[Clang] Give empty template parameter mapping an empty MLTAL

[zyn0217]

There are cases where atomic constraints are independent of template parameters, yet we still have a template parameter mapping.

We don't bother translating template arguments for them. Note that we retain an empty parameter mapping rather than none at all, as the former may improve cache hit rates (We don't profile MLTAL but profile the empty template argument list instead.)

This is a regression on trunk, so there's no release note. Also this corrects a spelling issue to make ourself consistent.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

There are cases where atomic constraints are independent of template parameters, yet we still have a template parameter mapping.

We don't bother translating template arguments for them. Note that we retain an empty parameter mapping rather than none at all, as the former may improve cache hit rates (We don't profile MLTAL but profile the empty template argument list instead.)

This is a regression on trunk, so there's no release note. Also this corrects a spelling issue to make ourself consistent.

---

Full diff: https://github.com/llvm/llvm-project/pull/164051.diff

1 Files Affected:

• (modified) clang/lib/Sema/SemaConcept.cpp (+33-19)

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 04a73181831d8..6781070cd0457 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -432,7 +432,7 @@ class ConstraintSatisfactionChecker {
   // XXX: It is SLOW! Use it very carefully.
   std::optional<MultiLevelTemplateArgumentList> SubstitutionInTemplateArguments(
       const NormalizedConstraintWithParamMapping &Constraint,
-      MultiLevelTemplateArgumentList MLTAL,
+      const MultiLevelTemplateArgumentList &MLTAL,
       llvm::SmallVector<TemplateArgument> &SubstitutedOuterMost);
 
   ExprResult EvaluateSlow(const AtomicConstraint &Constraint,
@@ -564,12 +564,17 @@ ExprResult ConstraintSatisfactionChecker::EvaluateAtomicConstraint(
 std::optional<MultiLevelTemplateArgumentList>
 ConstraintSatisfactionChecker::SubstitutionInTemplateArguments(
     const NormalizedConstraintWithParamMapping &Constraint,
-    MultiLevelTemplateArgumentList MLTAL,
-    llvm::SmallVector<TemplateArgument> &SubstitutedOuterMost) {
+    const MultiLevelTemplateArgumentList &MLTAL,
+    llvm::SmallVector<TemplateArgument> &SubstitutedOutermost) {
 
   if (!Constraint.hasParameterMapping())
     return std::move(MLTAL);
 
+  // The mapping is empty, meaning no template arguments are needed for
+  // evaluation.
+  if (Constraint.getParameterMapping().empty())
+    return MultiLevelTemplateArgumentList();
+
   TemplateDeductionInfo Info(Constraint.getBeginLoc());
   Sema::InstantiatingTemplate Inst(
       S, Constraint.getBeginLoc(),
@@ -607,7 +612,7 @@ ConstraintSatisfactionChecker::SubstitutionInTemplateArguments(
   // The empty MLTAL situation should only occur when evaluating non-dependent
   // constraints.
   if (MLTAL.getNumSubstitutedLevels())
-    SubstitutedOuterMost =
+    SubstitutedOutermost =
         llvm::to_vector_of<TemplateArgument>(MLTAL.getOutermost());
   unsigned Offset = 0;
   for (unsigned I = 0, MappedIndex = 0; I < Used.size(); I++) {
@@ -615,19 +620,19 @@ ConstraintSatisfactionChecker::SubstitutionInTemplateArguments(
     if (Used[I])
       Arg = S.Context.getCanonicalTemplateArgument(
           CTAI.SugaredConverted[MappedIndex++]);
-    if (I < SubstitutedOuterMost.size()) {
-      SubstitutedOuterMost[I] = Arg;
+    if (I < SubstitutedOutermost.size()) {
+      SubstitutedOutermost[I] = Arg;
       Offset = I + 1;
     } else {
-      SubstitutedOuterMost.push_back(Arg);
-      Offset = SubstitutedOuterMost.size();
+      SubstitutedOutermost.push_back(Arg);
+      Offset = SubstitutedOutermost.size();
     }
   }
-  if (Offset < SubstitutedOuterMost.size())
-    SubstitutedOuterMost.erase(SubstitutedOuterMost.begin() + Offset);
+  if (Offset < SubstitutedOutermost.size())
+    SubstitutedOutermost.erase(SubstitutedOutermost.begin() + Offset);
 
