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Jul 6, 2022
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RequirementMachine: Remove old minimal conformances algorithm #59900

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RequirementMachine: Remove old minimal conformances algorithm
slavapestov Jul 2, 2022
d3b7aab
RequirementMachine: Remove completion procedure hacks for old minimal…
slavapestov Jul 2, 2022
901cdaf
RequirementMachine: Relax assertions in verifyRewriteSystem()
slavapestov Jul 6, 2022
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118 changes: 6 additions & 112 deletions lib/AST/RequirementMachine/KnuthBendix.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -157,66 +157,8 @@ RewriteSystem::computeCriticalPair(ArrayRef<Symbol>::const_iterator from,
return false;
}

// If X == TUW for some W, then the critical pair is (TUW, TYV),
// and we have
// - lhs == (TUV => TUW)
// - rhs == (U => Y).
//
// We explicitly apply the rewrite step (Y => U) to the beginning of the
// rewrite path, transforming the critical pair to (TYW, TYV).
//
// In particular, if V == W.[P] for some protocol P, then we in fact have
// a property rule and a same-type rule:
//
// - lhs == (TUW.[P] => TUW)
// - rhs == (U => Y)
//
// Without this hack, the critical pair would be:
//
// (TUW => TYW.[P])
//
// With this hack, the critical pair becomes:
//
// (TYW.[P] => TYW)
//
// This ensures that the newly-added rule is itself a property rule;
// otherwise, this would only be the case if addRule() reduced TUW
// into TYW without immediately reducing some subterm of TUW first.
//
// While completion will eventually simplify all such rules down into
// property rules, their existence in the first place breaks subtle
// invariants in the minimal conformances algorithm, which expects
// homotopy generators describing redundant protocol conformance rules
// to have a certain structure.
if (t.size() + rhs.getLHS().size() <= x.size() &&
std::equal(rhs.getLHS().begin(),
rhs.getLHS().end(),
x.begin() + t.size())) {
// We have a path from TUW to TYV. Invert to get a path from TYV to
// TUW.
path.invert();

// Compute the term W.
MutableTerm w(x.begin() + t.size() + rhs.getLHS().size(), x.end());

// Now add a rewrite step T.(U => Y).W to get a path from TYV to
// TYW.
path.add(RewriteStep::forRewriteRule(/*startOffset=*/t.size(),
/*endOffset=*/w.size(),
getRuleID(rhs),
/*inverse=*/false));

// Compute the term TYW.
MutableTerm tyw(t);
tyw.append(rhs.getRHS());
tyw.append(w);

// Add the pair (TYV, TYW).
pairs.emplace_back(tyv, tyw, path);
} else {
// Add the pair (X, TYV).
pairs.emplace_back(x, tyv, path);
}
// Add the pair (X, TYV).
pairs.emplace_back(x, tyv, path);
} else {
// lhs == TU -> X, rhs == UV -> Y.

Expand Down Expand Up @@ -270,56 +212,8 @@ RewriteSystem::computeCriticalPair(ArrayRef<Symbol>::const_iterator from,
return false;
}

// If Y == UW for some W, then the critical pair is (XV, TUW),
// and we have
// - lhs == (TU -> X)
// - rhs == (UV -> UW).
//
// We explicitly apply the rewrite step (TU => X) to the rewrite path,
// transforming the critical pair to (XV, XW).
//
// In particular, if T == X, U == [P] for some protocol P, and
// V == W.[p] for some property symbol p, then we in fact have a pair
// of property rules:
//
// - lhs == (T.[P] => T)
// - rhs == ([P].W.[p] => [P].W)
//
// Without this hack, the critical pair would be:
//
// (T.W.[p] => T.[P].W)
//
// With this hack, the critical pair becomes:
//
// (T.W.[p] => T.W)
//
// This ensures that the newly-added rule is itself a property rule;
// otherwise, this would only be the case if addRule() reduced T.[P].W
// into T.W without immediately reducing some subterm of T first.
//
// While completion will eventually simplify all such rules down into
// property rules, their existence in the first place breaks subtle
// invariants in the minimal conformances algorithm, which expects
// homotopy generators describing redundant protocol conformance rules
// to have a certain structure.
if (lhs.getLHS().size() <= ty.size() &&
std::equal(lhs.getLHS().begin(),
lhs.getLHS().end(),
ty.begin())) {
unsigned endOffset = ty.size() - lhs.getLHS().size();
path.add(RewriteStep::forRewriteRule(/*startOffset=*/0,
endOffset,
getRuleID(lhs),
/*inverse=*/false));

// Compute the term XW.
MutableTerm xw(lhs.getRHS());
xw.append(ty.end() - endOffset, ty.end());

pairs.emplace_back(xv, xw, path);
} else {
pairs.emplace_back(xv, ty, path);
}
// Add the pair (XV, TY).
pairs.emplace_back(xv, ty, path);
}

return true;
Expand Down Expand Up @@ -347,7 +241,7 @@ RewriteSystem::computeConfluentCompletion(unsigned maxRuleCount,
assert(!Minimized);
assert(!Frozen);

// Complete might already be set, if we're re-running completion after
// 'Complete' might already be set, if we're re-running completion after
// adding new rules in the property map's concrete type unification procedure.
Complete = 1;

Expand All @@ -359,7 +253,7 @@ RewriteSystem::computeConfluentCompletion(unsigned maxRuleCount,
do {
ruleCount = Rules.size();

// For every rule, looking for other rules that overlap with this rule.
// For every rule, look for other rules that overlap with this rule.
for (unsigned i = FirstLocalRule, e = Rules.size(); i < e; ++i) {
const auto &lhs = getRule(i);
if (lhs.isLHSSimplified() ||
Expand Down
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