neon_umull:
+      if (!areExtractShuffleVectors(II->getOperand(0), II->getOperand(1)))
+        return false;
+      Ops.push_back(&II->getOperandUse(0));
+      Ops.push_back(&II->getOperandUse(1));
+      return true;
+    default:
+      return false;
+    }
+  }
+
+  switch (I->getOpcode()) {
+  case Instruction::Sub:
+  case Instruction::Add: {
+    if (!areExtractExts(I->getOperand(0), I->getOperand(1)))
+      return false;
+
+    // If the exts' operands extract either the lower or upper elements, we
+    // can sink them too.
+    auto Ext1 = cast<Instruction>(I->getOperand(0));
+    auto Ext2 = cast<Instruction>(I->getOperand(1));
+    if (areExtractShuffleVectors(Ext1, Ext2)) {
+      Ops.push_back(&Ext1->getOperandUse(0));
+      Ops.push_back(&Ext2->getOperandUse(0));
+    }
+
+    Ops.push_back(&I->getOperandUse(0));
+    Ops.push_back(&I->getOperandUse(1));
+
+    return true;
+  }
+  default:
+    return false;
+  }
+  return false;
+}
+
 bool AArch64TargetLowering::hasPairedLoad(EVT LoadedType,
                                           unsigned &RequiredAligment) const {
   if (!LoadedType.isSimple() ||

llvm/lib/Target/AArch64/AArch64ISelLowering.h

@@ -327,6 +327,9 @@ class AArch64TargetLowering : public TargetLowering {
   bool isZExtFree(EVT VT1, EVT VT2) const override;
   bool isZExtFree(SDValue Val, EVT VT2) const override;
 
+  bool shouldSinkOperands(Instruction *I,
+                          SmallVectorImpl<Use *> &Ops) const override;
+
   bool hasPairedLoad(EVT LoadedType, unsigned &RequiredAligment) const override;
 
   unsigned getMaxSupportedInterleaveFactor() const override { return 4; }