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MemorySSA: Move updater to its own file

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293357 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Daniel Berlin 2017-01-28 01:35:02 +00:00
parent f6dfcde86b
commit 92cf58ee36
6 changed files with 460 additions and 384 deletions
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45
include/llvm/Transforms/Utils/MemorySSA.h
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@ -752,51 +752,6 @@ private:
unsigned NextID;
};
// An automatic updater for MemorySSA that handles arbitrary insertion,
// deletion, and moves. It performs phi insertion where necessary, and
// automatically updates the MemorySSA IR to be correct.
// While updating loads or removing instructions is often easy enough to not
// need this, updating stores should generally not be attemped outside this
// API.
//
// Basic API usage:
// Create the memory access you want for the instruction (this is mainly so
// we know where it is, without having to duplicate the entire set of create
// functions MemorySSA supports).
// Call insertDef or insertUse depending on whether it's a MemoryUse or a
// MemoryDef.
// That's it.
//
// For moving, first, move the instruction itself using the normal SSA
// instruction moving API, then just call moveBefore or moveAfter with the right
// arguments.
//
class MemorySSAUpdater {
private:
MemorySSA *MSSA;
SmallVector<MemoryPhi *, 8> InsertedPHIs;
SmallPtrSet<BasicBlock *, 8> VisitedBlocks;
public:
MemorySSAUpdater(MemorySSA *MSSA) : MSSA(MSSA) {}
void insertDef(MemoryDef *Def);
void insertUse(MemoryUse *Use);
void moveBefore(MemoryUseOrDef *What, MemoryUseOrDef *Where);
void moveAfter(MemoryUseOrDef *What, MemoryUseOrDef *Where);
private:
void moveTo(MemoryUseOrDef *What, BasicBlock *BB,
MemorySSA::AccessList::iterator Where);
MemoryAccess *getPreviousDef(MemoryAccess *);
MemoryAccess *getPreviousDefInBlock(MemoryAccess *);
MemoryAccess *getPreviousDefFromEnd(BasicBlock *);
MemoryAccess *getPreviousDefRecursive(BasicBlock *);
MemoryAccess *recursePhi(MemoryAccess *Phi);
template <class RangeType>
MemoryAccess *tryRemoveTrivialPhi(MemoryPhi *Phi, RangeType &Operands);
void fixupDefs(const SmallVectorImpl<MemoryAccess *> &);
};
// This pass does eager building and then printing of MemorySSA. It is used by
// the tests to be able to build, dump, and verify Memory SSA.
class MemorySSAPrinterLegacyPass : public FunctionPass {

86
include/llvm/Transforms/Utils/MemorySSAUpdater.h Normal file
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@ -0,0 +1,86 @@
//===- MemorySSAUpdater.h - Memory SSA Updater-------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// \file
// \brief An automatic updater for MemorySSA that handles arbitrary insertion,
// deletion, and moves. It performs phi insertion where necessary, and
// automatically updates the MemorySSA IR to be correct.
// While updating loads or removing instructions is often easy enough to not
// need this, updating stores should generally not be attemped outside this
// API.
//
// Basic API usage:
// Create the memory access you want for the instruction (this is mainly so
// we know where it is, without having to duplicate the entire set of create
// functions MemorySSA supports).
// Call insertDef or insertUse depending on whether it's a MemoryUse or a
// MemoryDef.
// That's it.
//
// For moving, first, move the instruction itself using the normal SSA
// instruction moving API, then just call moveBefore or moveAfter with the right
// arguments.
//
// walk memory instructions using a use/def graph.
