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[Unroll] Switch from an eagerly populated SCEV cache to one that is

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lazily built.

Also, make it a much more generic SCEV cache, which today exposes only
a reduced GEP model description but could be extended in the future to
do other profitable caching of SCEV information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238124 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2015-05-25 01:00:46 +00:00
parent 8ef4be78cd
commit 3dd00ff834
1 changed files with 113 additions and 86 deletions
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199
lib/Transforms/Scalar/LoopUnrollPass.cpp
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@ -320,81 +320,110 @@ struct FindConstantPointers {
} // End anonymous namespace.
namespace {
/// \brief Struct to represent a GEP whose start and step are known fixed
/// offsets from a base address due to SCEV's analysis.
struct SCEVGEPDescriptor {
Value *BaseAddr;
unsigned Start;
unsigned Step;
/// \brief A cache of SCEV results used to optimize repeated queries to SCEV on
/// the same set of instructions.
///
/// The primary cost this saves is the cost of checking the validity of a SCEV
/// every time it is looked up. However, in some cases we can provide a reduced
/// and especially useful model for an instruction based upon SCEV that is
/// non-trivial to compute but more useful to clients.
class SCEVCache {
public:
/// \brief Struct to represent a GEP whose start and step are known fixed
/// offsets from a base address due to SCEV's analysis.
struct GEPDescriptor {
Value *BaseAddr = nullptr;
unsigned Start = 0;
unsigned Step = 0;
};
Optional<GEPDescriptor> getGEPDescriptor(GetElementPtrInst *GEP);
SCEVCache(const Loop &L, ScalarEvolution &SE) : L(L), SE(SE) {}
private:
const Loop &L;
ScalarEvolution &SE;
SmallDenseMap<GetElementPtrInst *, GEPDescriptor> GEPDescriptors;
};
} // End anonymous namespace.
/// \brief Build a cache of all the GEP instructions which SCEV can describe.
/// \brief Get a simplified descriptor for a GEP instruction.
///
/// Visit all GEPs in the loop and find those which after complete loop
/// unrolling would become a constant, or BaseAddress+Constant. For those where
/// we can identify small constant starts and steps from a base address, return
/// a map from the GEP to the base, start, and step relevant for that GEP. This
/// is essentially a simplified and fast to query form of the SCEV analysis
/// which we can afford to look into repeatedly for different iterations of the
/// loop.
static SmallDenseMap<Value *, SCEVGEPDescriptor>
buildSCEVGEPCache(const Loop &L, ScalarEvolution &SE) {
SmallDenseMap<Value *, SCEVGEPDescriptor> Cache;
/// Where possible, this produces a simplified descriptor for a GEP instruction
/// using SCEV analysis of the containing loop. If this isn't possible, it
/// returns an empty optional.
///
/// The model is a base address, an initial offset, and a per-iteration step.
/// This fits very common patterns of GEPs inside loops and is something we can
/// use to simulate the behavior of a particular iteration of a loop.
///
/// This is a cached interface. The first call may do non-trivial work to
/// compute the result, but all subsequent calls will return a fast answer
/// based on a cached result. This includes caching negative results.
Optional<SCEVCache::GEPDescriptor>
SCEVCache::getGEPDescriptor(GetElementPtrInst *GEP) {
decltype(GEPDescriptors)::iterator It;
bool Inserted;
for (auto BB : L.getBlocks()) {
for (Instruction &I : *BB) {
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(&I)) {
Value *V = cast<Value>(GEP);
if (!SE.isSCEVable(V->getType()))
continue;
const SCEV *S = SE.getSCEV(V);
std::tie(It, Inserted) = GEPDescriptors.insert({GEP, {}});
// FIXME: It'd be nice if the worklist and set used by the
// SCEVTraversal could be re-used between loop iterations, but the
// interface doesn't support that. There is no way to clear the visited
// sets between uses.
FindConstantPointers Visitor(&L, SE);
SCEVTraversal<FindConstantPointers> T(Visitor);
if (!Inserted) {
if (!It->second.BaseAddr)
return None;
// Try to find (BaseAddress+Step+Offset) tuple.
// If succeeded, save it to the cache - it might help in folding
// loads.
T.visitAll(S);
if (!Visitor.IndexIsConstant || !Visitor.BaseAddress)
continue;
const SCEV *BaseAddrSE = SE.getSCEV(Visitor.BaseAddress);
if (BaseAddrSE->getType() != S->getType())
continue;
const SCEV *OffSE = SE.getMinusSCEV(S, BaseAddrSE);
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(OffSE);
if (!AR)
continue;
const SCEVConstant *StepSE =
dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE));
const SCEVConstant *StartSE = dyn_cast<SCEVConstant>(AR->getStart());
if (!StepSE || !StartSE)
continue;
// Check and skip caching if doing so would require lots of bits to
// avoid overflow.
APInt Start = StartSE->getValue()->getValue();
APInt Step = StepSE->getValue()->getValue();
if (Start.getActiveBits() > 32 || Step.getActiveBits() > 32)
continue;
// We found a cacheable SCEV model for the GEP.
Cache[V] = {Visitor.BaseAddress,
(unsigned)Start.getLimitedValue(),
(unsigned)Step.getLimitedValue()};
}
}
return It->second;
}
return Cache;
// We've inserted a new record into the cache, so compute the GEP descriptor
// if possible.
