[IR] Simplify createReplacementInstr

[foad]

Summary

NFCI, although the test change shows that ConstantExpr::getAsInstruction
is better than the old implementation of createReplacementInstr because
it propagates things like the sdiv "exact" flag.

[foad]

Given how simple createReplacementInstr is now, this does make me wonder if it was really worth creating ReplaceConstant.{cpp,h} in the first place.

[yaxunl]

LGTM. Thanks.

clang/test/CodeGenCUDA/managed-var.cu

 // HOST:  %ld.managed = load [100 x %struct.vec]*, [100 x %struct.vec]** @v, align 16
 // HOST:  %0 = getelementptr inbounds [100 x %struct.vec], [100 x %struct.vec]* %ld.managed, i64 0, i64 1, i32 0
 // HOST:  %1 = ptrtoint float* %0 to i64
 // HOST:  %ld.managed1 = load <{ %struct.vec, [99 x %struct.vec] }>*, <{ %struct.vec, [99 x %struct.vec] }>** @v2, align 16
 // HOST:  %2 = bitcast <{ %struct.vec, [99 x %struct.vec] }>* %ld.managed1 to [100 x %struct.vec]*
 // HOST:  %3 = getelementptr inbounds [100 x %struct.vec], [100 x %struct.vec]* %2, i64 0, i64 1, i32 1
 // HOST:  %4 = ptrtoint float* %3 to i64
 // HOST:  %5 = sub i64 %4, %1
-// HOST:  %6 = sdiv i64 %5, 4
+// HOST:  %6 = sdiv exact i64 %5, 4
 // HOST:  %7 = sitofp i64 %6 to float
 // HOST:  ret float %7
 float addr_taken2() {
   return (float)reinterpret_cast<long>(&(v2[1].y)-&(v[1].x));
 }
 
 // COMMON-LABEL: define {{.*}}@_Z5load4v()
 // DEV:  %ld.managed = load i32 addrspace(1)*, i32 addrspace(1)* addrspace(1)* @ex, align 4

llvm/lib/IR/ReplaceConstant.cpp

 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/NoFolder.h"
 
 namespace llvm {
 // Replace a constant expression by instructions with equivalent operations at
 // a specified location.
 Instruction *createReplacementInstr(ConstantExpr *CE, Instruction *Instr) {
-  IRBuilder<NoFolder> Builder(Instr);
-  unsigned OpCode = CE->getOpcode();
-  switch (OpCode) {
-  case Instruction::GetElementPtr: {
-    SmallVector<Value *, 4> CEOpVec(CE->operands());
-    ArrayRef<Value *> CEOps(CEOpVec);
-    return dyn_cast<Instruction>(
-        Builder.CreateInBoundsGEP(cast<GEPOperator>(CE)->getSourceElementType(),
-                                  CEOps[0], CEOps.slice(1)));
-  }
-  case Instruction::Add:
-  case Instruction::Sub:
-  case Instruction::Mul:
-  case Instruction::UDiv:
-  case Instruction::SDiv:
-  case Instruction::FDiv:
-  case Instruction::URem:
-  case Instruction::SRem:
-  case Instruction::FRem:
-  case Instruction::Shl:
-  case Instruction::LShr:
-  case Instruction::AShr:
-  case Instruction::And:
-  case Instruction::Or:
-  case Instruction::Xor:
-    return dyn_cast<Instruction>(
-        Builder.CreateBinOp((Instruction::BinaryOps)OpCode, CE->getOperand(0),
-                            CE->getOperand(1), CE->getName()));
-  case Instruction::Trunc:
-  case Instruction::ZExt:
-  case Instruction::SExt:
-  case Instruction::FPToUI:
-  case Instruction::FPToSI:
-  case Instruction::UIToFP:
-  case Instruction::SIToFP:
-  case Instruction::FPTrunc:
-  case Instruction::FPExt:
-  case Instruction::PtrToInt:
-  case Instruction::IntToPtr:
-  case Instruction::BitCast:
-  case Instruction::AddrSpaceCast:
-    return dyn_cast<Instruction>(
-        Builder.CreateCast((Instruction::CastOps)OpCode, CE->getOperand(0),
-                           CE->getType(), CE->getName()));
-  default:
-    llvm_unreachable("Unhandled constant expression!\n");
-  }
+  auto *CEInstr = CE->getAsInstruction();
+  CEInstr->insertBefore(Instr);
+  return CEInstr;
 }
 
 void convertConstantExprsToInstructions(Instruction *I, ConstantExpr *CE,
                                         SmallPtrSetImpl<Instruction *> *Insts) {
   // Collect all reachable paths to CE from constant exprssion operands of I.
   std::map<Use *, std::vector<std::vector<ConstantExpr *>>> CEPaths;
   collectConstantExprPaths(I, CE, CEPaths);