//===----------------------------------------------------------------------===//
#ifndef LLVM_TRANSFORMS_UTILS_MEMORYSSAUPDATER_H
#define LLVM_TRANSFORMS_UTILS_MEMORYSSAUPDATER_H
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/OperandTraits.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Transforms/Utils/MemorySSA.h"
namespace llvm {
class Function;
class Instruction;
class MemoryAccess;
class LLVMContext;
class raw_ostream;
class MemorySSAUpdater {
private:
MemorySSA *MSSA;
SmallVector<MemoryPhi *, 8> InsertedPHIs;
SmallPtrSet<BasicBlock *, 8> VisitedBlocks;
public:
MemorySSAUpdater(MemorySSA *MSSA) : MSSA(MSSA) {}
void insertDef(MemoryDef *Def);
void insertUse(MemoryUse *Use);
void moveBefore(MemoryUseOrDef *What, MemoryUseOrDef *Where);
void moveAfter(MemoryUseOrDef *What, MemoryUseOrDef *Where);
private:
void moveTo(MemoryUseOrDef *What, BasicBlock *BB,
MemorySSA::AccessList::iterator Where);
MemoryAccess *getPreviousDef(MemoryAccess *);
MemoryAccess *getPreviousDefInBlock(MemoryAccess *);
MemoryAccess *getPreviousDefFromEnd(BasicBlock *);
MemoryAccess *getPreviousDefRecursive(BasicBlock *);
MemoryAccess *recursePhi(MemoryAccess *Phi);
template <class RangeType>
MemoryAccess *tryRemoveTrivialPhi(MemoryPhi *Phi, RangeType &Operands);
void fixupDefs(const SmallVectorImpl<MemoryAccess *> &);
};
} // end namespace llvm
#endif // LLVM_TRANSFORMS_UTILS_MEMORYSSAUPDATER_H

1
lib/Transforms/Utils/CMakeLists.txt
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@ -34,6 +34,7 @@ add_llvm_library(LLVMTransformUtils
LowerSwitch.cpp
Mem2Reg.cpp
MemorySSA.cpp
MemorySSAUpdater.cpp
MetaRenamer.cpp
ModuleUtils.cpp
NameAnonGlobals.cpp

339
lib/Transforms/Utils/MemorySSA.cpp
View File

@ -2397,343 +2397,4 @@ MemoryAccess *DoNothingMemorySSAWalker::getClobberingMemoryAccess(
return Use->getDefiningAccess();
return StartingAccess;
}
// This is the marker algorithm from "Simple and Efficient Construction of
// Static Single Assignment Form"
// The simple, non-marker algorithm places phi nodes at any join
// Here, we place markers, and only place phi nodes if they end up necessary.
// They are only necessary if they break a cycle (IE we recursively visit
// ourselves again), or we discover, while getting the value of the operands,
// that there are two or more definitions needing to be merged.
// This still will leave non-minimal form in the case of irreducible control
// flow, where phi nodes may be in cycles with themselves, but unnecessary.
MemoryAccess *MemorySSAUpdater::getPreviousDefRecursive(BasicBlock *BB) {
// Single predecessor case, just recurse, we can only have one definition.
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
return getPreviousDefFromEnd(Pred);
} else if (VisitedBlocks.count(BB)) {
// We hit our node again, meaning we had a cycle, we must insert a phi
// node to break it so we have an operand. The only case this will
// insert useless phis is if we have irreducible control flow.
return MSSA->createMemoryPhi(BB);
} else if (VisitedBlocks.insert(BB).second) {
// Mark us visited so we can detect a cycle
SmallVector<MemoryAccess *, 8> PhiOps;
// Recurse to get the values in our predecessors for placement of a
// potential phi node. This will insert phi nodes if we cycle in order to
// break the cycle and have an operand.
for (auto *Pred : predecessors(BB))
PhiOps.push_back(getPreviousDefFromEnd(Pred));
// Now try to simplify the ops to avoid placing a phi.
// This may return null if we never created a phi yet, that's okay
MemoryPhi *Phi = dyn_cast_or_null<MemoryPhi>(MSSA->getMemoryAccess(BB));
bool PHIExistsButNeedsUpdate = false;
// See if the existing phi operands match what we need.
// Unlike normal SSA, we only allow one phi node per block, so we can't just
// create a new one.
if (Phi && Phi->getNumOperands() != 0)
if (!std::equal(Phi->op_begin(), Phi->op_end(), PhiOps.begin())) {
PHIExistsButNeedsUpdate = true;
}
// See if we can avoid the phi by simplifying it.
auto *Result = tryRemoveTrivialPhi(Phi, PhiOps);
// If we couldn't simplify, we may have to create a phi
if (Result == Phi) {
if (!Phi)
Phi = MSSA->createMemoryPhi(BB);
// These will have been filled in by the recursive read we did above.
if (PHIExistsButNeedsUpdate) {
std::copy(PhiOps.begin(), PhiOps.end(), Phi->op_begin());
std::copy(pred_begin(BB), pred_end(BB), Phi->block_begin());
} else {
unsigned i = 0;
for (auto *Pred : predecessors(BB))
Phi->addIncoming(PhiOps[i++], Pred);
}
Result = Phi;
}
if (MemoryPhi *MP = dyn_cast<MemoryPhi>(Result))
InsertedPHIs.push_back(MP);
// Set ourselves up for the next variable by resetting visited state.
