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//===- ForwardOpTree.h ------------------------------------------*- 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
// Move instructions between statements.
#include "polly/ForwardOpTree.h"
#include "polly/Options.h"
#include "polly/ScopBuilder.h"
#include "polly/ScopInfo.h"
#include "polly/ScopPass.h"
#include "polly/Support/GICHelper.h"
#include "polly/Support/ISLOStream.h"
#include "polly/Support/ISLTools.h"
#include "polly/Support/VirtualInstruction.h"
#include "polly/ZoneAlgo.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "isl/ctx.h"
#include "isl/isl-noexceptions.h"
#include <cassert>
#include <memory>
#define DEBUG_TYPE "polly-optree"
using namespace llvm;
using namespace polly;
static cl::opt<bool>
cl::desc("Analyze array contents for load forwarding"),
cl::cat(PollyCategory), cl::init(true), cl::Hidden);
static cl::opt<bool>
cl::desc("Replace PHIs by their incoming values"),
cl::cat(PollyCategory), cl::init(false), cl::Hidden);
static cl::opt<unsigned>
cl::desc("Maximum number of ISL operations to invest for known "
"analysis; 0=no limit"),
cl::init(1000000), cl::cat(PollyCategory), cl::Hidden);
STATISTIC(KnownAnalyzed, "Number of successfully analyzed SCoPs");
"Analyses aborted because max_operations was reached");
STATISTIC(TotalInstructionsCopied, "Number of copied instructions");
"Number of forwarded loads because their value was known");
STATISTIC(TotalReloads, "Number of reloaded values");
STATISTIC(TotalReadOnlyCopied, "Number of copied read-only accesses");
STATISTIC(TotalForwardedTrees, "Number of forwarded operand trees");
"Number of statements with at least one forwarded tree");
STATISTIC(ScopsModified, "Number of SCoPs with at least one forwarded tree");
STATISTIC(NumValueWrites, "Number of scalar value writes after OpTree");
"Number of scalar value writes nested in affine loops after OpTree");
STATISTIC(NumPHIWrites, "Number of scalar phi writes after OpTree");
"Number of scalar phi writes nested in affine loops after OpTree");
STATISTIC(NumSingletonWrites, "Number of singleton writes after OpTree");
"Number of singleton writes nested in affine loops after OpTree");
namespace {
/// The state of whether an operand tree was/can be forwarded.
/// The items apply to an instructions and its operand tree with the instruction
/// as the root element. If the value in question is not an instruction in the
/// SCoP, it can be a leaf of an instruction's operand tree.
enum ForwardingDecision {
/// An uninitialized value.
/// The root instruction or value cannot be forwarded at all.
/// The root instruction or value can be forwarded as a leaf of a larger
/// operand tree.
/// It does not make sense to move the value itself, it would just replace it
/// by a use of itself. For instance, a constant "5" used in a statement can
/// be forwarded, but it would just replace it by the same constant "5".
/// However, it makes sense to move as an operand of
/// %add = add 5, 5
/// where "5" is moved as part of a larger operand tree. "5" would be placed
/// (disregarding for a moment that literal constants don't have a location
/// and can be used anywhere) into the same statement as %add would.
/// The root instruction can be forwarded and doing so avoids a scalar
/// dependency.
/// This can be either because the operand tree can be moved to the target
/// statement, or a memory access is redirected to read from a different
/// location.
/// A forwarding method cannot be applied to the operand tree.
/// The difference to FD_CannotForward is that there might be other methods
/// that can handle it.
/// Represents the evaluation of and action to taken when forwarding a value
/// from an operand tree.
struct ForwardingAction {
using KeyTy = std::pair<Value *, ScopStmt *>;
/// Evaluation of forwarding a value.
ForwardingDecision Decision = FD_Unknown;
/// Callback to execute the forwarding.
/// Returning true allows deleting the polly::MemoryAccess if the value is the
/// root of the operand tree (and its elimination the reason why the
/// forwarding is done). Return false if the MemoryAccess is reused or there
/// might be other users of the read accesses. In the letter case the
/// polly::SimplifyPass can remove dead MemoryAccesses.
std::function<bool()> Execute = []() -> bool {
llvm_unreachable("unspecified how to forward");
/// Other values that need to be forwarded if this action is executed. Their
/// actions are executed after this one.
