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327 lines
12 KiB
C++
327 lines
12 KiB
C++
//===- InlineCost.cpp - Cost analysis for inliner -------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements inline cost analysis.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/InlineCost.h"
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#include "llvm/Support/CallSite.h"
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#include "llvm/CallingConv.h"
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#include "llvm/IntrinsicInst.h"
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using namespace llvm;
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// CountCodeReductionForConstant - Figure out an approximation for how many
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// instructions will be constant folded if the specified value is constant.
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//
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unsigned InlineCostAnalyzer::FunctionInfo::
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CountCodeReductionForConstant(Value *V) {
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unsigned Reduction = 0;
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for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
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if (isa<BranchInst>(*UI))
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Reduction += 40; // Eliminating a conditional branch is a big win
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else if (SwitchInst *SI = dyn_cast<SwitchInst>(*UI))
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// Eliminating a switch is a big win, proportional to the number of edges
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// deleted.
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Reduction += (SI->getNumSuccessors()-1) * 40;
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else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
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// Turning an indirect call into a direct call is a BIG win
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Reduction += CI->getCalledValue() == V ? 500 : 0;
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} else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
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// Turning an indirect call into a direct call is a BIG win
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Reduction += II->getCalledValue() == V ? 500 : 0;
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} else {
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// Figure out if this instruction will be removed due to simple constant
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// propagation.
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Instruction &Inst = cast<Instruction>(**UI);
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// We can't constant propagate instructions which have effects or
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// read memory.
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//
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// FIXME: It would be nice to capture the fact that a load from a
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// pointer-to-constant-global is actually a *really* good thing to zap.
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// Unfortunately, we don't know the pointer that may get propagated here,
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// so we can't make this decision.
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if (Inst.mayReadFromMemory() || Inst.mayHaveSideEffects() ||
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isa<AllocationInst>(Inst))
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continue;
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bool AllOperandsConstant = true;
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for (unsigned i = 0, e = Inst.getNumOperands(); i != e; ++i)
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if (!isa<Constant>(Inst.getOperand(i)) && Inst.getOperand(i) != V) {
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AllOperandsConstant = false;
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break;
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}
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if (AllOperandsConstant) {
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// We will get to remove this instruction...
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Reduction += 7;
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// And any other instructions that use it which become constants
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// themselves.
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Reduction += CountCodeReductionForConstant(&Inst);
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}
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}
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return Reduction;
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}
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// CountCodeReductionForAlloca - Figure out an approximation of how much smaller
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// the function will be if it is inlined into a context where an argument
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// becomes an alloca.
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//
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unsigned InlineCostAnalyzer::FunctionInfo::
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CountCodeReductionForAlloca(Value *V) {
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if (!isa<PointerType>(V->getType())) return 0; // Not a pointer
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unsigned Reduction = 0;
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for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E;++UI){
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Instruction *I = cast<Instruction>(*UI);
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if (isa<LoadInst>(I) || isa<StoreInst>(I))
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Reduction += 10;
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else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I)) {
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// If the GEP has variable indices, we won't be able to do much with it.
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if (!GEP->hasAllConstantIndices())
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Reduction += CountCodeReductionForAlloca(GEP)+15;
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} else {
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// If there is some other strange instruction, we're not going to be able
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// to do much if we inline this.
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return 0;
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}
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}
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return Reduction;
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}
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/// analyzeFunction - Fill in the current structure with information gleaned
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/// from the specified function.
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void InlineCostAnalyzer::FunctionInfo::analyzeFunction(Function *F) {
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unsigned NumInsts = 0, NumBlocks = 0, NumVectorInsts = 0;
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// Look at the size of the callee. Each basic block counts as 20 units, and
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// each instruction counts as 5.
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for (Function::const_iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
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for (BasicBlock::const_iterator II = BB->begin(), E = BB->end();
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II != E; ++II) {
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if (isa<PHINode>(II)) continue; // PHI nodes don't count.
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// Special handling for calls.
