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llvm-6502/lib/Target/PowerPC/PPCCTRLoops.cpp
Hal Finkel b1fd3cd78f Implement PPC counter loops as a late IR-level pass 
The old PPCCTRLoops pass, like the Hexagon pass version from which it was
derived, could only handle some simple loops in canonical form. We cannot
directly adapt the new Hexagon hardware loops pass, however, because the
Hexagon pass contains a fundamental assumption that non-constant-trip-count
loops will contain a guard, and this is not always true (the result being that
incorrect negative counts can be generated). With this commit, we replace the
pass with a late IR-level pass which makes use of SE to calculate the
backedge-taken counts and safely generate the loop-count expressions (including
any necessary max() parts). This IR level pass inserts custom intrinsics that
are lowered into the desired decrement-and-branch instructions.

The most fragile part of this new implementation is that interfering uses of
the counter register must be detected on the IR level (and, on PPC, this also
includes any indirect branches in addition to function calls). Also, to make
all of this work, we need a variant of the mtctr instruction that is marked
as having side effects. Without this, machine-code level CSE, DCE, etc.
illegally transform the resulting code. Hopefully, this can be improved
in the future.

This new pass is smaller than the original (and much smaller than the new
Hexagon hardware loops pass), and can handle many additional cases correctly.
In addition, the preheader-creation code has been copied from LoopSimplify, and
after we decide on where it belongs, this code will be refactored so that it
can be explicitly shared (making this implementation even smaller).

The new test-case files ctrloop-{le,lt,ne}.ll have been adapted from tests for
the new Hexagon pass. There are a few classes of loops that this pass does not
transform (noted by FIXMEs in the files), but these deficiencies can be
addressed within the SE infrastructure (thus helping many other passes as well).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181927 91177308-0d34-0410-b5e6-96231b3b80d8
2013-05-15 21:37:41 +00:00

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//===-- PPCCTRLoops.cpp - Identify and generate CTR loops -----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass identifies loops where we can generate the PPC branch instructions
// that decrement and test the count register (CTR) (bdnz and friends).
//
// The pattern that defines the induction variable can changed depending on
// prior optimizations. For example, the IndVarSimplify phase run by 'opt'
// normalizes induction variables, and the Loop Strength Reduction pass
// run by 'llc' may also make changes to the induction variable.
//
// Criteria for CTR loops:
// - Countable loops (w/ ind. var for a trip count)
// - Try inner-most loops first
// - No nested CTR loops.
// - No function calls in loops.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "ctrloops"
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/PassSupport.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ValueHandle.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "PPCTargetMachine.h"
#include "PPC.h"
#include <algorithm>
#include <vector>
using namespace llvm;
#ifndef NDEBUG
static cl::opt<int> CTRLoopLimit("ppc-max-ctrloop", cl::Hidden, cl::init(-1));
#endif
STATISTIC(NumCTRLoops, "Number of loops converted to CTR loops");
namespace llvm {
void initializePPCCTRLoopsPass(PassRegistry&);
}
namespace {
struct PPCCTRLoops : public FunctionPass {
#ifndef NDEBUG
static int Counter;
#endif
public:
static char ID;
PPCCTRLoops() : FunctionPass(ID), TM(0) {
initializePPCCTRLoopsPass(*PassRegistry::getPassRegistry());
}
PPCCTRLoops(PPCTargetMachine &TM) : FunctionPass(ID), TM(&TM) {
initializePPCCTRLoopsPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<LoopInfo>();
AU.addPreserved<LoopInfo>();
AU.addRequired<DominatorTree>();
AU.addPreserved<DominatorTree>();
AU.addRequired<ScalarEvolution>();
}
private:
// FIXME: Copied from LoopSimplify.