SmallVector<KeyTy, 4> Depends;
/// Named ctor: The method creating this object does not apply to the kind of
/// value, but other methods may.
static ForwardingAction notApplicable() {
ForwardingAction Result;
Result.Decision = FD_NotApplicable;
return Result;
/// Named ctor: The value cannot be forwarded.
static ForwardingAction cannotForward() {
ForwardingAction Result;
Result.Decision = FD_CannotForward;
return Result;
/// Named ctor: The value can just be used without any preparation.
static ForwardingAction triviallyForwardable(bool IsProfitable, Value *Val) {
ForwardingAction Result;
Result.Decision =
IsProfitable ? FD_CanForwardProfitably : FD_CanForwardLeaf;
Result.Execute = [=]() {
LLVM_DEBUG(dbgs() << " trivially forwarded: " << *Val << "\n");
return true;
return Result;
/// Name ctor: The value can be forwarded by executing an action.
static ForwardingAction canForward(std::function<bool()> Execute,
ArrayRef<KeyTy> Depends,
bool IsProfitable) {
ForwardingAction Result;
Result.Decision =
IsProfitable ? FD_CanForwardProfitably : FD_CanForwardLeaf;
Result.Execute = std::move(Execute);
Result.Depends.append(Depends.begin(), Depends.end());
return Result;
/// Implementation of operand tree forwarding for a specific SCoP.
/// For a statement that requires a scalar value (through a value read
/// MemoryAccess), see if its operand can be moved into the statement. If so,
/// the MemoryAccess is removed and the all the operand tree instructions are
/// moved into the statement. All original instructions are left in the source
/// statements. The simplification pass can clean these up.
class ForwardOpTreeImpl : ZoneAlgorithm {
using MemoizationTy = DenseMap<ForwardingAction::KeyTy, ForwardingAction>;
/// Scope guard to limit the number of isl operations for this pass.
IslMaxOperationsGuard &MaxOpGuard;
/// How many instructions have been copied to other statements.
int NumInstructionsCopied = 0;
/// Number of loads forwarded because their value was known.
int NumKnownLoadsForwarded = 0;
/// Number of values reloaded from known array elements.
int NumReloads = 0;
/// How many read-only accesses have been copied.
int NumReadOnlyCopied = 0;
/// How many operand trees have been forwarded.
int NumForwardedTrees = 0;
/// Number of statements with at least one forwarded operand tree.
int NumModifiedStmts = 0;
/// Whether we carried out at least one change to the SCoP.
bool Modified = false;
/// Cache of how to forward values.
/// The key of this map is the llvm::Value to be forwarded and the
/// polly::ScopStmt it is forwarded from. This is because the same llvm::Value
/// can evaluate differently depending on where it is evaluate. For instance,
/// a synthesizable Scev represents a recurrence with an loop but the loop's
/// exit value if evaluated after the loop.
/// The cached results are only valid for the current TargetStmt.
/// CHECKME: ScalarEvolution::getScevAtScope should take care for getting the
/// exit value when instantiated outside of the loop. The primary concern is
/// ambiguity when crossing PHI nodes, which currently is not supported.
MemoizationTy ForwardingActions;
/// Contains the zones where array elements are known to contain a specific
/// value.
/// { [Element[] -> Zone[]] -> ValInst[] }
/// @see computeKnown()
isl::union_map Known;
/// Translator for newly introduced ValInsts to already existing ValInsts such
/// that new introduced load instructions can reuse the Known analysis of its
/// original load. { ValInst[] -> ValInst[] }
isl::union_map Translator;
/// Get list of array elements that do contain the same ValInst[] at Domain[].
/// @param ValInst { Domain[] -> ValInst[] }
/// The values for which we search for alternative locations,
/// per statement instance.
/// @return { Domain[] -> Element[] }
/// For each statement instance, the array elements that contain the
/// same ValInst.
isl::union_map findSameContentElements(isl::union_map ValInst) {
// { Domain[] }
isl::union_set Domain = ValInst.domain();
// { Domain[] -> Scatter[] }
isl::union_map Schedule = getScatterFor(Domain);
// { Element[] -> [Scatter[] -> ValInst[]] }
isl::union_map MustKnownCurried =
convertZoneToTimepoints(Known, isl::dim::in, false, true).curry();
// { [Domain[] -> ValInst[]] -> Scatter[] }
isl::union_map DomValSched = ValInst.domain_map().apply_range(Schedule);
// { [Scatter[] -> ValInst[]] -> [Domain[] -> ValInst[]] }
isl::union_map SchedValDomVal =
// { Element[] -> [Domain[] -> ValInst[]] }
isl::union_map MustKnownInst = MustKnownCurried.apply_range(SchedValDomVal);
// { Domain[] -> Element[] }
isl::union_map MustKnownMap =
return MustKnownMap;
/// Find a single array element for each statement instance, within a single
/// array.
/// @param MustKnown { Domain[] -> Element[] }
/// Set of candidate array elements.
/// @param Domain { Domain[] }
/// The statement instance for which we need elements for.
/// @return { Domain[] -> Element[] }
/// For each statement instance, an array element out of @p MustKnown.
/// All array elements must be in the same array (Polly does not yet
/// support reading from different accesses using the same
/// MemoryAccess). If no mapping for all of @p Domain exists, returns
/// null.
isl::map singleLocation(isl::union_map MustKnown, isl::set Domain) {
// { Domain[] -> Element[] }
isl::map Result;
// Make irrelevant elements not interfere.
