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llvm-project/llvm/lib/Analysis/LoadValueNumbering.cpp

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//===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a value numbering pass that value numbers load and call
// instructions. To do this, it finds lexically identical load instructions,
// and uses alias analysis to determine which loads are guaranteed to produce
// the same value. To value number call instructions, it looks for calls to
// functions that do not write to memory which do not have intervening
// instructions that clobber the memory that is read from.
//
// This pass builds off of another value numbering pass to implement value
// numbering for non-load and non-call instructions. It uses Alias Analysis so
// that it can disambiguate the load instructions. The more powerful these base
// analyses are, the more powerful the resultant value numbering will be.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/LoadValueNumbering.h"
#include "llvm/Constant.h"
#include "llvm/Function.h"
#include "llvm/iMemory.h"
#include "llvm/iOther.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"
#include "llvm/Analysis/ValueNumbering.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Support/CFG.h"
#include "llvm/Target/TargetData.h"
#include <set>
using namespace llvm;
namespace {
// FIXME: This should not be a FunctionPass.
struct LoadVN : public FunctionPass, public ValueNumbering {
/// Pass Implementation stuff. This doesn't do any analysis.
///
bool runOnFunction(Function &) { return false; }
/// getAnalysisUsage - Does not modify anything. It uses Value Numbering
/// and Alias Analysis.
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
/// getEqualNumberNodes - Return nodes with the same value number as the
/// specified Value. This fills in the argument vector with any equal
/// values.
///
virtual void getEqualNumberNodes(Value *V1,
std::vector<Value*> &RetVals) const;
/// getCallEqualNumberNodes - Given a call instruction, find other calls
/// that have the same value number.
void getCallEqualNumberNodes(CallInst *CI,
std::vector<Value*> &RetVals) const;
};
// Register this pass...
RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering");
// Declare that we implement the ValueNumbering interface
RegisterAnalysisGroup<ValueNumbering, LoadVN> Y;
}
Pass *llvm::createLoadValueNumberingPass() { return new LoadVN(); }
/// getAnalysisUsage - Does not modify anything. It uses Value Numbering and
/// Alias Analysis.
///
void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<AliasAnalysis>();
AU.addRequired<ValueNumbering>();
AU.addRequired<DominatorSet>();
AU.addRequired<TargetData>();
}
static bool isPathTransparentTo(BasicBlock *CurBlock, BasicBlock *Dom,
Value *Ptr, unsigned Size, AliasAnalysis &AA,
std::set<BasicBlock*> &Visited,
std::map<BasicBlock*, bool> &TransparentBlocks){
// If we have already checked out this path, or if we reached our destination,
// stop searching, returning success.
if (CurBlock == Dom || !Visited.insert(CurBlock).second)
return true;
// Check whether this block is known transparent or not.
std::map<BasicBlock*, bool>::iterator TBI =
TransparentBlocks.lower_bound(CurBlock);
if (TBI == TransparentBlocks.end() || TBI->first != CurBlock) {
// If this basic block can modify the memory location, then the path is not
// transparent!
if (AA.canBasicBlockModify(*CurBlock, Ptr, Size)) {
TransparentBlocks.insert(TBI, std::make_pair(CurBlock, false));
return false;
}
TransparentBlocks.insert(TBI, std::make_pair(CurBlock, true));
} else if (!TBI->second)
// This block is known non-transparent, so that path can't be either.
return false;
// The current block is known to be transparent. The entire path is
// transparent if all of the predecessors paths to the parent is also
// transparent to the memory location.
for (pred_iterator PI = pred_begin(CurBlock), E = pred_end(CurBlock);
PI != E; ++PI)
if (!isPathTransparentTo(*PI, Dom, Ptr, Size, AA, Visited,
TransparentBlocks))
return false;
return true;
}
/// getCallEqualNumberNodes - Given a call instruction, find other calls that
/// have the same value number.
void LoadVN::getCallEqualNumberNodes(CallInst *CI,
std::vector<Value*> &RetVals) const {
Function *CF = CI->getCalledFunction();
if (CF == 0) return; // Indirect call.
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
if (!AA.onlyReadsMemory(CF)) return; // Nothing we can do.
// Scan all of the arguments of the function, looking for one that is not
// global. In particular, we would prefer to have an argument or instruction
// operand to chase the def-use chains of.