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if (isa<CallInst>(II) || isa<InvokeInst>(II)) {
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if (isa<DbgInfoIntrinsic>(II))
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continue; // Debug intrinsics don't count as size.
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CallSite CS = CallSite::get(const_cast<Instruction*>(&*II));
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// If this function contains a call to setjmp or _setjmp, never inline
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// it. This is a hack because we depend on the user marking their local
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// variables as volatile if they are live across a setjmp call, and they
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// probably won't do this in callers.
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if (Function *F = CS.getCalledFunction())
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if (F->isDeclaration() &&
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(F->getName() == "setjmp" || F->getName() == "_setjmp")) {
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NeverInline = true;
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return;
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}
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// Calls often compile into many machine instructions. Bump up their
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// cost to reflect this.
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if (!isa<IntrinsicInst>(II))
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NumInsts += 5;
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}
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if (const AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
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if (!AI->isStaticAlloca())
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this->usesDynamicAlloca = true;
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}
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if (isa<ExtractElementInst>(II) || isa<VectorType>(II->getType()))
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++NumVectorInsts;
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// Noop casts, including ptr <-> int, don't count.
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if (const CastInst *CI = dyn_cast<CastInst>(II)) {
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if (CI->isLosslessCast() || isa<IntToPtrInst>(CI) ||
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isa<PtrToIntInst>(CI))
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continue;
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} else if (const GetElementPtrInst *GEPI =
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dyn_cast<GetElementPtrInst>(II)) {
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// If a GEP has all constant indices, it will probably be folded with
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// a load/store.
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if (GEPI->hasAllConstantIndices())
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continue;
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}
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++NumInsts;
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}
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++NumBlocks;
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}
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this->NumBlocks = NumBlocks;
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this->NumInsts = NumInsts;
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this->NumVectorInsts = NumVectorInsts;
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// Check out all of the arguments to the function, figuring out how much
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// code can be eliminated if one of the arguments is a constant.
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for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
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ArgumentWeights.push_back(ArgInfo(CountCodeReductionForConstant(I),
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CountCodeReductionForAlloca(I)));
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}
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// getInlineCost - The heuristic used to determine if we should inline the
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// function call or not.
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//
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InlineCost InlineCostAnalyzer::getInlineCost(CallSite CS,
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SmallPtrSet<const Function *, 16> &NeverInline) {
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Instruction *TheCall = CS.getInstruction();
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Function *Callee = CS.getCalledFunction();
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Function *Caller = TheCall->getParent()->getParent();
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// Don't inline functions which can be redefined at link-time to mean
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// something else.
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if (Callee->mayBeOverridden() ||
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// Don't inline functions marked noinline.
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Callee->hasFnAttr(Attribute::NoInline) || NeverInline.count(Callee))
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return llvm::InlineCost::getNever();
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// InlineCost - This value measures how good of an inline candidate this call
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// site is to inline. A lower inline cost make is more likely for the call to
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// be inlined. This value may go negative.
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//
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int InlineCost = 0;
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// If there is only one call of the function, and it has internal linkage,
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// make it almost guaranteed to be inlined.
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//
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if (Callee->hasLocalLinkage() && Callee->hasOneUse())
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InlineCost -= 15000;
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// If this function uses the coldcc calling convention, prefer not to inline
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// it.
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if (Callee->getCallingConv() == CallingConv::Cold)
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InlineCost += 2000;
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// If the instruction after the call, or if the normal destination of the
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// invoke is an unreachable instruction, the function is noreturn. As such,
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// there is little point in inlining this.
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if (InvokeInst *II = dyn_cast<InvokeInst>(TheCall)) {
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if (isa<UnreachableInst>(II->getNormalDest()->begin()))
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InlineCost += 10000;
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} else if (isa<UnreachableInst>(++BasicBlock::iterator(TheCall)))
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InlineCost += 10000;
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// Get information about the callee...
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FunctionInfo &CalleeFI = CachedFunctionInfo[Callee];
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// If we haven't calculated this information yet, do so now.