Domain = Domain.intersect_params(S->getContext());
// MemoryAccesses can read only elements from a single array
// (i.e. not: { Dom[0] -> A[0]; Dom[1] -> B[1] }).
// Look through all spaces until we find one that contains at least the
// wanted statement instance.s
for (isl::map Map : MustKnown.get_map_list()) {
// Get the array this is accessing.
isl::id ArrayId = Map.get_tuple_id(isl::dim::out);
ScopArrayInfo *SAI = static_cast<ScopArrayInfo *>(ArrayId.get_user());
// No support for generation of indirect array accesses.
if (SAI->getBasePtrOriginSAI())
// Determine whether this map contains all wanted values.
isl::set MapDom = Map.domain();
if (!Domain.is_subset(MapDom).is_true())
// There might be multiple array elements that contain the same value, but
// choose only one of them. lexmin is used because it returns a one-value
// mapping, we do not care about which one.
// TODO: Get the simplest access function.
Result = Map.lexmin();
return Result;
ForwardOpTreeImpl(Scop *S, LoopInfo *LI, IslMaxOperationsGuard &MaxOpGuard)
: ZoneAlgorithm("polly-optree", S, LI), MaxOpGuard(MaxOpGuard) {}
/// Compute the zones of known array element contents.
/// @return True if the computed #Known is usable.
bool computeKnownValues() {
isl::union_map MustKnown, KnownFromLoad, KnownFromInit;
// Check that nothing strange occurs.
IslQuotaScope QuotaScope = MaxOpGuard.enter();
if (NormalizePHIs)
Known = computeKnown(true, true);
// Preexisting ValInsts use the known content analysis of themselves.
Translator = makeIdentityMap(Known.range(), false);
if (Known.is_null() || Translator.is_null() || NormalizeMap.is_null()) {
assert(isl_ctx_last_error(IslCtx.get()) == isl_error_quota);
Known = {};
Translator = {};
NormalizeMap = {};
LLVM_DEBUG(dbgs() << "Known analysis exceeded max_operations\n");
return false;
LLVM_DEBUG(dbgs() << "All known: " << Known << "\n");
return true;
void printStatistics(raw_ostream &OS, int Indent = 0) {
OS.indent(Indent) << "Statistics {\n";
OS.indent(Indent + 4) << "Instructions copied: " << NumInstructionsCopied
<< '\n';
OS.indent(Indent + 4) << "Known loads forwarded: " << NumKnownLoadsForwarded
<< '\n';
OS.indent(Indent + 4) << "Reloads: " << NumReloads << '\n';
OS.indent(Indent + 4) << "Read-only accesses copied: " << NumReadOnlyCopied
<< '\n';
OS.indent(Indent + 4) << "Operand trees forwarded: " << NumForwardedTrees
<< '\n';
OS.indent(Indent + 4) << "Statements with forwarded operand trees: "
<< NumModifiedStmts << '\n';
OS.indent(Indent) << "}\n";
void printStatements(raw_ostream &OS, int Indent = 0) const {
OS.indent(Indent) << "After statements {\n";
for (auto &Stmt : *S) {
OS.indent(Indent + 4) << Stmt.getBaseName() << "\n";
for (auto *MA : Stmt)
OS.indent(Indent + 12);
OS.indent(Indent) << "}\n";
/// Create a new MemoryAccess of type read and MemoryKind::Array.
/// @param Stmt The statement in which the access occurs.
/// @param LI The instruction that does the access.
/// @param AccessRelation The array element that each statement instance
/// accesses.
/// @param The newly created access.
MemoryAccess *makeReadArrayAccess(ScopStmt *Stmt, LoadInst *LI,
isl::map AccessRelation) {
isl::id ArrayId = AccessRelation.get_tuple_id(isl::dim::out);
ScopArrayInfo *SAI = reinterpret_cast<ScopArrayInfo *>(ArrayId.get_user());
// Create a dummy SCEV access, to be replaced anyway.
SmallVector<const SCEV *, 4> Sizes;
SmallVector<const SCEV *, 4> Subscripts;
for (unsigned i = 0; i < SAI->getNumberOfDimensions(); i += 1) {
MemoryAccess *Access =
new MemoryAccess(Stmt, LI, MemoryAccess::READ, SAI->getBasePtr(),
LI->getType(), true, {}, Sizes, LI, MemoryKind::Array);
Stmt->addAccess(Access, true);
return Access;
/// Forward a load by reading from an array element that contains the same
/// value. Typically the location it was loaded from.
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param Inst The (possibly speculatable) instruction to forward.
/// @param UseStmt The statement that uses @p Inst.
/// @param UseLoop The loop @p Inst is used in.