Value *Op = CF;
for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
if (isa<Argument>(CI->getOperand(i)) ||
isa<Instruction>(CI->getOperand(i))) {
Op = CI->getOperand(i);
break;
}
// Identify all lexically identical calls in this function.
std::vector<CallInst*> IdenticalCalls;
Function *CIFunc = CI->getParent()->getParent();
for (Value::use_iterator UI = Op->use_begin(), E = Op->use_end(); UI != E;
++UI)
if (CallInst *C = dyn_cast<CallInst>(*UI))
if (C->getNumOperands() == CI->getNumOperands() &&
C->getOperand(0) == CI->getOperand(0) &&
C->getParent()->getParent() == CIFunc && C != CI) {
bool AllOperandsEqual = true;
for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
if (C->getOperand(i) != CI->getOperand(i)) {
AllOperandsEqual = false;
break;
}
if (AllOperandsEqual)
IdenticalCalls.push_back(C);
}
if (IdenticalCalls.empty()) return;
// Eliminate duplicates, which could occur if we chose a value that is passed
// into a call site multiple times.
std::sort(IdenticalCalls.begin(), IdenticalCalls.end());
IdenticalCalls.erase(std::unique(IdenticalCalls.begin(),IdenticalCalls.end()),
IdenticalCalls.end());
// If the call reads memory, we must make sure that there are no stores
// between the calls in question.
//
// FIXME: This should use mod/ref information. What we really care about it
// whether an intervening instruction could modify memory that is read, not
// ANY memory.
//
if (!AA.doesNotAccessMemory(CF)) {
DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
BasicBlock *CIBB = CI->getParent();
for (unsigned i = 0; i != IdenticalCalls.size(); ++i) {
CallInst *C = IdenticalCalls[i];
bool CantEqual = false;
if (DomSetInfo.dominates(CIBB, C->getParent())) {
// FIXME: we currently only handle the case where both calls are in the
// same basic block.
if (CIBB != C->getParent()) {
CantEqual = true;
} else {
Instruction *First = CI, *Second = C;
if (!DomSetInfo.dominates(CI, C))
std::swap(First, Second);
// Scan the instructions between the calls, checking for stores or
// calls to dangerous functions.
BasicBlock::iterator I = First;
for (++First; I != BasicBlock::iterator(Second); ++I) {
if (isa<StoreInst>(I)) {
// FIXME: We could use mod/ref information to make this much
// better!
CantEqual = true;
break;
} else if (CallInst *CI = dyn_cast<CallInst>(I)) {
if (CI->getCalledFunction() == 0 ||
!AA.onlyReadsMemory(CI->getCalledFunction())) {
CantEqual = true;
break;
}
} else if (I->mayWriteToMemory()) {
CantEqual = true;
break;
}
}
}
} else if (DomSetInfo.dominates(C->getParent(), CIBB)) {
// FIXME: We could implement this, but we don't for now.
CantEqual = true;
} else {
// FIXME: if one doesn't dominate the other, we can't tell yet.
CantEqual = true;
}
if (CantEqual) {
// This call does not produce the same value as the one in the query.
std::swap(IdenticalCalls[i--], IdenticalCalls.back());
IdenticalCalls.pop_back();
}
}
}
// Any calls that are identical and not destroyed will produce equal values!
for (unsigned i = 0, e = IdenticalCalls.size(); i != e; ++i)
RetVals.push_back(IdenticalCalls[i]);
}
// getEqualNumberNodes - Return nodes with the same value number as the
// specified Value. This fills in the argument vector with any equal values.
//
void LoadVN::getEqualNumberNodes(Value *V,
std::vector<Value*> &RetVals) const {
// If the alias analysis has any must alias information to share with us, we
// can definitely use it.
if (isa<PointerType>(V->getType()))
getAnalysis<AliasAnalysis>().getMustAliases(V, RetVals);
if (!isa<LoadInst>(V)) {
if (CallInst *CI = dyn_cast<CallInst>(V))
getCallEqualNumberNodes(CI, RetVals);
// Not a load instruction? Just chain to the base value numbering
// implementation to satisfy the request...
assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
"getAnalysis() returned this!");
return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
}
// Volatile loads cannot be replaced with the value of other loads.
LoadInst *LI = cast<LoadInst>(V);
if (LI->isVolatile())
return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
// If we have a load instruction, find all of the load and store instructions
// that use the same source operand. We implement this recursively, because
// there could be a load of a load of a load that are all identical. We are
// guaranteed that this cannot be an infinite recursion because load
// instructions would have to pass through a PHI node in order for there to be
// a cycle. The PHI node would be handled by the else case here, breaking the
// infinite recursion.
