forked from OSchip/llvm-project
494 lines
21 KiB
C++
494 lines
21 KiB
C++
//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
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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 transforms calls of the current function (self recursion) followed
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// by a return instruction with a branch to the entry of the function, creating
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// a loop. This pass also implements the following extensions to the basic
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// algorithm:
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//
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// 1. Trivial instructions between the call and return do not prevent the
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// transformation from taking place, though currently the analysis cannot
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// support moving any really useful instructions (only dead ones).
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// 2. This pass transforms functions that are prevented from being tail
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// recursive by an associative expression to use an accumulator variable,
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// thus compiling the typical naive factorial or 'fib' implementation into
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// efficient code.
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// 3. TRE is performed if the function returns void, if the return
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// returns the result returned by the call, or if the function returns a
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// run-time constant on all exits from the function. It is possible, though
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// unlikely, that the return returns something else (like constant 0), and
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// can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
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// the function return the exact same value.
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// 4. If it can prove that callees do not access theier caller stack frame,
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// they are marked as eligible for tail call elimination (by the code
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// generator).
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//
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// There are several improvements that could be made:
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//
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// 1. If the function has any alloca instructions, these instructions will be
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// moved out of the entry block of the function, causing them to be
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// evaluated each time through the tail recursion. Safely keeping allocas
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// in the entry block requires analysis to proves that the tail-called
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// function does not read or write the stack object.
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// 2. Tail recursion is only performed if the call immediately preceeds the
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// return instruction. It's possible that there could be a jump between
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// the call and the return.
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// 3. There can be intervening operations between the call and the return that
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// prevent the TRE from occurring. For example, there could be GEP's and
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// stores to memory that will not be read or written by the call. This
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// requires some substantial analysis (such as with DSA) to prove safe to
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// move ahead of the call, but doing so could allow many more TREs to be
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// performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
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// 4. The algorithm we use to detect if callees access their caller stack
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// frames is very primitive.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "tailcallelim"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Constants.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/Compiler.h"
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using namespace llvm;
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STATISTIC(NumEliminated, "Number of tail calls removed");
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STATISTIC(NumAccumAdded, "Number of accumulators introduced");
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namespace {
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struct VISIBILITY_HIDDEN TailCallElim : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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TailCallElim() : FunctionPass(&ID) {}
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virtual bool runOnFunction(Function &F);
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private:
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bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
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bool &TailCallsAreMarkedTail,
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std::vector<PHINode*> &ArgumentPHIs,
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bool CannotTailCallElimCallsMarkedTail);
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bool CanMoveAboveCall(Instruction *I, CallInst *CI);
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Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
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};
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}
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char TailCallElim::ID = 0;
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static RegisterPass<TailCallElim> X("tailcallelim", "Tail Call Elimination");
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// Public interface to the TailCallElimination pass
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FunctionPass *llvm::createTailCallEliminationPass() {
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return new TailCallElim();
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}
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/// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by
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/// callees of this function. We only do very simple analysis right now, this
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/// could be expanded in the future to use mod/ref information for particular
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/// call sites if desired.
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static bool AllocaMightEscapeToCalls(AllocaInst *AI) {
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// FIXME: do simple 'address taken' analysis.
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return true;
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}
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/// FunctionContainsAllocas - Scan the specified basic block for alloca
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/// instructions. If it contains any that might be accessed by calls, return
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/// true.
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static bool CheckForEscapingAllocas(BasicBlock *BB,
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bool &CannotTCETailMarkedCall) {
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bool RetVal = false;
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
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RetVal |= AllocaMightEscapeToCalls(AI);
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// If this alloca is in the body of the function, or if it is a variable
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// sized allocation, we cannot tail call eliminate calls marked 'tail'
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// with this mechanism.
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if (BB != &BB->getParent()->getEntryBlock() ||
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!isa<ConstantInt>(AI->getArraySize()))
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CannotTCETailMarkedCall = true;
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}
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return RetVal;
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}
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bool TailCallElim::runOnFunction(Function &F) {
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// If this function is a varargs function, we won't be able to PHI the args
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// right, so don't even try to convert it...