/// @param DefStmt The statement @p Inst is defined in.
/// @param DefLoop The loop which contains @p Inst.
/// @return A ForwardingAction object describing the feasibility and
/// profitability evaluation and the callback carrying-out the value
/// forwarding.
ForwardingAction forwardKnownLoad(ScopStmt *TargetStmt, Instruction *Inst,
ScopStmt *UseStmt, Loop *UseLoop,
ScopStmt *DefStmt, Loop *DefLoop) {
// Cannot do anything without successful known analysis.
if (Known.is_null() || Translator.is_null() ||
return ForwardingAction::notApplicable();
LoadInst *LI = dyn_cast<LoadInst>(Inst);
if (!LI)
return ForwardingAction::notApplicable();
ForwardingDecision OpDecision =
forwardTree(TargetStmt, LI->getPointerOperand(), DefStmt, DefLoop);
switch (OpDecision) {
case FD_CanForwardProfitably:
case FD_CanForwardLeaf:
case FD_CannotForward:
return ForwardingAction::cannotForward();
llvm_unreachable("Shouldn't return this");
MemoryAccess *Access = TargetStmt->getArrayAccessOrNULLFor(LI);
if (Access) {
// If the load is already in the statement, no forwarding is necessary.
// However, it might happen that the LoadInst is already present in the
// statement's instruction list. In that case we do as follows:
// - For the evaluation, we can trivially forward it as it is
// benefit of forwarding an already present instruction.
// - For the execution, prepend the instruction (to make it
// available to all instructions following in the instruction list), but
// do not add another MemoryAccess.
auto ExecAction = [this, TargetStmt, LI, Access]() -> bool {
dbgs() << " forwarded known load with preexisting MemoryAccess"
<< Access << "\n");
return true;
return ForwardingAction::canForward(
ExecAction, {{LI->getPointerOperand(), DefStmt}}, true);
// Allow the following Isl calculations (until we return the
// ForwardingAction, excluding the code inside the lambda that will be
// executed later) to fail.
IslQuotaScope QuotaScope = MaxOpGuard.enter();
// { DomainDef[] -> ValInst[] }
isl::map ExpectedVal = makeValInst(Inst, UseStmt, UseLoop);
assert(!isNormalized(ExpectedVal).is_false() &&
"LoadInsts are always normalized");
// { DomainUse[] -> DomainTarget[] }
isl::map UseToTarget = getDefToTarget(UseStmt, TargetStmt);
// { DomainTarget[] -> ValInst[] }
isl::map TargetExpectedVal = ExpectedVal.apply_domain(UseToTarget);
isl::union_map TranslatedExpectedVal =
// { DomainTarget[] -> Element[] }
isl::union_map Candidates = findSameContentElements(TranslatedExpectedVal);
isl::map SameVal = singleLocation(Candidates, getDomainFor(TargetStmt));
if (SameVal.is_null())
return ForwardingAction::notApplicable();
LLVM_DEBUG(dbgs() << " expected values where " << TargetExpectedVal
<< "\n");
LLVM_DEBUG(dbgs() << " candidate elements where " << Candidates
<< "\n");
// { ValInst[] }
isl::space ValInstSpace = ExpectedVal.get_space().range();
// After adding a new load to the SCoP, also update the Known content
// about it. The new load will have a known ValInst of
// { [DomainTarget[] -> Value[]] }
// but which -- because it is a copy of it -- has same value as the
// { [DomainDef[] -> Value[]] }
// that it replicates. Instead of cloning the known content of
// [DomainDef[] -> Value[]]
// for DomainTarget[], we add a 'translator' that maps
// [DomainTarget[] -> Value[]] to [DomainDef[] -> Value[]]
// before comparing to the known content.
// TODO: 'Translator' could also be used to map PHINodes to their incoming
// ValInsts.
isl::map LocalTranslator;
if (!ValInstSpace.is_wrapping().is_false()) {
// { DefDomain[] -> Value[] }
isl::map ValInsts = ExpectedVal.range().unwrap();
// { DefDomain[] }
isl::set DefDomain = ValInsts.domain();
// { Value[] }
isl::space ValSpace = ValInstSpace.unwrap().range();
// { Value[] -> Value[] }
isl::map ValToVal =
// { DomainDef[] -> DomainTarget[] }
isl::map DefToTarget = getDefToTarget(DefStmt, TargetStmt);
// { [TargetDomain[] -> Value[]] -> [DefDomain[] -> Value] }
LocalTranslator = DefToTarget.reverse().product(ValToVal);
LLVM_DEBUG(dbgs() << " local translator is " << LocalTranslator
<< "\n");
if (LocalTranslator.is_null())
return ForwardingAction::notApplicable();
auto ExecAction = [this, TargetStmt, LI, SameVal,
LocalTranslator]() -> bool {
MemoryAccess *Access = makeReadArrayAccess(TargetStmt, LI, SameVal);
LLVM_DEBUG(dbgs() << " forwarded known load with new MemoryAccess"
<< Access << "\n");
if (!LocalTranslator.is_null())
Translator = Translator.unite(LocalTranslator);
return true;
return ForwardingAction::canForward(
ExecAction, {{LI->getPointerOperand(), DefStmt}}, true);
/// Forward a scalar by redirecting the access to an array element that stores
/// the same value.