VisitedBlocks.erase(BB);
return Result;
}
llvm_unreachable("Should have hit one of the three cases above");
}
// This starts at the memory access, and goes backwards in the block to find the
// previous definition. If a definition is not found the block of the access,
// it continues globally, creating phi nodes to ensure we have a single
// definition.
MemoryAccess *MemorySSAUpdater::getPreviousDef(MemoryAccess *MA) {
auto *LocalResult = getPreviousDefInBlock(MA);
return LocalResult ? LocalResult : getPreviousDefRecursive(MA->getBlock());
}
// This starts at the memory access, and goes backwards in the block to the find
// the previous definition. If the definition is not found in the block of the
// access, it returns nullptr.
MemoryAccess *MemorySSAUpdater::getPreviousDefInBlock(MemoryAccess *MA) {
auto *Defs = MSSA->getWritableBlockDefs(MA->getBlock());
// It's possible there are no defs, or we got handed the first def to start.
if (Defs) {
// If this is a def, we can just use the def iterators.
if (!isa<MemoryUse>(MA)) {
auto Iter = MA->getReverseDefsIterator();
++Iter;
if (Iter != Defs->rend())
return &*Iter;
} else {
// Otherwise, have to walk the all access iterator.
auto Iter = MA->getReverseIterator();
++Iter;
while (&*Iter != &*Defs->begin()) {
if (!isa<MemoryUse>(*Iter))
return &*Iter;
--Iter;
}
// At this point it must be pointing at firstdef
assert(&*Iter == &*Defs->begin() &&
"Should have hit first def walking backwards");
return &*Iter;
}
}
return nullptr;
}
// This starts at the end of block
MemoryAccess *MemorySSAUpdater::getPreviousDefFromEnd(BasicBlock *BB) {
auto *Defs = MSSA->getWritableBlockDefs(BB);
if (Defs)
return &*Defs->rbegin();
return getPreviousDefRecursive(BB);
}
// Recurse over a set of phi uses to eliminate the trivial ones
MemoryAccess *MemorySSAUpdater::recursePhi(MemoryAccess *Phi) {
if (!Phi)
return nullptr;
TrackingVH<MemoryAccess> Res(Phi);
SmallVector<TrackingVH<Value>, 8> Uses;
std::copy(Phi->user_begin(), Phi->user_end(), std::back_inserter(Uses));
for (auto &U : Uses) {
if (MemoryPhi *UsePhi = dyn_cast<MemoryPhi>(&*U)) {
auto OperRange = UsePhi->operands();
tryRemoveTrivialPhi(UsePhi, OperRange);
}
}
return Res;
}
// Eliminate trivial phis
// Phis are trivial if they are defined either by themselves, or all the same
// argument.
// IE phi(a, a) or b = phi(a, b) or c = phi(a, a, c)
// We recursively try to remove them.
template <class RangeType>
MemoryAccess *MemorySSAUpdater::tryRemoveTrivialPhi(MemoryPhi *Phi,
RangeType &Operands) {
// Detect equal or self arguments
MemoryAccess *Same = nullptr;
for (auto &Op : Operands) {
// If the same or self, good so far
if (Op == Phi || Op == Same)
continue;
// not the same, return the phi since it's not eliminatable by us
if (Same)
return Phi;
Same = cast<MemoryAccess>(Op);
}
// Never found a non-self reference, the phi is undef
if (Same == nullptr)
return MSSA->getLiveOnEntryDef();
if (Phi) {
Phi->replaceAllUsesWith(Same);
MSSA->removeMemoryAccess(Phi);
}
// We should only end up recursing in case we replaced something, in which
// case, we may have made other Phis trivial.
return recursePhi(Same);
}
void MemorySSAUpdater::insertUse(MemoryUse *MU) {
InsertedPHIs.clear();
MU->setDefiningAccess(getPreviousDef(MU));
// Unlike for defs, there is no extra work to do. Because uses do not create
// new may-defs, there are only two cases:
//
// 1. There was a def already below us, and therefore, we should not have
// created a phi node because it was already needed for the def.