BasicBlock *InsertPreheaderForLoop(Loop *L);
void PlaceSplitBlockCarefully(BasicBlock *NewBB,
SmallVectorImpl<BasicBlock*> &SplitPreds,
Loop *L);
bool convertToCTRLoop(Loop *L);
private:
PPCTargetMachine *TM;
LoopInfo *LI;
ScalarEvolution *SE;
DataLayout *TD;
DominatorTree *DT;
const TargetLibraryInfo *LibInfo;
};
char PPCCTRLoops::ID = 0;
#ifndef NDEBUG
int PPCCTRLoops::Counter = 0;
#endif
} // end anonymous namespace
INITIALIZE_PASS_BEGIN(PPCCTRLoops, "ppc-ctr-loops", "PowerPC CTR Loops",
false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_END(PPCCTRLoops, "ppc-ctr-loops", "PowerPC CTR Loops",
false, false)
FunctionPass *llvm::createPPCCTRLoops(PPCTargetMachine &TM) {
return new PPCCTRLoops(TM);
}
bool PPCCTRLoops::runOnFunction(Function &F) {
LI = &getAnalysis<LoopInfo>();
SE = &getAnalysis<ScalarEvolution>();
DT = &getAnalysis<DominatorTree>();
TD = getAnalysisIfAvailable<DataLayout>();
LibInfo = getAnalysisIfAvailable<TargetLibraryInfo>();
bool MadeChange = false;
for (LoopInfo::iterator I = LI->begin(), E = LI->end();
I != E; ++I) {
Loop *L = *I;
if (!L->getParentLoop())
MadeChange |= convertToCTRLoop(L);
}
return MadeChange;
}
bool PPCCTRLoops::convertToCTRLoop(Loop *L) {
bool MadeChange = false;
Triple TT = Triple(L->getHeader()->getParent()->getParent()->
getTargetTriple());
if (!TT.isArch32Bit() && !TT.isArch64Bit())
return MadeChange; // Unknown arch. type.
// Process nested loops first.
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) {
MadeChange |= convertToCTRLoop(*I);
}
// If a nested loop has been converted, then we can't convert this loop.
if (MadeChange)
return MadeChange;
#ifndef NDEBUG
// Stop trying after reaching the limit (if any).
int Limit = CTRLoopLimit;
if (Limit >= 0) {
if (Counter >= CTRLoopLimit)
return false;
Counter++;
}
#endif
// We don't want to spill/restore the counter register, and so we don't
// want to use the counter register if the loop contains calls.
for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
I != IE; ++I) {
for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
J != JE; ++J) {
if (CallInst *CI = dyn_cast<CallInst>(J)) {
if (!TM)
return MadeChange;
const TargetLowering *TLI = TM->getTargetLowering();
if (Function *F = CI->getCalledFunction()) {
// Most intrinsics don't become function calls, but some might.
// sin, cos, exp and log are always calls.
unsigned Opcode;
if (F->getIntrinsicID() != Intrinsic::not_intrinsic) {
switch (F->getIntrinsicID()) {
default: continue;
case Intrinsic::setjmp:
case Intrinsic::longjmp:
case Intrinsic::memcpy:
case Intrinsic::memmove:
case Intrinsic::memset:
case Intrinsic::powi:
case Intrinsic::log:
case Intrinsic::log2:
case Intrinsic::log10:
case Intrinsic::exp:
case Intrinsic::exp2:
case Intrinsic::pow:
case Intrinsic::sin:
case Intrinsic::cos:
return MadeChange;
case Intrinsic::sqrt: Opcode = ISD::FSQRT; break;
case Intrinsic::floor: Opcode = ISD::FFLOOR; break;
case Intrinsic::ceil: Opcode = ISD::FCEIL; break;
case Intrinsic::trunc: Opcode = ISD::FTRUNC; break;
case Intrinsic::rint: Opcode = ISD::FRINT; break;
case Intrinsic::nearbyint: Opcode = ISD::FNEARBYINT; break;
}
}
// PowerPC does not use [US]DIVREM or other library calls for
// operations on regular types which are not otherwise library calls
// (i.e. soft float or atomics). If adapting for targets that do,
// additional care is required here.
LibFunc::Func Func;
if (!F->hasLocalLinkage() && F->hasName() && LibInfo &&
LibInfo->getLibFunc(F->getName(), Func) &&
LibInfo->hasOptimizedCodeGen(Func)) {
// Non-read-only functions are never treated as intrinsics.
if (!CI->onlyReadsMemory())
return MadeChange;
// Conversion happens only for FP calls.
if (!CI->getArgOperand(0)->getType()->isFloatingPointTy())
return MadeChange;
switch (Func) {
default: return MadeChange;
case LibFunc::copysign:
case LibFunc::copysignf:
case LibFunc::copysignl:
continue; // ISD::FCOPYSIGN is never a library call.
case LibFunc::fabs:
case LibFunc::fabsf:
case LibFunc::fabsl:
continue; // ISD::FABS is never a library call.