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param Inst The scalar to forward.
/// @param UseStmt The statement that uses @p Inst.
/// @param UseLoop The loop @p Inst is used in.
/// @param DefStmt The statement @p Inst is defined in.
/// @param DefLoop The loop which contains @p Inst.
/// @return A ForwardingAction object describing the feasibility and
/// profitability evaluation and the callback carrying-out the value
/// forwarding.
ForwardingAction reloadKnownContent(ScopStmt *TargetStmt, Instruction *Inst,
ScopStmt *UseStmt, Loop *UseLoop,
ScopStmt *DefStmt, Loop *DefLoop) {
// Cannot do anything without successful known analysis.
if (Known.is_null() || Translator.is_null() ||
return ForwardingAction::notApplicable();
// Don't spend too much time analyzing whether it can be reloaded.
IslQuotaScope QuotaScope = MaxOpGuard.enter();
// { DomainDef[] -> ValInst[] }
isl::union_map ExpectedVal = makeNormalizedValInst(Inst, UseStmt, UseLoop);
// { DomainUse[] -> DomainTarget[] }
isl::map UseToTarget = getDefToTarget(UseStmt, TargetStmt);
// { DomainTarget[] -> ValInst[] }
isl::union_map TargetExpectedVal = ExpectedVal.apply_domain(UseToTarget);
isl::union_map TranslatedExpectedVal =
// { DomainTarget[] -> Element[] }
isl::union_map Candidates = findSameContentElements(TranslatedExpectedVal);
isl::map SameVal = singleLocation(Candidates, getDomainFor(TargetStmt));
if (SameVal.is_null())
return ForwardingAction::notApplicable();
auto ExecAction = [this, TargetStmt, Inst, SameVal]() {
MemoryAccess *Access = TargetStmt->lookupInputAccessOf(Inst);
if (!Access)
Access = TargetStmt->ensureValueRead(Inst);
LLVM_DEBUG(dbgs() << " forwarded known content of " << *Inst
<< " which is " << SameVal << "\n");
return false;
return ForwardingAction::canForward(ExecAction, {}, true);
/// Forwards a speculatively executable instruction.
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param UseInst The (possibly speculatable) instruction to forward.
/// @param DefStmt The statement @p UseInst is defined in.
/// @param DefLoop The loop which contains @p UseInst.
/// @return A ForwardingAction object describing the feasibility and
/// profitability evaluation and the callback carrying-out the value
/// forwarding.
ForwardingAction forwardSpeculatable(ScopStmt *TargetStmt,
Instruction *UseInst, ScopStmt *DefStmt,
Loop *DefLoop) {
// PHIs, unless synthesizable, are not yet supported.
if (isa<PHINode>(UseInst))
return ForwardingAction::notApplicable();
// Compatible instructions must satisfy the following conditions:
// 1. Idempotent (instruction will be copied, not moved; although its
// original instance might be removed by simplification)
// 2. Not access memory (There might be memory writes between)
// 3. Not cause undefined behaviour (we might copy to a location when the
// original instruction was no executed; this is currently not possible
// because we do not forward PHINodes)
// 4. Not leak memory if executed multiple times (i.e. malloc)
// Instruction::mayHaveSideEffects is not sufficient because it considers
// malloc to not have side-effects. llvm::isSafeToSpeculativelyExecute is
// not sufficient because it allows memory accesses.
if (mayBeMemoryDependent(*UseInst))
return ForwardingAction::notApplicable();
SmallVector<ForwardingAction::KeyTy, 4> Depends;
for (Value *OpVal : UseInst->operand_values()) {
ForwardingDecision OpDecision =
forwardTree(TargetStmt, OpVal, DefStmt, DefLoop);
switch (OpDecision) {
case FD_CannotForward:
return ForwardingAction::cannotForward();
case FD_CanForwardLeaf:
case FD_CanForwardProfitably:
Depends.emplace_back(OpVal, DefStmt);
case FD_NotApplicable:
case FD_Unknown:
"forwardTree should never return FD_NotApplicable/FD_Unknown");
auto ExecAction = [this, TargetStmt, UseInst]() {
// To ensure the right order, prepend this instruction before its
// operands. This ensures that its operands are inserted before the
// instruction using them.
