blob: d7b202f831890e39f5e1509f6e63eb31ed07d33a [file] [log] [blame]
//===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements bookkeeping for "interesting" users of expressions
// computed from induction variables.
#include "llvm/Analysis/IVUsers.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "iv-users"
AnalysisKey IVUsersAnalysis::Key;
IVUsers IVUsersAnalysis::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR) {
return IVUsers(&L, &AR.AC, &AR.LI, &AR.DT, &AR.SE);
char IVUsersWrapperPass::ID = 0;
INITIALIZE_PASS_BEGIN(IVUsersWrapperPass, "iv-users",
"Induction Variable Users", false, true)
INITIALIZE_PASS_END(IVUsersWrapperPass, "iv-users", "Induction Variable Users",
false, true)
Pass *llvm::createIVUsersPass() { return new IVUsersWrapperPass(); }
/// isInteresting - Test whether the given expression is "interesting" when
/// used by the given expression, within the context of analyzing the
/// given loop.
static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
ScalarEvolution *SE, LoopInfo *LI) {
// An addrec is interesting if it's affine or if it has an interesting start.
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
// Keep things simple. Don't touch loop-variant strides unless they're
// only used outside the loop and we can simplify them.
if (AR->getLoop() == L)
return AR->isAffine() ||
(!L->contains(I) &&
SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
// Otherwise recurse to see if the start value is interesting, and that
// the step value is not interesting, since we don't yet know how to
// do effective SCEV expansions for addrecs with interesting steps.
return isInteresting(AR->getStart(), I, L, SE, LI) &&
!isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
// An add is interesting if exactly one of its operands is interesting.
if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
bool AnyInterestingYet = false;
for (const auto *Op : Add->operands())
if (isInteresting(Op, I, L, SE, LI)) {
if (AnyInterestingYet)
return false;
AnyInterestingYet = true;
return AnyInterestingYet;
// Nothing else is interesting here.
return false;
/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
/// and now we need to decide whether the user should use the preinc or post-inc
/// value. If this user should use the post-inc version of the IV, return true.
/// Choosing wrong here can break dominance properties (if we choose to use the
/// post-inc value when we cannot) or it can end up adding extra live-ranges to
/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
/// should use the post-inc value).
static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
const Loop *L, DominatorTree *DT) {
// If the user is in the loop, use the preinc value.
if (L->contains(User))
return false;
BasicBlock *LatchBlock = L->getLoopLatch();
if (!LatchBlock)
return false;
// Ok, the user is outside of the loop. If it is dominated by the latch
// block, use the post-inc value.
if (DT->dominates(LatchBlock, User->getParent()))
return true;
// There is one case we have to be careful of: PHI nodes. These little guys
// can live in blocks that are not dominated by the latch block, but (since
// their uses occur in the predecessor block, not the block the PHI lives in)
// should still use the post-inc value. Check for this case now.
PHINode *PN = dyn_cast<PHINode>(User);
if (!PN || !Operand)
return false; // not a phi, not dominated by latch block.
// Look at all of the uses of Operand by the PHI node. If any use corresponds
// to a block that is not dominated by the latch block, give up and use the
// preincremented value.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == Operand &&
!DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
return false;
// Okay, all uses of Operand by PN are in predecessor blocks that really are
// dominated by the latch block. Use the post-incremented value.
return true;
/// Inspect the specified instruction. If it is a reducible SCEV, recursively
/// add its users to the IVUsesByStride set and return true. Otherwise, return
/// false.
bool IVUsers::AddUsersIfInteresting(Instruction *I) {
const DataLayout &DL = I->getModule()->getDataLayout();
// Add this IV user to the Processed set before returning false to ensure that
// all IV users are members of the set. See IVUsers::isIVUserOrOperand.
if (!Processed.insert(I).second)
return true; // Instruction already handled.
if (!SE->isSCEVable(I->getType()))
return false; // Void and FP expressions cannot be reduced.
// IVUsers is used by LSR which assumes that all SCEV expressions are safe to
// pass to SCEVExpander. Expressions are not safe to expand if they represent
// operations that are not safe to speculate, namely integer division.
if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
return false;
// LSR is not APInt clean, do not touch integers bigger than 64-bits.
// Also avoid creating IVs of non-native types. For example, we don't want a
// 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
uint64_t Width = SE->getTypeSizeInBits(I->getType());
if (Width > 64 || !DL.isLegalInteger(Width))
return false;
// Don't attempt to promote ephemeral values to indvars. They will be removed
// later anyway.
if (EphValues.count(I))
return false;
// Get the symbolic expression for this instruction.
const SCEV *ISE = SE->getSCEV(I);
// If we've come to an uninteresting expression, stop the traversal and
// call this a user.
if (!isInteresting(ISE, I, L, SE, LI))
return false;
SmallPtrSet<Instruction *, 4> UniqueUsers;
for (Use &U : I->uses()) {
Instruction *User = cast<Instruction>(U.getUser());
if (!UniqueUsers.insert(User).second)
// Do not infinitely recurse on PHI nodes.
if (isa<PHINode>(User) && Processed.count(User))
// Descend recursively, but not into PHI nodes outside the current loop.
