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[SCEV] Move towards a verifier without false positives

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This change reboots SCEV's current (off by default) verification logic
to avoid false failures.  Instead of stringifying trip counts, it maps
old and new trip counts to the same ScalarEvolution "universe" and
asks ScalarEvolution to compute the difference between them.  If the
difference comes out to be a non-zero constant, then (barring some
corner cases) we *know* we messed up.

I've not yet enabled this by default since it hits an exponential time
issue in SCEV, but once I fix that, I'll flip it on by default in
EXPENSIVE_CHECKS builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301146 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das 2017-04-23 23:04:45 +00:00
parent 84468c22d8
commit 0f49a66c42
1 changed files with 60 additions and 69 deletions
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129
lib/Analysis/ScalarEvolution.cpp
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@ -10237,84 +10237,75 @@ void ScalarEvolution::forgetMemoizedResults(const SCEV *S) {
RemoveSCEVFromBackedgeMap(PredicatedBackedgeTakenCounts);
}
typedef DenseMap<const Loop *, std::string> VerifyMap;
/// replaceSubString - Replaces all occurrences of From in Str with To.
static void replaceSubString(std::string &Str, StringRef From, StringRef To) {
size_t Pos = 0;
while ((Pos = Str.find(From, Pos)) != std::string::npos) {
Str.replace(Pos, From.size(), To.data(), To.size());
Pos += To.size();
}
}
/// getLoopBackedgeTakenCounts - Helper method for verifyAnalysis.
static void
getLoopBackedgeTakenCounts(Loop *L, VerifyMap &Map, ScalarEvolution &SE) {
std::string &S = Map[L];
if (S.empty()) {
raw_string_ostream OS(S);
SE.getBackedgeTakenCount(L)->print(OS);
// false and 0 are semantically equivalent. This can happen in dead loops.
replaceSubString(OS.str(), "false", "0");
// Remove wrap flags, their use in SCEV is highly fragile.
// FIXME: Remove this when SCEV gets smarter about them.
replaceSubString(OS.str(), "<nw>", "");
replaceSubString(OS.str(), "<nsw>", "");
replaceSubString(OS.str(), "<nuw>", "");
}
for (auto *R : reverse(*L))
getLoopBackedgeTakenCounts(R, Map, SE); // recurse.
}
void ScalarEvolution::verify() const {
ScalarEvolution &SE = *const_cast<ScalarEvolution *>(this);
// Gather stringified backedge taken counts for all loops using SCEV's caches.
// FIXME: It would be much better to store actual values instead of strings,
// but SCEV pointers will change if we drop the caches.
VerifyMap BackedgeDumpsOld, BackedgeDumpsNew;
for (LoopInfo::reverse_iterator I = LI.rbegin(), E = LI.rend(); I != E; ++I)
getLoopBackedgeTakenCounts(*I, BackedgeDumpsOld, SE);
// Gather stringified backedge taken counts for all loops using a fresh
// ScalarEvolution object.
ScalarEvolution SE2(F, TLI, AC, DT, LI);
for (LoopInfo::reverse_iterator I = LI.rbegin(), E = LI.rend(); I != E; ++I)
getLoopBackedgeTakenCounts(*I, BackedgeDumpsNew, SE2);
// Now compare whether they're the same with and without caches. This allows
// verifying that no pass changed the cache.
assert(BackedgeDumpsOld.size() == BackedgeDumpsNew.size() &&
"New loops suddenly appeared!");
SmallVector<Loop *, 8> LoopStack(LI.begin(), LI.end());
for (VerifyMap::iterator OldI = BackedgeDumpsOld.begin(),
OldE = BackedgeDumpsOld.end(),
NewI = BackedgeDumpsNew.begin();
OldI != OldE; ++OldI, ++NewI) {
assert(OldI->first == NewI->first && "Loop order changed!");
// Map's SCEV expressions from one ScalarEvolution "universe" to another.
struct SCEVMapper : public SCEVRewriteVisitor<SCEVMapper> {
const SCEV *visitConstant(const SCEVConstant *Constant) {
return SE.getConstant(Constant->getAPInt());
}
const SCEV *visitUnknown(const SCEVUnknown *Expr) {
return SE.getUnknown(Expr->getValue());
}
// Compare the stringified SCEVs. We don't care if undef backedgetaken count
// changes.
// FIXME: We currently ignore SCEV changes from/to CouldNotCompute. This
// means that a pass is buggy or SCEV has to learn a new pattern but is
// usually not harmful.
if (OldI->second != NewI->second &&
OldI->second.find("undef") == std::string::npos &&
NewI->second.find("undef") == std::string::npos &&
OldI->second != "***COULDNOTCOMPUTE***" &&
NewI->second != "***COULDNOTCOMPUTE***") {
dbgs() << "SCEVValidator: SCEV for loop '"
<< OldI->first->getHeader()->getName()
<< "' changed from '" << OldI->second
<< "' to '" << NewI->second << "'!\n";
const SCEV *visitCouldNotCompute(const SCEVCouldNotCompute *Expr) {
return SE.getCouldNotCompute();
}
SCEVMapper(ScalarEvolution &SE) : SCEVRewriteVisitor<SCEVMapper>(SE) {}
};
SCEVMapper SCM(SE2);
while (!LoopStack.empty()) {
auto *L = LoopStack.pop_back_val();
LoopStack.insert(LoopStack.end(), L->begin(), L->end());
auto *CurBECount = SCM.visit(
const_cast<ScalarEvolution *>(this)->getBackedgeTakenCount(L));
auto *NewBECount = SE2.getBackedgeTakenCount(L);
if (CurBECount == SE2.getCouldNotCompute() ||
NewBECount == SE2.getCouldNotCompute()) {
// NB! This situation is legal, but is very suspicious -- whatever pass
// change the loop to make a trip count go from could not compute to
// computable or vice-versa *should have* invalidated SCEV. However, we
// choose not to assert here (for now) since we don't want false
// positives.
continue;
}
if (containsUndefs(CurBECount) || containsUndefs(NewBECount)) {
// SCEV treats "undef" as an unknown but consistent value (i.e. it does
// not propagate undef aggressively). This means we can (and do) fail
// verification in cases where a transform makes the trip count of a loop
// go from "undef" to "undef+1" (say). The transform is fine, since in
// both cases the loop iterates "undef" times, but SCEV thinks we
// increased the trip count of the loop by 1 incorrectly.
continue;
}
if (SE.getTypeSizeInBits(CurBECount->getType()) >
SE.getTypeSizeInBits(NewBECount->getType()))
NewBECount = SE2.getZeroExtendExpr(NewBECount, CurBECount->getType());
else if (SE.getTypeSizeInBits(CurBECount->getType()) <
SE.getTypeSizeInBits(NewBECount->getType()))
CurBECount = SE2.getZeroExtendExpr(CurBECount, NewBECount->getType());
auto *ConstantDelta =
dyn_cast<SCEVConstant>(SE2.getMinusSCEV(CurBECount, NewBECount));
if (ConstantDelta && ConstantDelta->getAPInt() != 0) {
dbgs() << "Trip Count Changed!\n";
dbgs() << "Old: " << *CurBECount << "\n";
dbgs() << "New: " << *NewBECount << "\n";
dbgs() << "Delta: " << *ConstantDelta << "\n";
std::abort();
}
}
// TODO: Verify more things.
}
bool ScalarEvolution::invalidate(