case LibFunc::sqrt:
case LibFunc::sqrtf:
case LibFunc::sqrtl:
Opcode = ISD::FSQRT; break;
case LibFunc::floor:
case LibFunc::floorf:
case LibFunc::floorl:
Opcode = ISD::FFLOOR; break;
case LibFunc::nearbyint:
case LibFunc::nearbyintf:
case LibFunc::nearbyintl:
Opcode = ISD::FNEARBYINT; break;
case LibFunc::ceil:
case LibFunc::ceilf:
case LibFunc::ceill:
Opcode = ISD::FCEIL; break;
case LibFunc::rint:
case LibFunc::rintf:
case LibFunc::rintl:
Opcode = ISD::FRINT; break;
case LibFunc::trunc:
case LibFunc::truncf:
case LibFunc::truncl:
Opcode = ISD::FTRUNC; break;
}
MVT VTy =
TLI->getSimpleValueType(CI->getArgOperand(0)->getType(), true);
if (VTy == MVT::Other)
return MadeChange;
if (TLI->isOperationLegalOrCustom(Opcode, VTy))
continue;
else if (VTy.isVector() &&
TLI->isOperationLegalOrCustom(Opcode, VTy.getScalarType()))
continue;
return MadeChange;
}
}
return MadeChange;
} else if (isa<BinaryOperator>(J) &&
J->getType()->getScalarType()->isPPC_FP128Ty()) {
// Most operations on ppc_f128 values become calls.
return MadeChange;
} else if (isa<UIToFPInst>(J) || isa<SIToFPInst>(J) ||
isa<FPToUIInst>(J) || isa<FPToSIInst>(J)) {
CastInst *CI = cast<CastInst>(J);
if (CI->getSrcTy()->getScalarType()->isPPC_FP128Ty() ||
CI->getDestTy()->getScalarType()->isPPC_FP128Ty())
return MadeChange;
} else if (isa<IndirectBrInst>(J) || isa<InvokeInst>(J)) {
// On PowerPC, indirect jumps use the counter register.
return MadeChange;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(J)) {
if (!TM)
return MadeChange;
const TargetLowering *TLI = TM->getTargetLowering();
if (TLI->supportJumpTables() &&
SI->getNumCases()+1 >= (unsigned) TLI->getMinimumJumpTableEntries())
return MadeChange;
}
}
}
SmallVector<BasicBlock*, 4> ExitingBlocks;
L->getExitingBlocks(ExitingBlocks);
BasicBlock *CountedExitBlock = 0;
const SCEV *ExitCount = 0;
BranchInst *CountedExitBranch = 0;
for (SmallVector<BasicBlock*, 4>::iterator I = ExitingBlocks.begin(),
IE = ExitingBlocks.end(); I != IE; ++I) {
const SCEV *EC = SE->getExitCount(L, *I);
DEBUG(dbgs() << "Exit Count for " << *L << " from block " <<
(*I)->getName() << ": " << *EC << "\n");
if (isa<SCEVCouldNotCompute>(EC))
continue;
if (const SCEVConstant *ConstEC = dyn_cast<SCEVConstant>(EC)) {
if (ConstEC->getValue()->isZero())
continue;
} else if (!SE->isLoopInvariant(EC, L))
continue;
// We now have a loop-invariant count of loop iterations (which is not the
// constant zero) for which we know that this loop will not exit via this
// exisiting block.
// We need to make sure that this block will run on every loop iteration.
// For this to be true, we must dominate all blocks with backedges. Such
// blocks are in-loop predecessors to the header block.
bool NotAlways = false;
for (pred_iterator PI = pred_begin(L->getHeader()),
PIE = pred_end(L->getHeader()); PI != PIE; ++PI) {
if (!L->contains(*PI))
continue;
if (!DT->dominates(*I, *PI)) {
NotAlways = true;
break;
}
}
if (NotAlways)
continue;
// Make sure this blocks ends with a conditional branch.
Instruction *TI = (*I)->getTerminator();
if (!TI)
continue;
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (!BI->isConditional())
continue;
CountedExitBranch = BI;
} else
continue;
// Note that this block may not be the loop latch block, even if the loop
// has a latch block.