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if (F.getFunctionType()->isVarArg()) return false;
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BasicBlock *OldEntry = 0;
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bool TailCallsAreMarkedTail = false;
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std::vector<PHINode*> ArgumentPHIs;
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bool MadeChange = false;
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bool FunctionContainsEscapingAllocas = false;
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// CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
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// marked with the 'tail' attribute, because doing so would cause the stack
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// size to increase (real TCE would deallocate variable sized allocas, TCE
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// doesn't).
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bool CannotTCETailMarkedCall = false;
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// Loop over the function, looking for any returning blocks, and keeping track
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// of whether this function has any non-trivially used allocas.
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
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if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
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break;
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FunctionContainsEscapingAllocas |=
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CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
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}
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/// FIXME: The code generator produces really bad code when an 'escaping
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/// alloca' is changed from being a static alloca to being a dynamic alloca.
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/// Until this is resolved, disable this transformation if that would ever
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/// happen. This bug is PR962.
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if (FunctionContainsEscapingAllocas)
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return false;
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// Second pass, change any tail calls to loops.
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
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MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
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ArgumentPHIs,CannotTCETailMarkedCall);
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// If we eliminated any tail recursions, it's possible that we inserted some
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// silly PHI nodes which just merge an initial value (the incoming operand)
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// with themselves. Check to see if we did and clean up our mess if so. This
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// occurs when a function passes an argument straight through to its tail
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// call.
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if (!ArgumentPHIs.empty()) {
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for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
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PHINode *PN = ArgumentPHIs[i];
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// If the PHI Node is a dynamic constant, replace it with the value it is.
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if (Value *PNV = PN->hasConstantValue()) {
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PN->replaceAllUsesWith(PNV);
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PN->eraseFromParent();
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}
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}
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}
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// Finally, if this function contains no non-escaping allocas, mark all calls
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// in the function as eligible for tail calls (there is no stack memory for
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// them to access).
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if (!FunctionContainsEscapingAllocas)
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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if (CallInst *CI = dyn_cast<CallInst>(I)) {
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CI->setTailCall();
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MadeChange = true;
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}
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return MadeChange;
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}
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/// CanMoveAboveCall - Return true if it is safe to move the specified
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/// instruction from after the call to before the call, assuming that all
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/// instructions between the call and this instruction are movable.
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///
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bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
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// FIXME: We can move load/store/call/free instructions above the call if the
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// call does not mod/ref the memory location being processed.
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if (I->mayHaveSideEffects()) // This also handles volatile loads.
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return false;
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if (LoadInst* L = dyn_cast<LoadInst>(I)) {
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// Loads may always be moved above calls without side effects.
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if (CI->mayHaveSideEffects()) {
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// Non-volatile loads may be moved above a call with side effects if it
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// does not write to memory and the load provably won't trap.
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// FIXME: Writes to memory only matter if they may alias the pointer
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// being loaded from.
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if (CI->mayWriteToMemory() ||
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!isSafeToLoadUnconditionally(L->getPointerOperand(), L))
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return false;
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}
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}
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// Otherwise, if this is a side-effect free instruction, check to make sure
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// that it does not use the return value of the call. If it doesn't use the
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// return value of the call, it must only use things that are defined before
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// the call, or movable instructions between the call and the instruction
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// itself.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
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if (I->getOperand(i) == CI)
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return false;
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return true;
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}
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// isDynamicConstant - Return true if the specified value is the same when the
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// return would exit as it was when the initial iteration of the recursive
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// function was executed.
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//
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// We currently handle static constants and arguments that are not modified as
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// part of the recursion.
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//
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static bool isDynamicConstant(Value *V, CallInst *CI) {
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if (isa<Constant>(V)) return true; // Static constants are always dyn consts
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// Check to see if this is an immutable argument, if so, the value
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// will be available to initialize the accumulator.
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if (Argument *Arg = dyn_cast<Argument>(V)) {
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// Figure out which argument number this is...
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unsigned ArgNo = 0;
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Function *F = CI->getParent()->getParent();
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for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
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++ArgNo;
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// If we are passing this argument into call as the corresponding
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// argument operand, then the argument is dynamically constant.
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// Otherwise, we cannot transform this function safely.
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if (CI->getOperand(ArgNo+1) == Arg)
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return true;
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}
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// Not a constant or immutable argument, we can't safely transform.
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return false;
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}
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// getCommonReturnValue - Check to see if the function containing the specified
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// return instruction and tail call consistently returns the same
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// runtime-constant value at all exit points. If so, return the returned value.
