Disable indvar widening if arithmetics on the wider type are more expensive

[jingyue]

Summary

IndVarSimplify should not widen an indvar if arithmetics on the wider
indvar are more expensive than those on the narrower indvar. For
instance, although NVPTX64 treats i64 as a legal type, an ADD on i64 is
twice as expensive as that on i32, because the hardware needs to
simulate a 64-bit integer using two 32-bit integers.

Split from D6188, and based on D6195 which adds NVPTXTargetTransformInfo.

Fixes PR21148.

[jingyue]

+ Tom Stellard

This is separated from D6188, and focuses mostly on IndVarSimplify.

Jingyue

[jingyue]

rebase against the latest version

[atrick]

LGTM.

I don't like the fact that TTI info silently changes behavior of an early IR pass while not required to be present; that's a broader issue though that I'd rather not debate in this context. This is actually a pretty benign use of TTI.

[jingyue]

Thanks Andrew!

I don't like that either. I saw there's effort on making DataLayout mandatory, and think maybe the same thing will eventually happen to TargetTransformInfo. We already have a default "boring" NoTTI, so the switch wouldn't be too hard.

Jingyue

lib/Target/NVPTX/NVPTXTargetTransformInfo.cpp

 // pass constructor initialization.
 namespace llvm {
 void initializeNVPTXTTIPass(PassRegistry &);
 }
 
 namespace {
 
 class NVPTXTTI final : public ImmutablePass, public TargetTransformInfo {
+  const NVPTXTargetLowering *TLI;
 public:
-  NVPTXTTI() : ImmutablePass(ID) {
+  NVPTXTTI() : ImmutablePass(ID), TLI(nullptr) {
     llvm_unreachable("This pass cannot be directly constructed");
   }
 
-  NVPTXTTI(const NVPTXTargetMachine *TM) : ImmutablePass(ID) {
+  NVPTXTTI(const NVPTXTargetMachine *TM)
+      : ImmutablePass(ID), TLI(TM->getSubtargetImpl()->getTargetLowering()) {
     initializeNVPTXTTIPass(*PassRegistry::getPassRegistry());
   }
 
   void initializePass() override { pushTTIStack(this); }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     TargetTransformInfo::getAnalysisUsage(AU);
   }
 
   /// Pass identification.
   static char ID;
 
   /// Provide necessary pointer adjustments for the two base classes.
   void *getAdjustedAnalysisPointer(const void *ID) override {
     if (ID == &TargetTransformInfo::ID)
       return (TargetTransformInfo *)this;
     return this;
   }
 
   bool hasBranchDivergence() const override;
 
+  unsigned getArithmeticInstrCost(
+      unsigned Opcode, Type *Ty, OperandValueKind Opd1Info = OK_AnyValue,
+      OperandValueKind Opd2Info = OK_AnyValue,
+      OperandValueProperties Opd1PropInfo = OP_None,
+      OperandValueProperties Opd2PropInfo = OP_None) const override;
+
   /// @}
 };
 
 } // end anonymous namespace
 
 INITIALIZE_AG_PASS(NVPTXTTI, TargetTransformInfo, "NVPTXtti",
                    "NVPTX Target Transform Info", true, true, false)
 char NVPTXTTI::ID = 0;
 
 ImmutablePass *
 llvm::createNVPTXTargetTransformInfoPass(const NVPTXTargetMachine *TM) {
   return new NVPTXTTI(TM);
 }
 
 bool NVPTXTTI::hasBranchDivergence() const { return true; }
+
+unsigned NVPTXTTI::getArithmeticInstrCost(
+    unsigned Opcode, Type *Ty, OperandValueKind Opd1Info,
+    OperandValueKind Opd2Info, OperandValueProperties Opd1PropInfo,
+    OperandValueProperties Opd2PropInfo) const {
+  // Legalize the type.
+  std::pair<unsigned, MVT> LT = TLI->getTypeLegalizationCost(Ty);
+
+  int ISD = TLI->InstructionOpcodeToISD(Opcode);
+
+  switch (ISD) {
+  default:
+    return TargetTransformInfo::getArithmeticInstrCost(
+        Opcode, Ty, Opd1Info, Opd2Info, Opd1PropInfo, Opd2PropInfo);
+  case ISD::ADD:
+  case ISD::MUL:
+  case ISD::XOR:
+  case ISD::OR:
+  case ISD::AND:
+    // The machine code (SASS) simulates an i64 with two i32. Therefore, we
+    // estimate that arithmetic operations on i64 are twice as expensive as
+    // those on types that can fit into one machine register.
+    if (LT.second.SimpleTy == MVT::i64)
+      return 2 * LT.first;
+    // Delegate other cases to the basic TTI.
+    return TargetTransformInfo::getArithmeticInstrCost(
+        Opcode, Ty, Opd1Info, Opd2Info, Opd1PropInfo, Opd2PropInfo);
+  }
+}