LLVM_DEBUG(dbgs() << " forwarded speculable instruction: " << *UseInst
<< "\n");
return true;
return ForwardingAction::canForward(ExecAction, Depends, true);
/// Determines whether an operand tree can be forwarded and returns
/// instructions how to do so in the form of a ForwardingAction object.
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param UseVal The value (usually an instruction) which is root of an
/// operand tree.
/// @param UseStmt The statement that uses @p UseVal.
/// @param UseLoop The loop @p UseVal is used in.
/// @return A ForwardingAction object describing the feasibility and
/// profitability evaluation and the callback carrying-out the value
/// forwarding.
ForwardingAction forwardTreeImpl(ScopStmt *TargetStmt, Value *UseVal,
ScopStmt *UseStmt, Loop *UseLoop) {
ScopStmt *DefStmt = nullptr;
Loop *DefLoop = nullptr;
// { DefDomain[] -> TargetDomain[] }
isl::map DefToTarget;
VirtualUse VUse = VirtualUse::create(UseStmt, UseLoop, UseVal, true);
switch (VUse.getKind()) {
case VirtualUse::Constant:
case VirtualUse::Block:
case VirtualUse::Hoisted:
// These can be used anywhere without special considerations.
return ForwardingAction::triviallyForwardable(false, UseVal);
case VirtualUse::Synthesizable: {
// Check if the value is synthesizable at the new location as well. This
// might be possible when leaving a loop for which ScalarEvolution is
// unable to derive the exit value for.
// TODO: If there is a LCSSA PHI at the loop exit, use that one.
// If the SCEV contains a SCEVAddRecExpr, we currently depend on that we
// do not forward past its loop header. This would require us to use a
// previous loop induction variable instead the current one. We currently
// do not allow forwarding PHI nodes, thus this should never occur (the
// only exception where no phi is necessary being an unreachable loop
// without edge from the outside).
VirtualUse TargetUse = VirtualUse::create(
S, TargetStmt, TargetStmt->getSurroundingLoop(), UseVal, true);
if (TargetUse.getKind() == VirtualUse::Synthesizable)
return ForwardingAction::triviallyForwardable(false, UseVal);
dbgs() << " Synthesizable would not be synthesizable anymore: "
<< *UseVal << "\n");
return ForwardingAction::cannotForward();
case VirtualUse::ReadOnly: {
if (!ModelReadOnlyScalars)
return ForwardingAction::triviallyForwardable(false, UseVal);
// If we model read-only scalars, we need to create a MemoryAccess for it.
auto ExecAction = [this, TargetStmt, UseVal]() {
LLVM_DEBUG(dbgs() << " forwarded read-only value " << *UseVal
<< "\n");
// Note that we cannot return true here. With a operand tree
// depth of 0, UseVal is the use in TargetStmt that we try to replace.
// With -polly-analyze-read-only-scalars=true we would ensure the
// existence of a MemoryAccess (which already exists for a leaf) and be
// removed again by tryForwardTree because it's goal is to remove this
// scalar MemoryAccess. It interprets FD_CanForwardTree as the
// permission to do so.
return false;
return ForwardingAction::canForward(ExecAction, {}, false);
case VirtualUse::Intra:
// Knowing that UseStmt and DefStmt are the same statement instance, just
// reuse the information about UseStmt for DefStmt
DefStmt = UseStmt;
case VirtualUse::Inter:
Instruction *Inst = cast<Instruction>(UseVal);
if (!DefStmt) {
DefStmt = S->getStmtFor(Inst);
if (!DefStmt)
return ForwardingAction::cannotForward();
DefLoop = LI->getLoopFor(Inst->getParent());
ForwardingAction SpeculativeResult =
forwardSpeculatable(TargetStmt, Inst, DefStmt, DefLoop);
if (SpeculativeResult.Decision != FD_NotApplicable)
return SpeculativeResult;
ForwardingAction KnownResult = forwardKnownLoad(
TargetStmt, Inst, UseStmt, UseLoop, DefStmt, DefLoop);
if (KnownResult.Decision != FD_NotApplicable)
return KnownResult;
ForwardingAction ReloadResult = reloadKnownContent(
TargetStmt, Inst, UseStmt, UseLoop, DefStmt, DefLoop);
if (ReloadResult.Decision != FD_NotApplicable)
return ReloadResult;
// When no method is found to forward the operand tree, we effectively
// cannot handle it.
LLVM_DEBUG(dbgs() << " Cannot forward instruction: " << *Inst << "\n");
return ForwardingAction::cannotForward();
llvm_unreachable("Case unhandled");
/// Determines whether an operand tree can be forwarded. Previous evaluations
/// are cached.
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param UseVal The value (usually an instruction) which is root of an
/// operand tree.
/// @param UseStmt The statement that uses @p UseVal.
/// @param UseLoop The loop @p UseVal is used in.
/// @return FD_CannotForward if @p UseVal cannot be forwarded.
