Use the basic cost if a GEP is not used as addressing mode

Summary:
Currently, getGEPCost() returns TCC_FREE whenever a GEP is a legal addressing mode in the target.
However, since it doesn't check its actual users, it will return FREE even in cases
where the GEP cannot be folded away as a part of actual addressing mode.
For example, if an user of the GEP is a call instruction taking the GEP as a parameter,
then the GEP may not be folded in isel.

Reviewers: hfinkel, efriedma, mcrosier, jingyue, haicheng

Reviewed By: hfinkel

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D38085

llvm-svn: 314517

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -193,6 +193,13 @@ class TargetTransformInfo {
   int getGEPCost(Type *PointeeType, const Value *Ptr,
                  ArrayRef<const Value *> Operands) const;
 
+  /// \brief Estimate the cost of a GEP operation when lowered.
+  ///
+  /// This user-based overload adds the ability to check if the GEP can be
+  /// folded into its users.
+  int getGEPCost(const GEPOperator *GEP,
+                 ArrayRef<const Value *> Operands) const;
+
   /// \brief Estimate the cost of a EXT operation when lowered.
   ///
   /// The contract for this function is the same as \c getOperationCost except
@@ -251,9 +258,9 @@ class TargetTransformInfo {
   /// \brief Estimate the cost of a given IR user when lowered.
   ///
   /// This can estimate the cost of either a ConstantExpr or Instruction when
-  /// lowered. It has two primary advantages over the \c getOperationCost and
-  /// \c getGEPCost above, and one significant disadvantage: it can only be
-  /// used when the IR construct has already been formed.
+  /// lowered. It has two primary advantages over the \c getOperationCost above,
+  /// and one significant disadvantage: it can only be used when the IR
+  /// construct has already been formed.
   ///
   /// The advantages are that it can inspect the SSA use graph to reason more
   /// accurately about the cost. For example, all-constant-GEPs can often be
@@ -932,6 +939,8 @@ class TargetTransformInfo::Concept {
   virtual int getOperationCost(unsigned Opcode, Type *Ty, Type *OpTy) = 0;
   virtual int getGEPCost(Type *PointeeType, const Value *Ptr,
                          ArrayRef<const Value *> Operands) = 0;
+  virtual int getGEPCost(const GEPOperator *GEP,
+                         ArrayRef<const Value *> Operands) = 0;
   virtual int getExtCost(const Instruction *I, const Value *Src) = 0;
   virtual int getCallCost(FunctionType *FTy, int NumArgs) = 0;
   virtual int getCallCost(const Function *F, int NumArgs) = 0;
@@ -1113,6 +1122,10 @@ class TargetTransformInfo::Model final : public TargetTransformInfo::Concept {
                  ArrayRef<const Value *> Operands) override {
     return Impl.getGEPCost(PointeeType, Ptr, Operands);
   }
+  int getGEPCost(const GEPOperator *GEP,
+                 ArrayRef<const Value *> Operands) override {
+    return Impl.getGEPCost(GEP, Operands);
+  }
   int getExtCost(const Instruction *I, const Value *Src) override {
     return Impl.getExtCost(I, Src);
   }

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

@@ -726,6 +726,35 @@ class TargetTransformInfoImplCRTPBase : public TargetTransformInfoImplBase {
     return TTI::TCC_Basic;
   }
 
+  int getGEPCost(const GEPOperator *GEP, ArrayRef<const Value *> Operands) {
+    Type *PointeeType = GEP->getSourceElementType();
+    const Value *Ptr = GEP->getPointerOperand();
+
+    if (getGEPCost(PointeeType, Ptr, Operands) == TTI::TCC_Free) {
+      // Should check if the GEP is actually used in load / store instructions.
+      // For simplicity, we check only direct users of the GEP.
+      //
+      // FIXME: GEPs could also be folded away as a part of addressing mode in
+      // load/store instructions together with other instructions (e.g., other
+      // GEPs). Handling all such cases must be expensive to be performed
+      // in this function, so we stay conservative for now.
+      for (const User *U : GEP->users()) {
+        const Operator *UOP = cast<Operator>(U);
+        const Value *PointerOperand = nullptr;
+        if (auto *LI = dyn_cast<LoadInst>(UOP))
+          PointerOperand = LI->getPointerOperand();
+        else if (auto *SI = dyn_cast<StoreInst>(UOP))
+          PointerOperand = SI->getPointerOperand();
+
+        if ((!PointerOperand || PointerOperand != GEP) &&
+            !GEP->hasAllZeroIndices())
+          return TTI::TCC_Basic;
+      }
+      return TTI::TCC_Free;
+    }
+    return TTI::TCC_Basic;
+  }
+
   using BaseT::getIntrinsicCost;
 
   unsigned getIntrinsicCost(Intrinsic::ID IID, Type *RetTy,
@@ -749,11 +778,8 @@ class TargetTransformInfoImplCRTPBase : public TargetTransformInfoImplBase {
       if (A->isStaticAlloca())
         return TTI::TCC_Free;
 
-    if (const GEPOperator *GEP = dyn_cast<GEPOperator>(U)) {
-      return static_cast<T *>(this)->getGEPCost(GEP->getSourceElementType(),
-                                                GEP->getPointerOperand(),
-                                                Operands.drop_front());
-    }
+    if (const GEPOperator *GEP = dyn_cast<GEPOperator>(U))
+      return static_cast<T *>(this)->getGEPCost(GEP, Operands.drop_front());
 
     if (auto CS = ImmutableCallSite(U)) {
       const Function *F = CS.getCalledFunction();

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -151,6 +151,10 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
     return BaseT::getGEPCost(PointeeType, Ptr, Operands);
   }
 