// It's important to see the entire expression outside the loop to get
// choices that depend on addressing mode use right, although we won't
// consider references outside the loop in all cases.
// If User is already in Processed, we don't want to recurse into it again,
// but do want to record a second reference in the same instruction.
bool AddUserToIVUsers = false;
if (LI->getLoopFor(User->getParent()) != L) {
if (isa<PHINode>(User) || Processed.count(User) ||
!AddUsersIfInteresting(User)) {
LLVM_DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
<< " OF SCEV: " << *ISE << '\n');
AddUserToIVUsers = true;
} else if (Processed.count(User) || !AddUsersIfInteresting(User)) {
LLVM_DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
<< " OF SCEV: " << *ISE << '\n');
AddUserToIVUsers = true;
if (AddUserToIVUsers) {
// Okay, we found a user that we cannot reduce.
IVStrideUse &NewUse = AddUser(User, I);
// Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
// The regular return value here is discarded; instead of recording
// it, we just recompute it when we need it.
const SCEV *OriginalISE = ISE;
auto NormalizePred = [&](const SCEVAddRecExpr *AR) {
auto *L = AR->getLoop();
bool Result = IVUseShouldUsePostIncValue(User, I, L, DT);
if (Result)
return Result;
ISE = normalizeForPostIncUseIf(ISE, NormalizePred, *SE);
// PostIncNormalization effectively simplifies the expression under
// pre-increment assumptions. Those assumptions (no wrapping) might not
// hold for the post-inc value. Catch such cases by making sure the
// transformation is invertible.
if (OriginalISE != ISE) {
const SCEV *DenormalizedISE =
denormalizeForPostIncUse(ISE, NewUse.PostIncLoops, *SE);
// If we normalized the expression, but denormalization doesn't give the
// original one, discard this user.
if (OriginalISE != DenormalizedISE) {
<< *ISE << '\n');
return false;
LLVM_DEBUG(if (SE->getSCEV(I) != ISE) dbgs()
<< " NORMALIZED TO: " << *ISE << '\n');
return true;
IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
IVUses.push_back(new IVStrideUse(this, User, Operand));
return IVUses.back();
IVUsers::IVUsers(Loop *L, AssumptionCache *AC, LoopInfo *LI, DominatorTree *DT,
ScalarEvolution *SE)
: L(L), AC(AC), LI(LI), DT(DT), SE(SE), IVUses() {
// Collect ephemeral values so that AddUsersIfInteresting skips them.
CodeMetrics::collectEphemeralValues(L, AC, EphValues);
// Find all uses of induction variables in this loop, and categorize
// them by stride. Start by finding all of the PHI nodes in the header for
// this loop. If they are induction variables, inspect their uses.
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
void IVUsers::print(raw_ostream &OS, const Module *M) const {
OS << "IV Users for loop ";
L->getHeader()->printAsOperand(OS, false);
if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
OS << " with backedge-taken count " << *SE->getBackedgeTakenCount(L);
OS << ":\n";
for (const IVStrideUse &IVUse : IVUses) {
OS << " ";
IVUse.getOperandValToReplace()->printAsOperand(OS, false);
OS << " = " << *getReplacementExpr(IVUse);
for (auto PostIncLoop : IVUse.PostIncLoops) {
OS << " (post-inc with loop ";
PostIncLoop->getHeader()->printAsOperand(OS, false);
OS << ")";
OS << " in ";
if (IVUse.getUser())
OS << "Printing <null> User";
OS << '\n';
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void IVUsers::dump() const { print(dbgs()); }
void IVUsers::releaseMemory() {
IVUsersWrapperPass::IVUsersWrapperPass() : LoopPass(ID) {
void IVUsersWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
bool IVUsersWrapperPass::runOnLoop(Loop *L, LPPassManager &LPM) {
auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
IU.reset(new IVUsers(L, AC, LI, DT, SE));
return false;
void IVUsersWrapperPass::print(raw_ostream &OS, const Module *M) const {
IU->print(OS, M);
void IVUsersWrapperPass::releaseMemory() { IU->releaseMemory(); }
/// getReplacementExpr - Return a SCEV expression which computes the
/// value of the OperandValToReplace.
const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
return SE->getSCEV(IU.getOperandValToReplace());
/// getExpr - Return the expression for the use.
const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
return normalizeForPostIncUse(getReplacementExpr(IU), IU.getPostIncLoops(),
static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
if (AR->getLoop() == L)
return AR;
return findAddRecForLoop(AR->getStart(), L);
if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
for (const auto *Op : Add->operands())
if (const SCEVAddRecExpr *AR = findAddRecForLoop(Op, L))
return AR;
return nullptr;
return nullptr;
const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
return AR->getStepRecurrence(*SE);
return nullptr;
void IVStrideUse::transformToPostInc(const Loop *L) {
void IVStrideUse::deleted() {
// Remove this user from the list.
// this now dangles!