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if (CalleeFI.NumBlocks == 0)
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CalleeFI.analyzeFunction(Callee);
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// If we should never inline this, return a huge cost.
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if (CalleeFI.NeverInline)
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return InlineCost::getNever();
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// FIXME: It would be nice to kill off CalleeFI.NeverInline. Then we
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// could move this up and avoid computing the FunctionInfo for
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// things we are going to just return always inline for. This
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// requires handling setjmp somewhere else, however.
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if (!Callee->isDeclaration() && Callee->hasFnAttr(Attribute::AlwaysInline))
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return InlineCost::getAlways();
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if (CalleeFI.usesDynamicAlloca) {
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// Get infomation about the caller...
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FunctionInfo &CallerFI = CachedFunctionInfo[Caller];
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// If we haven't calculated this information yet, do so now.
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if (CallerFI.NumBlocks == 0)
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CallerFI.analyzeFunction(Caller);
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// Don't inline a callee with dynamic alloca into a caller without them.
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// Functions containing dynamic alloca's are inefficient in various ways;
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// don't create more inefficiency.
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if (!CallerFI.usesDynamicAlloca)
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return InlineCost::getNever();
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}
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// Add to the inline quality for properties that make the call valuable to
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// inline. This includes factors that indicate that the result of inlining
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// the function will be optimizable. Currently this just looks at arguments
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// passed into the function.
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//
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unsigned ArgNo = 0;
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for (CallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
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I != E; ++I, ++ArgNo) {
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// Each argument passed in has a cost at both the caller and the callee
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// sides. This favors functions that take many arguments over functions
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// that take few arguments.
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InlineCost -= 20;
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// If this is a function being passed in, it is very likely that we will be
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// able to turn an indirect function call into a direct function call.
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if (isa<Function>(I))
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InlineCost -= 100;
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// If an alloca is passed in, inlining this function is likely to allow
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// significant future optimization possibilities (like scalar promotion, and
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// scalarization), so encourage the inlining of the function.
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//
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else if (isa<AllocaInst>(I)) {
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if (ArgNo < CalleeFI.ArgumentWeights.size())
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InlineCost -= CalleeFI.ArgumentWeights[ArgNo].AllocaWeight;
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// If this is a constant being passed into the function, use the argument
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// weights calculated for the callee to determine how much will be folded
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// away with this information.
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} else if (isa<Constant>(I)) {
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if (ArgNo < CalleeFI.ArgumentWeights.size())
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InlineCost -= CalleeFI.ArgumentWeights[ArgNo].ConstantWeight;
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}
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}
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// Now that we have considered all of the factors that make the call site more
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// likely to be inlined, look at factors that make us not want to inline it.
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// Don't inline into something too big, which would make it bigger.
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//
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InlineCost += Caller->size()/15;
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// Look at the size of the callee. Each instruction counts as 5.
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InlineCost += CalleeFI.NumInsts*5;
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return llvm::InlineCost::get(InlineCost);
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}
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// getInlineFudgeFactor - Return a > 1.0 factor if the inliner should use a
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// higher threshold to determine if the function call should be inlined.
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float InlineCostAnalyzer::getInlineFudgeFactor(CallSite CS) {
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Function *Callee = CS.getCalledFunction();
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// Get information about the callee...
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FunctionInfo &CalleeFI = CachedFunctionInfo[Callee];
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// If we haven't calculated this information yet, do so now.
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if (CalleeFI.NumBlocks == 0)
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CalleeFI.analyzeFunction(Callee);
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float Factor = 1.0f;
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// Single BB functions are often written to be inlined.
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if (CalleeFI.NumBlocks == 1)
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Factor += 0.5f;
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// Be more aggressive if the function contains a good chunk (if it mades up
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// at least 10% of the instructions) of vector instructions.
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if (CalleeFI.NumVectorInsts > CalleeFI.NumInsts/2)
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Factor += 2.0f;
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else if (CalleeFI.NumVectorInsts > CalleeFI.NumInsts/10)
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Factor += 1.5f;
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return Factor;
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}
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