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[SCEV] Introduce field for storing SymbolicMaxNotTaken. NFCI 

ritht is initialized with either exact (if available) or
with constant max exit count. In the future, this can be improved.

Hypothetically this is not an NFC (it is possible that exact is not
known and max is known for a particular exit), but for how we use
it now it seems be an NFC (or at least I could not find an example
where it differs). constant max exit count. In the future, this can
be improved.

Differential Revision: https://reviews.llvm.org/D138699
Reviewed By: lebedev.ri
This commit is contained in:
Max Kazantsev 2022-11-25 13:10:24 +07:00
parent baca3b382b
commit 0b74cb4231
2 changed files with 19 additions and 8 deletions
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7
llvm/include/llvm/Analysis/ScalarEvolution.h
View File

@ -1314,6 +1314,7 @@ private:
const SCEV *ExactNotTaken; // The exit is not taken exactly this many times
const SCEV *ConstantMaxNotTaken; // The exit is not taken at most this many
// times
const SCEV *SymbolicMaxNotTaken;
// Not taken either exactly ConstantMaxNotTaken or zero times
bool MaxOrZero = false;
@ -1360,14 +1361,16 @@ private:
PoisoningVH<BasicBlock> ExitingBlock;
const SCEV *ExactNotTaken;
const SCEV *ConstantMaxNotTaken;
const SCEV *SymbolicMaxNotTaken;
SmallPtrSet<const SCEVPredicate *, 4> Predicates;
explicit ExitNotTakenInfo(
PoisoningVH<BasicBlock> ExitingBlock, const SCEV *ExactNotTaken,
const SCEV *ConstantMaxNotTaken,
const SCEV *ConstantMaxNotTaken, const SCEV *SymbolicMaxNotTaken,
const SmallPtrSet<const SCEVPredicate *, 4> &Predicates)
: ExitingBlock(ExitingBlock), ExactNotTaken(ExactNotTaken),
ConstantMaxNotTaken(ConstantMaxNotTaken), Predicates(Predicates) {}
ConstantMaxNotTaken(ConstantMaxNotTaken),
SymbolicMaxNotTaken(SymbolicMaxNotTaken), Predicates(Predicates) {}
bool hasAlwaysTruePredicate() const {
return Predicates.empty();

20
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -8560,8 +8560,11 @@ const SCEV *ScalarEvolution::BackedgeTakenInfo::getConstantMax(
const SCEV *ScalarEvolution::BackedgeTakenInfo::getSymbolicMax(
const BasicBlock *ExitingBlock, ScalarEvolution *SE) const {
// FIXME: Need to implement this. Return exact for now.
return getExact(ExitingBlock, SE);
for (const auto &ENT : ExitNotTaken)
if (ENT.ExitingBlock == ExitingBlock && ENT.hasAlwaysTruePredicate())
return ENT.SymbolicMaxNotTaken;
return SE->getCouldNotCompute();
}
/// getConstantMax - Get the constant max backedge taken count for the loop.
@ -8611,6 +8614,13 @@ ScalarEvolution::ExitLimit::ExitLimit(
if (ConstantMaxNotTaken->isZero())
ExactNotTaken = ConstantMaxNotTaken;
// FIXME: For now, SymbolicMaxNotTaken is either exact (if available) or
// constant max. In the future, we are planning to make it more powerful.
if (isa<SCEVCouldNotCompute>(ExactNotTaken))
SymbolicMaxNotTaken = ConstantMaxNotTaken;
else
SymbolicMaxNotTaken = ExactNotTaken;
assert((isa<SCEVCouldNotCompute>(ExactNotTaken) ||
!isa<SCEVCouldNotCompute>(ConstantMaxNotTaken)) &&
"Exact is not allowed to be less precise than Max");
@ -8647,7 +8657,8 @@ ScalarEvolution::BackedgeTakenInfo::BackedgeTakenInfo(
BasicBlock *ExitBB = EEI.first;
const ExitLimit &EL = EEI.second;
return ExitNotTakenInfo(ExitBB, EL.ExactNotTaken,
EL.ConstantMaxNotTaken, EL.Predicates);
EL.ConstantMaxNotTaken, EL.SymbolicMaxNotTaken,
EL.Predicates);
});
assert((isa<SCEVCouldNotCompute>(ConstantMax) ||
isa<SCEVConstant>(ConstantMax)) &&
@ -14752,9 +14763,6 @@ ScalarEvolution::computeSymbolicMaxBackedgeTakenCount(const Loop *L) {
for (BasicBlock *ExitingBB : ExitingBlocks) {
const SCEV *ExitCount =
getExitCount(L, ExitingBB, ScalarEvolution::SymbolicMaximum);
if (isa<SCEVCouldNotCompute>(ExitCount))
ExitCount = getExitCount(L, ExitingBB,
ScalarEvolution::ConstantMaximum);
if (!isa<SCEVCouldNotCompute>(ExitCount)) {
assert(DT.dominates(ExitingBB, L->getLoopLatch()) &&
"We should only have known counts for exiting blocks that "