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//
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static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) {
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Function *F = TheRI->getParent()->getParent();
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Value *ReturnedValue = 0;
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// TODO: Handle multiple value ret instructions;
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if (isa<StructType>(F->getReturnType()))
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return 0;
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for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
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if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
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if (RI != TheRI) {
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Value *RetOp = RI->getOperand(0);
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// We can only perform this transformation if the value returned is
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// evaluatable at the start of the initial invocation of the function,
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// instead of at the end of the evaluation.
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//
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if (!isDynamicConstant(RetOp, CI))
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return 0;
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if (ReturnedValue && RetOp != ReturnedValue)
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return 0; // Cannot transform if differing values are returned.
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ReturnedValue = RetOp;
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}
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return ReturnedValue;
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}
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/// CanTransformAccumulatorRecursion - If the specified instruction can be
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/// transformed using accumulator recursion elimination, return the constant
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/// which is the start of the accumulator value. Otherwise return null.
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///
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Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
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CallInst *CI) {
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if (!I->isAssociative()) return 0;
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assert(I->getNumOperands() == 2 &&
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"Associative operations should have 2 args!");
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// Exactly one operand should be the result of the call instruction...
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if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
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(I->getOperand(0) != CI && I->getOperand(1) != CI))
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return 0;
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// The only user of this instruction we allow is a single return instruction.
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if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
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return 0;
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// Ok, now we have to check all of the other return instructions in this
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// function. If they return non-constants or differing values, then we cannot
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// transform the function safely.
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return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
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}
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bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
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bool &TailCallsAreMarkedTail,
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std::vector<PHINode*> &ArgumentPHIs,
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bool CannotTailCallElimCallsMarkedTail) {
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BasicBlock *BB = Ret->getParent();
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Function *F = BB->getParent();
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if (&BB->front() == Ret) // Make sure there is something before the ret...
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return false;
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// If the return is in the entry block, then making this transformation would
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// turn infinite recursion into an infinite loop. This transformation is ok
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// in theory, but breaks some code like:
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// double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
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// disable this xform in this case, because the code generator will lower the
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// call to fabs into inline code.
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if (BB == &F->getEntryBlock())
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return false;
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// Scan backwards from the return, checking to see if there is a tail call in
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// this block. If so, set CI to it.
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CallInst *CI;
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BasicBlock::iterator BBI = Ret;
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while (1) {
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CI = dyn_cast<CallInst>(BBI);
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if (CI && CI->getCalledFunction() == F)
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break;
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if (BBI == BB->begin())
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return false; // Didn't find a potential tail call.
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--BBI;
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}
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// If this call is marked as a tail call, and if there are dynamic allocas in
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// the function, we cannot perform this optimization.
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if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
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return false;
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// If we are introducing accumulator recursion to eliminate associative
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// operations after the call instruction, this variable contains the initial
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// value for the accumulator. If this value is set, we actually perform
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// accumulator recursion elimination instead of simple tail recursion
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// elimination.
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Value *AccumulatorRecursionEliminationInitVal = 0;
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Instruction *AccumulatorRecursionInstr = 0;
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// Ok, we found a potential tail call. We can currently only transform the
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// tail call if all of the instructions between the call and the return are
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// movable to above the call itself, leaving the call next to the return.
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// Check that this is the case now.
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for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI)
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if (!CanMoveAboveCall(BBI, CI)) {
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// If we can't move the instruction above the call, it might be because it
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// is an associative operation that could be tranformed using accumulator
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// recursion elimination. Check to see if this is the case, and if so,
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// remember the initial accumulator value for later.
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if ((AccumulatorRecursionEliminationInitVal =
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CanTransformAccumulatorRecursion(BBI, CI))) {
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// Yes, this is accumulator recursion. Remember which instruction
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// accumulates.
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AccumulatorRecursionInstr = BBI;
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} else {
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return false; // Otherwise, we cannot eliminate the tail recursion!
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}
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}
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// We can only transform call/return pairs that either ignore the return value
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// of the call and return void, ignore the value of the call and return a
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// constant, return the value returned by the tail call, or that are being
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// accumulator recursion variable eliminated.