lib/Transforms/Scalar/IndVarSimplify.cpp

 
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopPass.h"
 #include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/LLVMContext.h"
static cl::opt<bool> VerifyIndvars(
   "verify-indvars", cl::Hidden,
   cl::desc("Verify the ScalarEvolution result after running indvars"));
 
 static cl::opt<bool> ReduceLiveIVs("liv-reduce", cl::Hidden,
   cl::desc("Reduce live induction variables."));
 
 namespace {
   class IndVarSimplify : public LoopPass {
-    LoopInfo        *LI;
-    ScalarEvolution *SE;
-    DominatorTree   *DT;
-    const DataLayout *DL;
-    TargetLibraryInfo *TLI;
+    LoopInfo                  *LI;
+    ScalarEvolution           *SE;
+    DominatorTree             *DT;
+    const DataLayout          *DL;
+    TargetLibraryInfo         *TLI;
+    const TargetTransformInfo *TTI;
 
     SmallVector<WeakVH, 16> DeadInsts;
     bool Changed;
   public:
 
     static char ID; // Pass identification, replacement for typeid
     IndVarSimplify() : LoopPass(ID), LI(nullptr), SE(nullptr), DT(nullptr),
                        DL(nullptr), Changed(false) {
WideIVInfo() : NarrowIV(nullptr), WidestNativeType(nullptr),
                    IsSigned(false) {}
   };
 }
 
 /// visitCast - Update information about the induction variable that is
 /// extended by this sign or zero extend operation. This is used to determine
 /// the final width of the IV before actually widening it.
 static void visitIVCast(CastInst *Cast, WideIVInfo &WI, ScalarEvolution *SE,
-                        const DataLayout *DL) {
+                        const DataLayout *DL, const TargetTransformInfo *TTI) {
   bool IsSigned = Cast->getOpcode() == Instruction::SExt;
   if (!IsSigned && Cast->getOpcode() != Instruction::ZExt)
     return;
 
   Type *Ty = Cast->getType();
   uint64_t Width = SE->getTypeSizeInBits(Ty);
   if (DL && !DL->isLegalInteger(Width))
     return;
 
+  // Cast is either an sext or zext up to this point.
+  // We should not widen an indvar if arithmetics on the wider indvar are more
+  // expensive than those on the narrower indvar. We check only the cost of ADD
+  // because at least an ADD is required to increment the induction variable. We
+  // could compute more comprehensively the cost of all instructions on the
+  // induction variable when necessary.
+  if (TTI &&
+      TTI->getArithmeticInstrCost(Instruction::Add, Ty) >
+          TTI->getArithmeticInstrCost(Instruction::Add,
+                                      Cast->getOperand(0)->getType())) {
+    return;
+  }
+
   if (!WI.WidestNativeType) {
     WI.WidestNativeType = SE->getEffectiveSCEVType(Ty);
     WI.IsSigned = IsSigned;
     return;
   }
 
   // We extend the IV to satisfy the sign of its first user, arbitrarily.
   if (WI.IsSigned != IsSigned)

 //===----------------------------------------------------------------------===//
 //  Simplification of IV users based on SCEV evaluation.
 //===----------------------------------------------------------------------===//
 
 namespace {
   class IndVarSimplifyVisitor : public IVVisitor {
     ScalarEvolution *SE;
     const DataLayout *DL;
+    const TargetTransformInfo *TTI;
     PHINode *IVPhi;
 
   public:
     WideIVInfo WI;
 