//
// 2. There is no def below us, and therefore, there is no extra renaming work
// to do.
}
void setMemoryPhiValueForBlock(MemoryPhi *MP, const BasicBlock *BB,
MemoryAccess *NewDef) {
// Replace any operand with us an incoming block with the new defining
// access.
int i = MP->getBasicBlockIndex(BB);
assert(i != -1 && "Should have found the basic block in the phi");
while (MP->getIncomingBlock(i) == BB) {
// Unlike above, there is already a phi node here, so we only need
// to set the right value.
MP->setIncomingValue(i, NewDef);
++i;
}
}
// A brief description of the algorithm:
// First, we compute what should define the new def, using the SSA
// construction algorithm.
// Then, we update the defs below us (and any new phi nodes) in the graph to
// point to the correct new defs, to ensure we only have one variable, and no
// disconnected stores.
void MemorySSAUpdater::insertDef(MemoryDef *MD) {
InsertedPHIs.clear();
// See if we had a local def, and if not, go hunting.
MemoryAccess *DefBefore = getPreviousDefInBlock(MD);
bool DefBeforeSameBlock = DefBefore != nullptr;
if (!DefBefore)
DefBefore = getPreviousDefRecursive(MD->getBlock());
// There is a def before us, which means we can replace any store/phi uses
// of that thing with us, since we are in the way of whatever was there
// before.
// We now define that def's memorydefs and memoryphis
for (auto UI = DefBefore->use_begin(), UE = DefBefore->use_end(); UI != UE;) {
Use &U = *UI++;
// Leave the uses alone
if (isa<MemoryUse>(U.getUser()))
continue;
U.set(MD);
}
// and that def is now our defining access.
// We change them in this order otherwise we will appear in the use list
// above and reset ourselves.
MD->setDefiningAccess(DefBefore);
SmallVector<MemoryAccess *, 8> FixupList(InsertedPHIs.begin(),
InsertedPHIs.end());
if (!DefBeforeSameBlock) {
// If there was a local def before us, we must have the same effect it
// did. Because every may-def is the same, any phis/etc we would create, it
// would also have created. If there was no local def before us, we
// performed a global update, and have to search all successors and make
// sure we update the first def in each of them (following all paths until
// we hit the first def along each path). This may also insert phi nodes.
// TODO: There are other cases we can skip this work, such as when we have a
// single successor, and only used a straight line of single pred blocks
// backwards to find the def. To make that work, we'd have to track whether
// getDefRecursive only ever used the single predecessor case. These types
// of paths also only exist in between CFG simplifications.
FixupList.push_back(MD);
}
while (!FixupList.empty()) {
unsigned StartingPHISize = InsertedPHIs.size();
fixupDefs(FixupList);
FixupList.clear();
// Put any new phis on the fixup list, and process them
FixupList.append(InsertedPHIs.end() - StartingPHISize, InsertedPHIs.end());
}
}
void MemorySSAUpdater::fixupDefs(const SmallVectorImpl<MemoryAccess *> &Vars) {
SmallPtrSet<const BasicBlock *, 8> Seen;
SmallVector<const BasicBlock *, 16> Worklist;
for (auto *NewDef : Vars) {
// First, see if there is a local def after the operand.
auto *Defs = MSSA->getWritableBlockDefs(NewDef->getBlock());
auto DefIter = NewDef->getDefsIterator();
// If there is a local def after us, we only have to rename that.
if (++DefIter != Defs->end()) {
cast<MemoryDef>(DefIter)->setDefiningAccess(NewDef);
continue;
}
// Otherwise, we need to search down through the CFG.