/// FD_CanForwardLeaf if @p UseVal is forwardable, but not
/// profitable.
/// FD_CanForwardProfitably if @p UseVal is forwardable and useful to
/// do.
ForwardingDecision forwardTree(ScopStmt *TargetStmt, Value *UseVal,
ScopStmt *UseStmt, Loop *UseLoop) {
// Lookup any cached evaluation.
auto It = ForwardingActions.find({UseVal, UseStmt});
if (It != ForwardingActions.end())
return It->second.Decision;
// Make a new evaluation.
ForwardingAction Action =
forwardTreeImpl(TargetStmt, UseVal, UseStmt, UseLoop);
ForwardingDecision Result = Action.Decision;
// Remember for the next time.
assert(!ForwardingActions.count({UseVal, UseStmt}) &&
"circular dependency?");
ForwardingActions.insert({{UseVal, UseStmt}, std::move(Action)});
return Result;
/// Forward an operand tree using cached actions.
/// @param Stmt Statement the operand tree is moved into.
/// @param UseVal Root of the operand tree within @p Stmt.
/// @param RA The MemoryAccess for @p UseVal that the forwarding intends
/// to remove.
void applyForwardingActions(ScopStmt *Stmt, Value *UseVal, MemoryAccess *RA) {
using ChildItTy =
using EdgeTy = std::pair<ForwardingAction *, ChildItTy>;
DenseSet<ForwardingAction::KeyTy> Visited;
SmallVector<EdgeTy, 32> Stack;
SmallVector<ForwardingAction *, 32> Ordered;
// Seed the tree search using the root value.
assert(ForwardingActions.count({UseVal, Stmt}));
ForwardingAction *RootAction = &ForwardingActions[{UseVal, Stmt}];
Stack.emplace_back(RootAction, RootAction->Depends.begin());
// Compute the postorder of the operand tree: all operands of an instruction
// must be visited before the instruction itself. As an additional
// requirement, the topological ordering must be 'compact': Any subtree node
// must not be interleaved with nodes from a non-shared subtree. This is
// because the same llvm::Instruction can be materialized multiple times as
// used at different ScopStmts which might be different values. Intersecting
// these lifetimes may result in miscompilations.
// FIXME: Intersecting lifetimes might still be possible for the roots
// themselves, since instructions are just prepended to a ScopStmt's
// instruction list.
while (!Stack.empty()) {
EdgeTy &Top = Stack.back();
ForwardingAction *TopAction = Top.first;
ChildItTy &TopEdge = Top.second;
if (TopEdge == TopAction->Depends.end()) {
// Postorder sorting
ForwardingAction::KeyTy Key = *TopEdge;
// Next edge for this level
auto VisitIt = Visited.insert(Key);
if (!VisitIt.second)
assert(ForwardingActions.count(Key) &&
"Must not insert new actions during execution phase");
ForwardingAction *ChildAction = &ForwardingActions[Key];
Stack.emplace_back(ChildAction, ChildAction->Depends.begin());
// Actually, we need the reverse postorder because actions prepend new
// instructions. Therefore, the first one will always be the action for the
// operand tree's root.
assert(Ordered.back() == RootAction);
if (RootAction->Execute())
for (auto DepAction : reverse(Ordered)) {
assert(DepAction->Decision != FD_Unknown &&
DepAction->Decision != FD_CannotForward);
assert(DepAction != RootAction);
/// Try to forward an operand tree rooted in @p RA.
bool tryForwardTree(MemoryAccess *RA) {
LLVM_DEBUG(dbgs() << "Trying to forward operand tree " << RA << "...\n");
ScopStmt *Stmt = RA->getStatement();
Loop *InLoop = Stmt->getSurroundingLoop();
isl::map TargetToUse;
if (!Known.is_null()) {
isl::space DomSpace = Stmt->getDomainSpace();
TargetToUse =
ForwardingDecision Assessment =
forwardTree(Stmt, RA->getAccessValue(), Stmt, InLoop);
// If considered feasible and profitable, forward it.
bool Changed = false;
if (Assessment == FD_CanForwardProfitably) {
applyForwardingActions(Stmt, RA->getAccessValue(), RA);
Changed = true;
return Changed;
/// Return which SCoP this instance is processing.
Scop *getScop() const { return S; }
/// Run the algorithm: Use value read accesses as operand tree roots and try
/// to forward them into the statement.
bool forwardOperandTrees() {
for (ScopStmt &Stmt : *S) {
bool StmtModified = false;
// Because we are modifying the MemoryAccess list, collect them first to
// avoid iterator invalidation.