     IndVarSimplifyVisitor(PHINode *IV, ScalarEvolution *SCEV,
-                          const DataLayout *DL, const DominatorTree *DTree):
-      SE(SCEV), DL(DL), IVPhi(IV) {
+                          const DataLayout *DL, const TargetTransformInfo *TTI,
+                          const DominatorTree *DTree)
+        : SE(SCEV), DL(DL), TTI(TTI), IVPhi(IV) {
       DT = DTree;
       WI.NarrowIV = IVPhi;
       if (ReduceLiveIVs)
         setSplitOverflowIntrinsics();
     }
 
     // Implement the interface used by simplifyUsersOfIV.
-    void visitCast(CastInst *Cast) override { visitIVCast(Cast, WI, SE, DL); }
+    void visitCast(CastInst *Cast) override {
+      visitIVCast(Cast, WI, SE, DL, TTI);
+    }
   };
 }
 
 /// SimplifyAndExtend - Iteratively perform simplification on a worklist of IV
 /// users. Each successive simplification may push more users which may
 /// themselves be candidates for simplification.
 ///
 /// Sign/Zero extend elimination is interleaved with IV simplification.
while (!LoopPhis.empty()) {
     // extension. The first time SCEV attempts to normalize sign/zero extension,
     // the result becomes final. So for the most predictable results, we delay
     // evaluation of sign/zero extend evaluation until needed, and avoid running
     // other SCEV based analysis prior to SimplifyAndExtend.
     do {
       PHINode *CurrIV = LoopPhis.pop_back_val();
 
       // Information about sign/zero extensions of CurrIV.
-      IndVarSimplifyVisitor Visitor(CurrIV, SE, DL, DT);
+      IndVarSimplifyVisitor Visitor(CurrIV, SE, DL, TTI, DT);
 
       Changed |= simplifyUsersOfIV(CurrIV, SE, &LPM, DeadInsts, &Visitor);
 
       if (Visitor.WI.WidestNativeType) {
         WideIVs.push_back(Visitor.WI);
       }
     } while(!LoopPhis.empty());
 
if (!L->isLoopSimplifyForm())
     return false;
 
   LI = &getAnalysis<LoopInfo>();
   SE = &getAnalysis<ScalarEvolution>();
   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
   DL = DLP ? &DLP->getDataLayout() : nullptr;
   TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
+  TTI = getAnalysisIfAvailable<TargetTransformInfo>();
 
   DeadInsts.clear();
   Changed = false;
 
   // If there are any floating-point recurrences, attempt to
   // transform them to use integer recurrences.
   RewriteNonIntegerIVs(L);
 

test/Transforms/IndVarSimplify/no-widen-expensive.ll

+; RUN: opt < %s -indvars -S | FileCheck %s
+
+target triple = "nvptx64-unknown-unknown"
+
+; For the nvptx64 architecture, the cost of an arithmetic instruction on a
+; 64-bit integer is twice as expensive as that on a 32-bit integer, because the
+; hardware needs to simulate a 64-bit integer using two 32-bit integers.
+; Therefore, in this particular architecture, we should not widen induction
+; variables to 64-bit integers even though i64 is a legal type in the 64-bit
+; PTX ISA.
+
+define void @indvar_32_bit(i32 %n, i32* nocapture %output) {
+; CHECK-LABEL: @indvar_32_bit
+entry:
+  %cmp5 = icmp sgt i32 %n, 0
+  br i1 %cmp5, label %for.body.preheader, label %for.end
+
+for.body.preheader:                               ; preds = %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.preheader, %for.body
+  %i.06 = phi i32 [ 0, %for.body.preheader ], [ %add, %for.body ]
+; CHECK: phi i32
+  %mul = mul nsw i32 %i.06, %i.06
+  %0 = sext i32 %i.06 to i64
+  %arrayidx = getelementptr inbounds i32* %output, i64 %0
+  store i32 %mul, i32* %arrayidx, align 4
+  %add = add nsw i32 %i.06, 3
+  %cmp = icmp slt i32 %add, %n
+  br i1 %cmp, label %for.body, label %for.end.loopexit
+
+for.end.loopexit:                                 ; preds = %for.body
+  br label %for.end
+
+for.end:                                          ; preds = %for.end.loopexit, %entry
+  ret void
+}