// For each of our successors, handle it directly if their is a phi, or
// place on the fixup worklist.
for (const auto *S : successors(NewDef->getBlock())) {
if (auto *MP = MSSA->getMemoryAccess(S))
setMemoryPhiValueForBlock(MP, NewDef->getBlock(), NewDef);
else
Worklist.push_back(S);
}
while (!Worklist.empty()) {
const BasicBlock *FixupBlock = Worklist.back();
Worklist.pop_back();
// Get the first def in the block that isn't a phi node.
if (auto *Defs = MSSA->getWritableBlockDefs(FixupBlock)) {
auto *FirstDef = &*Defs->begin();
// The loop above and below should have taken care of phi nodes
assert(!isa<MemoryPhi>(FirstDef) &&
"Should have already handled phi nodes!");
// We are now this def's defining access, make sure we actually dominate
// it
assert(MSSA->dominates(NewDef, FirstDef) &&
"Should have dominated the new access");
// This may insert new phi nodes, because we are not guaranteed the
// block we are processing has a single pred, and depending where the
// store was inserted, it may require phi nodes below it.
cast<MemoryDef>(FirstDef)->setDefiningAccess(getPreviousDef(FirstDef));
return;
}
// We didn't find a def, so we must continue.
for (const auto *S : successors(FixupBlock)) {
// If there is a phi node, handle it.
// Otherwise, put the block on the worklist
if (auto *MP = MSSA->getMemoryAccess(S))
setMemoryPhiValueForBlock(MP, FixupBlock, NewDef);
else {
// If we cycle, we should have ended up at a phi node that we already
// processed. FIXME: Double check this
if (!Seen.insert(S).second)
continue;
Worklist.push_back(S);
}
}
}
}
}
// Move What before Where in the MemorySSA IR.
void MemorySSAUpdater::moveTo(MemoryUseOrDef *What, BasicBlock *BB,
MemorySSA::AccessList::iterator Where) {
// Replace all our users with our defining access.
What->replaceAllUsesWith(What->getDefiningAccess());
// Let MemorySSA take care of moving it around in the lists.
MSSA->moveTo(What, BB, Where);
// Now reinsert it into the IR and do whatever fixups needed.
if (auto *MD = dyn_cast<MemoryDef>(What))
insertDef(MD);
else
insertUse(cast<MemoryUse>(What));
}
// Move What before Where in the MemorySSA IR.
void MemorySSAUpdater::moveBefore(MemoryUseOrDef *What, MemoryUseOrDef *Where) {
moveTo(What, Where->getBlock(), Where->getIterator());
}
// Move What after Where in the MemorySSA IR.
void MemorySSAUpdater::moveAfter(MemoryUseOrDef *What, MemoryUseOrDef *Where) {
moveTo(What, Where->getBlock(), ++Where->getIterator());
}
} // namespace llvm

372
lib/Transforms/Utils/MemorySSAUpdater.cpp Normal file
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@ -0,0 +1,372 @@
//===-- MemorySSAUpdater.cpp - Memory SSA Updater--------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------===//
//
// This file implements the MemorySSAUpdater class.
//
//===----------------------------------------------------------------===//
#include "llvm/Transforms/Utils/MemorySSAUpdater.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/FormattedStream.h"
#include "llvm/Transforms/Utils/MemorySSA.h"
#include <algorithm>
#define DEBUG_TYPE "memoryssa"
using namespace llvm;
namespace llvm {
// This is the marker algorithm from "Simple and Efficient Construction of
// Static Single Assignment Form"
// The simple, non-marker algorithm places phi nodes at any join
// Here, we place markers, and only place phi nodes if they end up necessary.
// They are only necessary if they break a cycle (IE we recursively visit
// ourselves again), or we discover, while getting the value of the operands,
// that there are two or more definitions needing to be merged.
// This still will leave non-minimal form in the case of irreducible control
// flow, where phi nodes may be in cycles with themselves, but unnecessary.
MemoryAccess *MemorySSAUpdater::getPreviousDefRecursive(BasicBlock *BB) {
// Single predecessor case, just recurse, we can only have one definition.