//
std::vector<Value*> PointerSources;
getEqualNumberNodes(LI->getOperand(0), PointerSources);
PointerSources.push_back(LI->getOperand(0));
BasicBlock *LoadBB = LI->getParent();
Function *F = LoadBB->getParent();
// Now that we know the set of equivalent source pointers for the load
// instruction, look to see if there are any load or store candidates that are
// identical.
//
std::map<BasicBlock*, std::vector<LoadInst*> > CandidateLoads;
std::map<BasicBlock*, std::vector<StoreInst*> > CandidateStores;
std::set<AllocationInst*> Allocations;
while (!PointerSources.empty()) {
Value *Source = PointerSources.back();
PointerSources.pop_back(); // Get a source pointer...
if (AllocationInst *AI = dyn_cast<AllocationInst>(Source))
Allocations.insert(AI);
for (Value::use_iterator UI = Source->use_begin(), UE = Source->use_end();
UI != UE; ++UI)
if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) {// Is a load of source?
if (Cand->getParent()->getParent() == F && // In the same function?
Cand != LI && !Cand->isVolatile()) // Not LI itself?
CandidateLoads[Cand->getParent()].push_back(Cand); // Got one...
} else if (StoreInst *Cand = dyn_cast<StoreInst>(*UI)) {
if (Cand->getParent()->getParent() == F && !Cand->isVolatile() &&
Cand->getOperand(1) == Source) // It's a store THROUGH the ptr...
CandidateStores[Cand->getParent()].push_back(Cand);
}
}
// Get alias analysis & dominators.
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
Value *LoadPtr = LI->getOperand(0);
// Find out how many bytes of memory are loaded by the load instruction...
unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LI->getType());
// Find all of the candidate loads and stores that are in the same block as
// the defining instruction.
std::set<Instruction*> Instrs;
Instrs.insert(CandidateLoads[LoadBB].begin(), CandidateLoads[LoadBB].end());
CandidateLoads.erase(LoadBB);
Instrs.insert(CandidateStores[LoadBB].begin(), CandidateStores[LoadBB].end());
CandidateStores.erase(LoadBB);
// Figure out if the load is invalidated from the entry of the block it is in
// until the actual instruction. This scans the block backwards from LI. If
// we see any candidate load or store instructions, then we know that the
// candidates have the same value # as LI.
bool LoadInvalidatedInBBBefore = false;
for (BasicBlock::iterator I = LI; I != LoadBB->begin(); ) {
--I;
// If this instruction is a candidate load before LI, we know there are no
// invalidating instructions between it and LI, so they have the same value
// number.
if (isa<LoadInst>(I) && Instrs.count(I)) {
RetVals.push_back(I);
Instrs.erase(I);
} else if (AllocationInst *AI = dyn_cast<AllocationInst>(I)) {
// If we run into an allocation of the value being loaded, then the
// contenxt are not initialized. We can return any value, so we will
// return a zero.
if (Allocations.count(AI)) {
LoadInvalidatedInBBBefore = true;
RetVals.push_back(Constant::getNullValue(LI->getType()));
break;
}
}
if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
// If the invalidating instruction is a store, and its in our candidate
// set, then we can do store-load forwarding: the load has the same value
// # as the stored value.
if (isa<StoreInst>(I) && Instrs.count(I)) {
Instrs.erase(I);
RetVals.push_back(I->getOperand(0));
}
LoadInvalidatedInBBBefore = true;
break;
}
}
// Figure out if the load is invalidated between the load and the exit of the
// block it is defined in. While we are scanning the current basic block, if
// we see any candidate loads, then we know they have the same value # as LI.
//
bool LoadInvalidatedInBBAfter = false;
for (BasicBlock::iterator I = LI->getNext(); I != LoadBB->end(); ++I) {
// If this instruction is a load, then this instruction returns the same
// value as LI.
if (isa<LoadInst>(I) && Instrs.count(I)) {
RetVals.push_back(I);
Instrs.erase(I);
}
if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
LoadInvalidatedInBBAfter = true;
break;
}
}
// If there is anything left in the Instrs set, it could not possibly equal
// LI.