Value *V = cast<Value>(GEP);
if (!SE.isSCEVable(V->getType()))
return None;
const SCEV *S = SE.getSCEV(V);
// FIXME: It'd be nice if the worklist and set used by the
// SCEVTraversal could be re-used between loop iterations, but the
// interface doesn't support that. There is no way to clear the visited
// sets between uses.
FindConstantPointers Visitor(&L, SE);
SCEVTraversal<FindConstantPointers> T(Visitor);
// Try to find (BaseAddress+Step+Offset) tuple.
// If succeeded, save it to the cache - it might help in folding
// loads.
T.visitAll(S);
if (!Visitor.IndexIsConstant || !Visitor.BaseAddress)
return None;
const SCEV *BaseAddrSE = SE.getSCEV(Visitor.BaseAddress);
if (BaseAddrSE->getType() != S->getType())
return None;
const SCEV *OffSE = SE.getMinusSCEV(S, BaseAddrSE);
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(OffSE);
if (!AR)
return None;
const SCEVConstant *StepSE =
dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE));
const SCEVConstant *StartSE = dyn_cast<SCEVConstant>(AR->getStart());
if (!StepSE || !StartSE)
return None;
// Check and skip caching if doing so would require lots of bits to
// avoid overflow.
APInt Start = StartSE->getValue()->getValue();
APInt Step = StepSE->getValue()->getValue();
if (Start.getActiveBits() > 32 || Step.getActiveBits() > 32)
return None;
// We found a cacheable SCEV model for the GEP.
It->second.BaseAddr = Visitor.BaseAddress;
It->second.Start = Start.getLimitedValue();
It->second.Step = Step.getLimitedValue();
return It->second;
}
namespace {
@ -421,9 +450,8 @@ class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
public:
UnrolledInstAnalyzer(unsigned Iteration,
DenseMap<Value *, Constant *> &SimplifiedValues,
SmallDenseMap<Value *, SCEVGEPDescriptor> &SCEVGEPCache)
: Iteration(Iteration), SimplifiedValues(SimplifiedValues),
SCEVGEPCache(SCEVGEPCache) {}
SCEVCache &SC)
: Iteration(Iteration), SimplifiedValues(SimplifiedValues), SC(SC) {}
// Allow access to the initial visit method.
using Base::visit;
@ -443,10 +471,8 @@ private:
// post-unrolling.
DenseMap<Value *, Constant *> &SimplifiedValues;
// To avoid requesting SCEV info on every iteration, request it once, and
// for each value that would become ConstAddress+Constant after loop
// unrolling, save the corresponding data.
SmallDenseMap<Value *, SCEVGEPDescriptor> &SCEVGEPCache;
// We use a cache to wrap all our SCEV queries.
SCEVCache &SC;
/// Base case for the instruction visitor.
bool visitInstruction(Instruction &I) { return false; };
@ -487,12 +513,14 @@ private:
if (Constant *SimplifiedAddrOp = SimplifiedValues.lookup(AddrOp))
AddrOp = SimplifiedAddrOp;
auto It = SCEVGEPCache.find(AddrOp);
if (It == SCEVGEPCache.end())
auto *GEP = dyn_cast<GetElementPtrInst>(AddrOp);
if (!GEP)
return false;
auto OptionalGEPDesc = SC.getGEPDescriptor(GEP);
if (!OptionalGEPDesc)
return false;
SCEVGEPDescriptor GEPDesc = It->second;
auto GV = dyn_cast<GlobalVariable>(GEPDesc.BaseAddr);
auto GV = dyn_cast<GlobalVariable>(OptionalGEPDesc->BaseAddr);
// We're only interested in loads that can be completely folded to a
// constant.
if (!GV || !GV->hasInitializer())
@ -507,9 +535,9 @@ private:
// low and both the start and step are 32-bit integers. We use signed
// integers so that UBSan will catch if a bug sneaks into the code.
int ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
int64_t Index = ((int64_t)GEPDesc.Start +
(int64_t)GEPDesc.Step * (int64_t)Iteration) /
ElemSize;
int64_t Index = ((int64_t)OptionalGEPDesc->Start +
(int64_t)OptionalGEPDesc->Step * (int64_t)Iteration) /
ElemSize;
if (Index >= CDS->getNumElements()) {
// FIXME: For now we conservatively ignore out of bound accesses, but
// we're allowed to perform the optimization in this case.
@ -562,14 +590,13 @@ analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
TripCount > UnrollMaxIterationsCountToAnalyze)
return None;
// To avoid compute SCEV-expressions on every iteration, compute them once
// and store interesting to us in SCEVGEPCache.
SmallDenseMap<Value *, SCEVGEPDescriptor> SCEVGEPCache =
buildSCEVGEPCache(*L, SE);
SmallSetVector<BasicBlock *, 16> BBWorklist;
DenseMap<Value *, Constant *> SimplifiedValues;
// Use a cache to access SCEV expressions so that we don't pay the cost on
// each iteration. This cache is lazily self-populating.
SCEVCache SC(*L, SE);
unsigned NumberOfOptimizedInstructions = 0;
unsigned UnrolledLoopSize = 0;
@ -579,7 +606,7 @@ analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
// we literally have to go through all loop's iterations.
for (unsigned Iteration = 0; Iteration < TripCount; ++Iteration) {
SimplifiedValues.clear();
UnrolledInstAnalyzer Analyzer(Iteration, SimplifiedValues, SCEVGEPCache);
UnrolledInstAnalyzer Analyzer(Iteration, SimplifiedValues, SC);
BBWorklist.clear();
BBWorklist.insert(L->getHeader());