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
return getPreviousDefFromEnd(Pred);
} else if (VisitedBlocks.count(BB)) {
// We hit our node again, meaning we had a cycle, we must insert a phi
// node to break it so we have an operand. The only case this will
// insert useless phis is if we have irreducible control flow.
return MSSA->createMemoryPhi(BB);
} else if (VisitedBlocks.insert(BB).second) {
// Mark us visited so we can detect a cycle
SmallVector<MemoryAccess *, 8> PhiOps;
// Recurse to get the values in our predecessors for placement of a
// potential phi node. This will insert phi nodes if we cycle in order to
// break the cycle and have an operand.
for (auto *Pred : predecessors(BB))
PhiOps.push_back(getPreviousDefFromEnd(Pred));
// Now try to simplify the ops to avoid placing a phi.
// This may return null if we never created a phi yet, that's okay
MemoryPhi *Phi = dyn_cast_or_null<MemoryPhi>(MSSA->getMemoryAccess(BB));
bool PHIExistsButNeedsUpdate = false;
// See if the existing phi operands match what we need.
// Unlike normal SSA, we only allow one phi node per block, so we can't just
// create a new one.
if (Phi && Phi->getNumOperands() != 0)
if (!std::equal(Phi->op_begin(), Phi->op_end(), PhiOps.begin())) {
PHIExistsButNeedsUpdate = true;
}
// See if we can avoid the phi by simplifying it.
auto *Result = tryRemoveTrivialPhi(Phi, PhiOps);
// If we couldn't simplify, we may have to create a phi
if (Result == Phi) {
if (!Phi)
Phi = MSSA->createMemoryPhi(BB);
// These will have been filled in by the recursive read we did above.
if (PHIExistsButNeedsUpdate) {
std::copy(PhiOps.begin(), PhiOps.end(), Phi->op_begin());
std::copy(pred_begin(BB), pred_end(BB), Phi->block_begin());
} else {
unsigned i = 0;
for (auto *Pred : predecessors(BB))
Phi->addIncoming(PhiOps[i++], Pred);
}
Result = Phi;
}
if (MemoryPhi *MP = dyn_cast<MemoryPhi>(Result))
InsertedPHIs.push_back(MP);
// Set ourselves up for the next variable by resetting visited state.
VisitedBlocks.erase(BB);
return Result;
}
llvm_unreachable("Should have hit one of the three cases above");
}
// This starts at the memory access, and goes backwards in the block to find the
// previous definition. If a definition is not found the block of the access,
// it continues globally, creating phi nodes to ensure we have a single
// definition.
MemoryAccess *MemorySSAUpdater::getPreviousDef(MemoryAccess *MA) {
auto *LocalResult = getPreviousDefInBlock(MA);
return LocalResult ? LocalResult : getPreviousDefRecursive(MA->getBlock());
}
// This starts at the memory access, and goes backwards in the block to the find
// the previous definition. If the definition is not found in the block of the
// access, it returns nullptr.
MemoryAccess *MemorySSAUpdater::getPreviousDefInBlock(MemoryAccess *MA) {
auto *Defs = MSSA->getWritableBlockDefs(MA->getBlock());
// It's possible there are no defs, or we got handed the first def to start.
if (Defs) {
// If this is a def, we can just use the def iterators.
if (!isa<MemoryUse>(MA)) {
auto Iter = MA->getReverseDefsIterator();
++Iter;
if (Iter != Defs->rend())
return &*Iter;
} else {
// Otherwise, have to walk the all access iterator.
auto Iter = MA->getReverseIterator();
++Iter;
while (&*Iter != &*Defs->begin()) {
if (!isa<MemoryUse>(*Iter))
return &*Iter;
--Iter;
}
// At this point it must be pointing at firstdef
assert(&*Iter == &*Defs->begin() &&
"Should have hit first def walking backwards");
return &*Iter;
}
}
return nullptr;
}
// This starts at the end of block
MemoryAccess *MemorySSAUpdater::getPreviousDefFromEnd(BasicBlock *BB) {
auto *Defs = MSSA->getWritableBlockDefs(BB);
if (Defs)
return &*Defs->rbegin();
return getPreviousDefRecursive(BB);
}
// Recurse over a set of phi uses to eliminate the trivial ones
MemoryAccess *MemorySSAUpdater::recursePhi(MemoryAccess *Phi) {
if (!Phi)
return nullptr;
TrackingVH<MemoryAccess> Res(Phi);
SmallVector<TrackingVH<Value>, 8> Uses;
std::copy(Phi->user_begin(), Phi->user_end(), std::back_inserter(Uses));
for (auto &U : Uses) {
if (MemoryPhi *UsePhi = dyn_cast<MemoryPhi>(&*U)) {
auto OperRange = UsePhi->operands();
tryRemoveTrivialPhi(UsePhi, OperRange);
}
}
return Res;
}
// Eliminate trivial phis
// Phis are trivial if they are defined either by themselves, or all the same
// argument.