SmallVector<MemoryAccess *, 16> Accs(Stmt.begin(), Stmt.end());
for (MemoryAccess *RA : Accs) {
if (!RA->isRead())
if (!RA->isLatestScalarKind())
if (tryForwardTree(RA)) {
Modified = true;
StmtModified = true;
if (StmtModified) {
if (Modified) {
return Modified;
/// Print the pass result, performed transformations and the SCoP after the
/// transformation.
void print(raw_ostream &OS, int Indent = 0) {
printStatistics(OS, Indent);
if (!Modified) {
// This line can easily be checked in regression tests.
OS << "ForwardOpTree executed, but did not modify anything\n";
printStatements(OS, Indent);
bool isModified() const { return Modified; }
static std::unique_ptr<ForwardOpTreeImpl> runForwardOpTree(Scop &S,
LoopInfo &LI) {
std::unique_ptr<ForwardOpTreeImpl> Impl;
IslMaxOperationsGuard MaxOpGuard(S.getIslCtx().get(), MaxOps, false);
Impl = std::make_unique<ForwardOpTreeImpl>(&S, &LI, MaxOpGuard);
if (AnalyzeKnown) {
LLVM_DEBUG(dbgs() << "Prepare forwarders...\n");
LLVM_DEBUG(dbgs() << "Forwarding operand trees...\n");
if (MaxOpGuard.hasQuotaExceeded()) {
LLVM_DEBUG(dbgs() << "Not all operations completed because of "
"max_operations exceeded\n");
LLVM_DEBUG(dbgs() << "\nFinal Scop:\n");
LLVM_DEBUG(dbgs() << S);
// Update statistics
Scop::ScopStatistics ScopStats = S.getStatistics();
NumValueWrites += ScopStats.NumValueWrites;
NumValueWritesInLoops += ScopStats.NumValueWritesInLoops;
NumPHIWrites += ScopStats.NumPHIWrites;
NumPHIWritesInLoops += ScopStats.NumPHIWritesInLoops;
NumSingletonWrites += ScopStats.NumSingletonWrites;
NumSingletonWritesInLoops += ScopStats.NumSingletonWritesInLoops;
return Impl;
static PreservedAnalyses
runForwardOpTreeUsingNPM(Scop &S, ScopAnalysisManager &SAM,
ScopStandardAnalysisResults &SAR, SPMUpdater &U,
raw_ostream *OS) {
LoopInfo &LI = SAR.LI;
std::unique_ptr<ForwardOpTreeImpl> Impl = runForwardOpTree(S, LI);
if (OS) {
*OS << "Printing analysis 'Polly - Forward operand tree' for region: '"
<< S.getName() << "' in function '" << S.getFunction().getName()
<< "':\n";
if (Impl) {
assert(Impl->getScop() == &S);
if (!Impl->isModified())
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
/// Pass that redirects scalar reads to array elements that are known to contain
/// the same value.
/// This reduces the number of scalar accesses and therefore potentially
/// increases the freedom of the scheduler. In the ideal case, all reads of a
/// scalar definition are redirected (We currently do not care about removing
/// the write in this case). This is also useful for the main DeLICM pass as
/// there are less scalars to be mapped.
class ForwardOpTreeWrapperPass : public ScopPass {
/// The pass implementation, also holding per-scop data.
std::unique_ptr<ForwardOpTreeImpl> Impl;
static char ID;
explicit ForwardOpTreeWrapperPass() : ScopPass(ID) {}
ForwardOpTreeWrapperPass(const ForwardOpTreeWrapperPass &) = delete;
ForwardOpTreeWrapperPass &
operator=(const ForwardOpTreeWrapperPass &) = delete;
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnScop(Scop &S) override {
// Free resources for previous SCoP's computation, if not yet done.
LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
Impl = runForwardOpTree(S, LI);
return false;
void printScop(raw_ostream &OS, Scop &S) const override {
if (!Impl)
assert(Impl->getScop() == &S);
void releaseMemory() override { Impl.reset(); }
}; // class ForwardOpTree
char ForwardOpTreeWrapperPass::ID;
} // namespace
Pass *polly::createForwardOpTreeWrapperPass() {
return new ForwardOpTreeWrapperPass();
INITIALIZE_PASS_BEGIN(ForwardOpTreeWrapperPass, "polly-optree",
"Polly - Forward operand tree", false, false)
INITIALIZE_PASS_END(ForwardOpTreeWrapperPass, "polly-optree",
"Polly - Forward operand tree", false, false)
llvm::PreservedAnalyses ForwardOpTreePass::run(Scop &S,
ScopAnalysisManager &SAM,
ScopStandardAnalysisResults &SAR,
SPMUpdater &U) {
return runForwardOpTreeUsingNPM(S, SAM, SAR, U, nullptr);
ForwardOpTreePrinterPass::run(Scop &S, ScopAnalysisManager &SAM,
ScopStandardAnalysisResults &SAR, SPMUpdater &U) {
return runForwardOpTreeUsingNPM(S, SAM, SAR, U, &OS);