CountedExitBlock = *I;
ExitCount = EC;
break;
}
if (!CountedExitBlock)
return MadeChange;
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader)
Preheader = InsertPreheaderForLoop(L);
if (!Preheader)
return MadeChange;
DEBUG(dbgs() << "Preheader for exit count: " << Preheader->getName() << "\n");
// Insert the count into the preheader and replace the condition used by the
// selected branch.
MadeChange = true;
SCEVExpander SCEVE(*SE, "loopcnt");
LLVMContext &C = SE->getContext();
Type *CountType = TT.isArch64Bit() ? Type::getInt64Ty(C) :
Type::getInt32Ty(C);
if (!ExitCount->getType()->isPointerTy() &&
ExitCount->getType() != CountType)
ExitCount = SE->getZeroExtendExpr(ExitCount, CountType);
ExitCount = SE->getAddExpr(ExitCount,
SE->getConstant(CountType, 1));
Value *ECValue = SCEVE.expandCodeFor(ExitCount, CountType,
Preheader->getTerminator());
IRBuilder<> CountBuilder(Preheader->getTerminator());
Module *M = Preheader->getParent()->getParent();
Value *MTCTRFunc = Intrinsic::getDeclaration(M, Intrinsic::ppc_mtctr,
CountType);
CountBuilder.CreateCall(MTCTRFunc, ECValue);
IRBuilder<> CondBuilder(CountedExitBranch);
Value *DecFunc =
Intrinsic::getDeclaration(M, Intrinsic::ppc_is_decremented_ctr_nonzero);
Value *NewCond = CondBuilder.CreateCall(DecFunc);
Value *OldCond = CountedExitBranch->getCondition();
CountedExitBranch->setCondition(NewCond);
// The false branch must exit the loop.
if (!L->contains(CountedExitBranch->getSuccessor(0)))
CountedExitBranch->swapSuccessors();
// The old condition may be dead now, and may have even created a dead PHI
// (the original induction variable).
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
DeleteDeadPHIs(CountedExitBlock);
++NumCTRLoops;
return MadeChange;
}
// FIXME: Copied from LoopSimplify.
BasicBlock *PPCCTRLoops::InsertPreheaderForLoop(Loop *L) {
BasicBlock *Header = L->getHeader();
// Compute the set of predecessors of the loop that are not in the loop.
SmallVector<BasicBlock*, 8> OutsideBlocks;
for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
PI != PE; ++PI) {
BasicBlock *P = *PI;
if (!L->contains(P)) { // Coming in from outside the loop?
// If the loop is branched to from an indirect branch, we won't
// be able to fully transform the loop, because it prohibits
// edge splitting.
if (isa<IndirectBrInst>(P->getTerminator())) return 0;
// Keep track of it.
OutsideBlocks.push_back(P);
}
}
// Split out the loop pre-header.
BasicBlock *PreheaderBB;
if (!Header->isLandingPad()) {
PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader",
this);
} else {
SmallVector<BasicBlock*, 2> NewBBs;
SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader",
".split-lp", this, NewBBs);
PreheaderBB = NewBBs[0];
}
PreheaderBB->getTerminator()->setDebugLoc(
Header->getFirstNonPHI()->getDebugLoc());
DEBUG(dbgs() << "Creating pre-header "
<< PreheaderBB->getName() << "\n");
// Make sure that NewBB is put someplace intelligent, which doesn't mess up
// code layout too horribly.
PlaceSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
return PreheaderBB;
}
void PPCCTRLoops::PlaceSplitBlockCarefully(BasicBlock *NewBB,
SmallVectorImpl<BasicBlock*> &SplitPreds,
Loop *L) {
// Check to see if NewBB is already well placed.
Function::iterator BBI = NewBB; --BBI;
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
if (&*BBI == SplitPreds[i])
return;
}
// If it isn't already after an outside block, move it after one. This is
// always good as it makes the uncond branch from the outside block into a
// fall-through.
// Figure out *which* outside block to put this after. Prefer an outside
// block that neighbors a BB actually in the loop.
BasicBlock *FoundBB = 0;
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
Function::iterator BBI = SplitPreds[i];
if (++BBI != NewBB->getParent()->end() &&
L->contains(BBI)) {
FoundBB = SplitPreds[i];
break;
}
}
// If our heuristic for a *good* bb to place this after doesn't find
// anything, just pick something. It's likely better than leaving it within
// the loop.
if (!FoundBB)
FoundBB = SplitPreds[0];
NewBB->moveAfter(FoundBB);
}
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