Instrs.clear();
// TransparentBlocks - For each basic block the load/store is alive across,
// figure out if the pointer is invalidated or not. If it is invalidated, the
// boolean is set to false, if it's not it is set to true. If we don't know
// yet, the entry is not in the map.
std::map<BasicBlock*, bool> TransparentBlocks;
// Loop over all of the basic blocks that also load the value. If the value
// is live across the CFG from the source to destination blocks, and if the
// value is not invalidated in either the source or destination blocks, add it
// to the equivalence sets.
for (std::map<BasicBlock*, std::vector<LoadInst*> >::iterator
I = CandidateLoads.begin(), E = CandidateLoads.end(); I != E; ++I) {
bool CantEqual = false;
// Right now we only can handle cases where one load dominates the other.
// FIXME: generalize this!
BasicBlock *BB1 = I->first, *BB2 = LoadBB;
if (DomSetInfo.dominates(BB1, BB2)) {
// The other load dominates LI. If the loaded value is killed entering
// the LoadBB block, we know the load is not live.
if (LoadInvalidatedInBBBefore)
CantEqual = true;
} else if (DomSetInfo.dominates(BB2, BB1)) {
std::swap(BB1, BB2); // Canonicalize
// LI dominates the other load. If the loaded value is killed exiting
// the LoadBB block, we know the load is not live.
if (LoadInvalidatedInBBAfter)
CantEqual = true;
} else {
// None of these loads can VN the same.
CantEqual = true;
}
if (!CantEqual) {
// Ok, at this point, we know that BB1 dominates BB2, and that there is
// nothing in the LI block that kills the loaded value. Check to see if
// the value is live across the CFG.
std::set<BasicBlock*> Visited;
for (pred_iterator PI = pred_begin(BB2), E = pred_end(BB2); PI!=E; ++PI)
if (!isPathTransparentTo(*PI, BB1, LoadPtr, LoadSize, AA,
Visited, TransparentBlocks)) {
// None of these loads can VN the same.
CantEqual = true;
break;
}
}
// If the loads can equal so far, scan the basic block that contains the
// loads under consideration to see if they are invalidated in the block.
// For any loads that are not invalidated, add them to the equivalence
// set!
if (!CantEqual) {
Instrs.insert(I->second.begin(), I->second.end());
if (BB1 == LoadBB) {
// If LI dominates the block in question, check to see if any of the
// loads in this block are invalidated before they are reached.
for (BasicBlock::iterator BBI = I->first->begin(); ; ++BBI) {
if (isa<LoadInst>(BBI) && Instrs.count(BBI)) {
// The load is in the set!
RetVals.push_back(BBI);
Instrs.erase(BBI);
if (Instrs.empty()) break;
} else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
& AliasAnalysis::Mod) {
// If there is a modifying instruction, nothing below it will value
// # the same.
break;
}
}
} else {
// If the block dominates LI, make sure that the loads in the block are
// not invalidated before the block ends.
BasicBlock::iterator BBI = I->first->end();
while (1) {
--BBI;
if (isa<LoadInst>(BBI) && Instrs.count(BBI)) {
// The load is in the set!
RetVals.push_back(BBI);
Instrs.erase(BBI);
if (Instrs.empty()) break;
} else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
& AliasAnalysis::Mod) {
// If there is a modifying instruction, nothing above it will value
// # the same.
break;
}
}
}
Instrs.clear();
}
}
// Handle candidate stores. If the loaded location is clobbered on entrance
// to the LoadBB, no store outside of the LoadBB can value number equal, so
// quick exit.
if (LoadInvalidatedInBBBefore)
return;
for (std::map<BasicBlock*, std::vector<StoreInst*> >::iterator
I = CandidateStores.begin(), E = CandidateStores.end(); I != E; ++I)
if (DomSetInfo.dominates(I->first, LoadBB)) {
// Check to see if the path from the store to the load is transparent
// w.r.t. the memory location.
bool CantEqual = false;
std::set<BasicBlock*> Visited;
for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB);
PI != E; ++PI)
if (!isPathTransparentTo(*PI, I->first, LoadPtr, LoadSize, AA,
Visited, TransparentBlocks)) {
// None of these stores can VN the same.
CantEqual = true;
break;
}
Visited.clear();
if (!CantEqual) {
// Okay, the path from the store block to the load block is clear, and
// we know that there are no invalidating instructions from the start
// of the load block to the load itself. Now we just scan the store
// block.
BasicBlock::iterator BBI = I->first->end();
while (1) {
--BBI;
if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)& AliasAnalysis::Mod){
// If the invalidating instruction is one of the candidates,
// then it provides the value the load loads.
if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
if (std::find(I->second.begin(), I->second.end(), SI) !=
I->second.end())
RetVals.push_back(SI->getOperand(0));
break;
}
}
}
}
}
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