// IE phi(a, a) or b = phi(a, b) or c = phi(a, a, c)
// We recursively try to remove them.
template <class RangeType>
MemoryAccess *MemorySSAUpdater::tryRemoveTrivialPhi(MemoryPhi *Phi,
RangeType &Operands) {
// Detect equal or self arguments
MemoryAccess *Same = nullptr;
for (auto &Op : Operands) {
// If the same or self, good so far
if (Op == Phi || Op == Same)
continue;
// not the same, return the phi since it's not eliminatable by us
if (Same)
return Phi;
Same = cast<MemoryAccess>(Op);
}
// Never found a non-self reference, the phi is undef
if (Same == nullptr)
return MSSA->getLiveOnEntryDef();
if (Phi) {
Phi->replaceAllUsesWith(Same);
MSSA->removeMemoryAccess(Phi);
}
// We should only end up recursing in case we replaced something, in which
// case, we may have made other Phis trivial.
return recursePhi(Same);
}
void MemorySSAUpdater::insertUse(MemoryUse *MU) {
InsertedPHIs.clear();
MU->setDefiningAccess(getPreviousDef(MU));
// Unlike for defs, there is no extra work to do. Because uses do not create
// new may-defs, there are only two cases:
//
// 1. There was a def already below us, and therefore, we should not have
// created a phi node because it was already needed for the def.
//
// 2. There is no def below us, and therefore, there is no extra renaming work
// to do.
}
void setMemoryPhiValueForBlock(MemoryPhi *MP, const BasicBlock *BB,
MemoryAccess *NewDef) {
// Replace any operand with us an incoming block with the new defining
// access.
int i = MP->getBasicBlockIndex(BB);
assert(i != -1 && "Should have found the basic block in the phi");
while (MP->getIncomingBlock(i) == BB) {
// Unlike above, there is already a phi node here, so we only need
// to set the right value.
MP->setIncomingValue(i, NewDef);
++i;
}
}
// A brief description of the algorithm:
// First, we compute what should define the new def, using the SSA
// construction algorithm.
// Then, we update the defs below us (and any new phi nodes) in the graph to
// point to the correct new defs, to ensure we only have one variable, and no
// disconnected stores.
void MemorySSAUpdater::insertDef(MemoryDef *MD) {
InsertedPHIs.clear();
// See if we had a local def, and if not, go hunting.
MemoryAccess *DefBefore = getPreviousDefInBlock(MD);
bool DefBeforeSameBlock = DefBefore != nullptr;
if (!DefBefore)
DefBefore = getPreviousDefRecursive(MD->getBlock());
// There is a def before us, which means we can replace any store/phi uses
// of that thing with us, since we are in the way of whatever was there
// before.
// We now define that def's memorydefs and memoryphis
for (auto UI = DefBefore->use_begin(), UE = DefBefore->use_end(); UI != UE;) {
Use &U = *UI++;
// Leave the uses alone
if (isa<MemoryUse>(U.getUser()))
continue;
U.set(MD);
}
// and that def is now our defining access.
// We change them in this order otherwise we will appear in the use list
// above and reset ourselves.
MD->setDefiningAccess(DefBefore);
SmallVector<MemoryAccess *, 8> FixupList(InsertedPHIs.begin(),
InsertedPHIs.end());
if (!DefBeforeSameBlock) {
// If there was a local def before us, we must have the same effect it
// did. Because every may-def is the same, any phis/etc we would create, it
// would also have created. If there was no local def before us, we
// performed a global update, and have to search all successors and make
// sure we update the first def in each of them (following all paths until
// we hit the first def along each path). This may also insert phi nodes.
// TODO: There are other cases we can skip this work, such as when we have a
// single successor, and only used a straight line of single pred blocks
// backwards to find the def. To make that work, we'd have to track whether
// getDefRecursive only ever used the single predecessor case. These types
// of paths also only exist in between CFG simplifications.