+  int getGEPCost(const GEPOperator *GEP, ArrayRef<const Value *> Operands) {
+    return BaseT::getGEPCost(GEP, Operands);
+  }
+
   int getExtCost(const Instruction *I, const Value *Src) {
     if (getTLI()->isExtFree(I))
       return TargetTransformInfo::TCC_Free;

llvm/include/llvm/IR/Operator.h

@@ -456,6 +456,8 @@ class GEPOperator
       if (ConstantInt *C = dyn_cast<ConstantInt>(I))
         if (C->isZero())
           continue;
+      if (isa<ConstantAggregateZero>(I))
+        continue;
       return false;
     }
     return true;

llvm/lib/Analysis/TargetTransformInfo.cpp

@@ -88,6 +88,11 @@ int TargetTransformInfo::getGEPCost(Type *PointeeType, const Value *Ptr,
   return TTIImpl->getGEPCost(PointeeType, Ptr, Operands);
 }
 
+int TargetTransformInfo::getGEPCost(const GEPOperator *GEP,
+                                    ArrayRef<const Value *> Operands) const {
+  return TTIImpl->getGEPCost(GEP, Operands);
+}
+
 int TargetTransformInfo::getExtCost(const Instruction *I,
                                     const Value *Src) const {
   return TTIImpl->getExtCost(I, Src);

llvm/lib/Transforms/Scalar/NaryReassociate.cpp

@@ -264,7 +264,7 @@ static bool isGEPFoldable(GetElementPtrInst *GEP,
   SmallVector<const Value*, 4> Indices;
   for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
     Indices.push_back(*I);
-  return TTI->getGEPCost(GEP->getSourceElementType(), GEP->getPointerOperand(),
+  return TTI->getGEPCost(cast<GEPOperator>(GEP),
                          Indices) == TargetTransformInfo::TCC_Free;
 }
 

llvm/lib/Transforms/Scalar/StraightLineStrengthReduce.cpp

@@ -239,7 +239,7 @@ static bool isGEPFoldable(GetElementPtrInst *GEP,
   SmallVector<const Value*, 4> Indices;
   for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
     Indices.push_back(*I);
-  return TTI->getGEPCost(GEP->getSourceElementType(), GEP->getPointerOperand(),
+  return TTI->getGEPCost(cast<GEPOperator>(GEP),
                          Indices) == TargetTransformInfo::TCC_Free;
 }
 

llvm/test/Analysis/CostModel/AArch64/gep.ll

@@ -290,3 +290,49 @@ define i64 @test36(i64* %p) {
   %v = load i64, i64* %a
   ret i64 %v
 }
+
+; CHECK-LABEL: test37
+; CHECK: cost of 1 for instruction:  {{.*}} = getelementptr inbounds i8*, i8**
+define i8 @test37(i64 %j, i8** readonly %P) {
+entry:
+  %arrayidx0 = getelementptr inbounds i8*, i8** %P, i64 %j
+  %l1 = call i8* @func(i8** %arrayidx0)
+  ret i8 0
+}
+
+; CHECK-LABEL: test38
+; CHECK: cost of 1 for instruction: {{.*}} = getelementptr inbounds i8*, i8**
+define i8 @test38(i8** readonly %P) {
+entry:
+  %arrayidx0 = getelementptr inbounds i8*, i8** %P, i64 10
+  %l1 = call i8* @func(i8** %arrayidx0)
+  ret i8 0
+}
+
+; CHECK-LABEL:test39
+; CHECK: cost of 0 for instruction: {{.*}} = getelementptr inbounds i8*, i8**
+define i8 @test39(i8** readonly %P) {
+entry:
+  %arrayidx0 = getelementptr inbounds i8*, i8** %P, i64 0
+  %l1 = call i8* @func(i8** %arrayidx0)
+  ret i8 0
+}
+
+; CHECK-LABEL:test40
+; CHECK: cost of 1 for instruction: {{.*}} = getelementptr inbounds i8*, i8**
+define i8** @test40(i8** readonly %P) {
+entry:
+  %arrayidx0 = getelementptr inbounds i8*, i8** %P, i64 10
+  ret i8** %arrayidx0
+}
+
+; CHECK-LABEL:test41
+; CHECK: cost of 1 for instruction: {{.*}} = getelementptr inbounds i8, i8*
+define i8 @test41(i8* %V, i8** readonly %P) {
+entry:
+  %arrayidx0 = getelementptr inbounds i8, i8* %V, i64 10
+  store i8* %arrayidx0, i8** %P
+  ret i8 0
+}
+
+declare i8* @func(i8**)

llvm/test/Analysis/CostModel/X86/vector_gep.ll

@@ -10,7 +10,7 @@ define <4 x i32> @foov(<4 x %struct.S*> %s, i64 %base){
   %vector = shufflevector <4 x i64> %temp, <4 x i64> undef, <4 x i32> zeroinitializer
 ;CHECK: cost of 0 for instruction: {{.*}} getelementptr inbounds %struct.S
   %B = getelementptr inbounds %struct.S, <4 x %struct.S*> %s, <4 x i32> zeroinitializer, <4 x i32> zeroinitializer
-;CHECK: cost of 0 for instruction: {{.*}} getelementptr inbounds [1000 x i32]
+;CHECK: cost of 1 for instruction: {{.*}} getelementptr inbounds [1000 x i32]
   %arrayidx = getelementptr inbounds [1000 x i32], <4 x [1000 x i32]*> %B, <4 x i64> zeroinitializer, <4 x i64> %vector
   %res = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %arrayidx, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
   ret <4 x i32> %res