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if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
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!isa<UndefValue>(Ret->getReturnValue()) &&
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AccumulatorRecursionEliminationInitVal == 0 &&
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!getCommonReturnValue(Ret, CI))
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return false;
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// OK! We can transform this tail call. If this is the first one found,
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// create the new entry block, allowing us to branch back to the old entry.
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if (OldEntry == 0) {
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OldEntry = &F->getEntryBlock();
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BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
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NewEntry->takeName(OldEntry);
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OldEntry->setName("tailrecurse");
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BranchInst::Create(OldEntry, NewEntry);
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// If this tail call is marked 'tail' and if there are any allocas in the
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// entry block, move them up to the new entry block.
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TailCallsAreMarkedTail = CI->isTailCall();
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if (TailCallsAreMarkedTail)
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// Move all fixed sized allocas from OldEntry to NewEntry.
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for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
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NEBI = NewEntry->begin(); OEBI != E; )
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if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
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if (isa<ConstantInt>(AI->getArraySize()))
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AI->moveBefore(NEBI);
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// Now that we have created a new block, which jumps to the entry
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// block, insert a PHI node for each argument of the function.
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// For now, we initialize each PHI to only have the real arguments
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// which are passed in.
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Instruction *InsertPos = OldEntry->begin();
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for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
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I != E; ++I) {
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PHINode *PN = PHINode::Create(I->getType(),
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I->getName() + ".tr", InsertPos);
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I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
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PN->addIncoming(I, NewEntry);
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ArgumentPHIs.push_back(PN);
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}
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}
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// If this function has self recursive calls in the tail position where some
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// are marked tail and some are not, only transform one flavor or another. We
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// have to choose whether we move allocas in the entry block to the new entry
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// block or not, so we can't make a good choice for both. NOTE: We could do
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// slightly better here in the case that the function has no entry block
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// allocas.
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if (TailCallsAreMarkedTail && !CI->isTailCall())
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return false;
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// Ok, now that we know we have a pseudo-entry block WITH all of the
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// required PHI nodes, add entries into the PHI node for the actual
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// parameters passed into the tail-recursive call.
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for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
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ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
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// If we are introducing an accumulator variable to eliminate the recursion,
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// do so now. Note that we _know_ that no subsequent tail recursion
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// eliminations will happen on this function because of the way the
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// accumulator recursion predicate is set up.
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//
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if (AccumulatorRecursionEliminationInitVal) {
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Instruction *AccRecInstr = AccumulatorRecursionInstr;
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// Start by inserting a new PHI node for the accumulator.
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PHINode *AccPN = PHINode::Create(AccRecInstr->getType(), "accumulator.tr",
|
|
OldEntry->begin());
|
|
|
|
// Loop over all of the predecessors of the tail recursion block. For the
|
|
// real entry into the function we seed the PHI with the initial value,
|
|
// computed earlier. For any other existing branches to this block (due to
|
|
// other tail recursions eliminated) the accumulator is not modified.
|
|
// Because we haven't added the branch in the current block to OldEntry yet,
|
|
// it will not show up as a predecessor.
|
|
for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
|
|
PI != PE; ++PI) {
|
|
if (*PI == &F->getEntryBlock())
|
|
AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI);
|
|
else
|
|
AccPN->addIncoming(AccPN, *PI);
|
|
}
|
|
|
|
// Add an incoming argument for the current block, which is computed by our
|
|
// associative accumulator instruction.
|
|
AccPN->addIncoming(AccRecInstr, BB);
|
|
|
|
// Next, rewrite the accumulator recursion instruction so that it does not
|
|
// use the result of the call anymore, instead, use the PHI node we just
|
|
// inserted.
|
|
AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
|
|
|
|
// Finally, rewrite any return instructions in the program to return the PHI
|
|
// node instead of the "initval" that they do currently. This loop will
|
|
// actually rewrite the return value we are destroying, but that's ok.
|
|
for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
|
|
if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
|
|
RI->setOperand(0, AccPN);
|
|
++NumAccumAdded;
|
|
}
|
|
|
|
// Now that all of the PHI nodes are in place, remove the call and
|
|
// ret instructions, replacing them with an unconditional branch.
|
|
BranchInst::Create(OldEntry, Ret);
|
|
BB->getInstList().erase(Ret); // Remove return.
|
|
BB->getInstList().erase(CI); // Remove call.
|
|
++NumEliminated;
|
|
return true;
|
|
}
|