FixupList.push_back(MD);
}
while (!FixupList.empty()) {
unsigned StartingPHISize = InsertedPHIs.size();
fixupDefs(FixupList);
FixupList.clear();
// Put any new phis on the fixup list, and process them
FixupList.append(InsertedPHIs.end() - StartingPHISize, InsertedPHIs.end());
}
}
void MemorySSAUpdater::fixupDefs(const SmallVectorImpl<MemoryAccess *> &Vars) {
SmallPtrSet<const BasicBlock *, 8> Seen;
SmallVector<const BasicBlock *, 16> Worklist;
for (auto *NewDef : Vars) {
// First, see if there is a local def after the operand.
auto *Defs = MSSA->getWritableBlockDefs(NewDef->getBlock());
auto DefIter = NewDef->getDefsIterator();
// If there is a local def after us, we only have to rename that.
if (++DefIter != Defs->end()) {
cast<MemoryDef>(DefIter)->setDefiningAccess(NewDef);
continue;
}
// Otherwise, we need to search down through the CFG.
// For each of our successors, handle it directly if their is a phi, or
// place on the fixup worklist.
for (const auto *S : successors(NewDef->getBlock())) {
if (auto *MP = MSSA->getMemoryAccess(S))
setMemoryPhiValueForBlock(MP, NewDef->getBlock(), NewDef);
else
Worklist.push_back(S);
}
while (!Worklist.empty()) {
const BasicBlock *FixupBlock = Worklist.back();
Worklist.pop_back();
// Get the first def in the block that isn't a phi node.
if (auto *Defs = MSSA->getWritableBlockDefs(FixupBlock)) {
auto *FirstDef = &*Defs->begin();
// The loop above and below should have taken care of phi nodes
assert(!isa<MemoryPhi>(FirstDef) &&
"Should have already handled phi nodes!");
// We are now this def's defining access, make sure we actually dominate
// it
assert(MSSA->dominates(NewDef, FirstDef) &&
"Should have dominated the new access");
// This may insert new phi nodes, because we are not guaranteed the
// block we are processing has a single pred, and depending where the
// store was inserted, it may require phi nodes below it.
cast<MemoryDef>(FirstDef)->setDefiningAccess(getPreviousDef(FirstDef));
return;
}
// We didn't find a def, so we must continue.
for (const auto *S : successors(FixupBlock)) {
// If there is a phi node, handle it.
// Otherwise, put the block on the worklist
if (auto *MP = MSSA->getMemoryAccess(S))
setMemoryPhiValueForBlock(MP, FixupBlock, NewDef);
else {
// If we cycle, we should have ended up at a phi node that we already
// processed. FIXME: Double check this
if (!Seen.insert(S).second)
continue;
Worklist.push_back(S);
}
}
}
}
}
// Move What before Where in the MemorySSA IR.
void MemorySSAUpdater::moveTo(MemoryUseOrDef *What, BasicBlock *BB,
MemorySSA::AccessList::iterator Where) {
// Replace all our users with our defining access.
What->replaceAllUsesWith(What->getDefiningAccess());
// Let MemorySSA take care of moving it around in the lists.
MSSA->moveTo(What, BB, Where);
// Now reinsert it into the IR and do whatever fixups needed.
if (auto *MD = dyn_cast<MemoryDef>(What))
insertDef(MD);
else
insertUse(cast<MemoryUse>(What));
}
// Move What before Where in the MemorySSA IR.
void MemorySSAUpdater::moveBefore(MemoryUseOrDef *What, MemoryUseOrDef *Where) {
moveTo(What, Where->getBlock(), Where->getIterator());
}
// Move What after Where in the MemorySSA IR.
void MemorySSAUpdater::moveAfter(MemoryUseOrDef *What, MemoryUseOrDef *Where) {
moveTo(What, Where->getBlock(), ++Where->getIterator());
}
} // namespace llvm

1
unittests/Transforms/Utils/MemorySSA.cpp
View File

@ -15,6 +15,7 @@
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Transforms/Utils/MemorySSAUpdater.h"
#include "gtest/gtest.h"
using namespace llvm;