   MultiLevelTemplateArgumentList SubstitutedTemplateArgs;
-  SubstitutedTemplateArgs.addOuterTemplateArguments(TD, SubstitutedOuterMost,
+  SubstitutedTemplateArgs.addOuterTemplateArguments(TD, SubstitutedOutermost,
                                                     /*Final=*/false);
   return std::move(SubstitutedTemplateArgs);
 }
@@ -636,9 +641,9 @@ ExprResult ConstraintSatisfactionChecker::EvaluateSlow(
     const AtomicConstraint &Constraint,
     const MultiLevelTemplateArgumentList &MLTAL) {
 
-  llvm::SmallVector<TemplateArgument> SubstitutedOuterMost;
+  llvm::SmallVector<TemplateArgument> SubstitutedOutermost;
   std::optional<MultiLevelTemplateArgumentList> SubstitutedArgs =
-      SubstitutionInTemplateArguments(Constraint, MLTAL, SubstitutedOuterMost);
+      SubstitutionInTemplateArguments(Constraint, MLTAL, SubstitutedOutermost);
   if (!SubstitutedArgs) {
     Satisfaction.IsSatisfied = false;
     return ExprEmpty();
@@ -786,13 +791,13 @@ ExprResult ConstraintSatisfactionChecker::EvaluateSlow(
                      FoldExpandedConstraint::FoldOperatorKind::And;
   unsigned EffectiveDetailEndIndex = Satisfaction.Details.size();
 
-  llvm::SmallVector<TemplateArgument> SubstitutedOuterMost;
+  llvm::SmallVector<TemplateArgument> SubstitutedOutermost;
   // FIXME: Is PackSubstitutionIndex correct?
   llvm::SaveAndRestore _(PackSubstitutionIndex, S.ArgPackSubstIndex);
   std::optional<MultiLevelTemplateArgumentList> SubstitutedArgs =
       SubstitutionInTemplateArguments(
           static_cast<const NormalizedConstraintWithParamMapping &>(Constraint),
-          MLTAL, SubstitutedOuterMost);
+          MLTAL, SubstitutedOutermost);
   if (!SubstitutedArgs) {
     Satisfaction.IsSatisfied = false;
     return ExprError();
@@ -880,9 +885,9 @@ ExprResult ConstraintSatisfactionChecker::EvaluateSlow(
     const MultiLevelTemplateArgumentList &MLTAL, unsigned Size) {
   const ConceptReference *ConceptId = Constraint.getConceptId();
 
-  llvm::SmallVector<TemplateArgument> SubstitutedOuterMost;
+  llvm::SmallVector<TemplateArgument> SubstitutedOutermost;
   std::optional<MultiLevelTemplateArgumentList> SubstitutedArgs =
-      SubstitutionInTemplateArguments(Constraint, MLTAL, SubstitutedOuterMost);
+      SubstitutionInTemplateArguments(Constraint, MLTAL, SubstitutedOutermost);
 
   if (!SubstitutedArgs) {
     Satisfaction.IsSatisfied = false;
@@ -2017,8 +2022,13 @@ void SubstituteParameterMappings::buildParameterMapping(
       SemaRef.MarkUsedTemplateParameters(Args->arguments(),
                                          /*Depth=*/0, OccurringIndices);
   }
+  unsigned Size = OccurringIndices.count();
+  // It's OK when Size is 0. We build an empty parameter mapping when the atomic
+  // constraint is independent of any template parameters, so we can distinguish
+  // it from cases where no mapping exists at all, e.g. when there are only
+  // atomic constraints.
   TemplateArgumentLoc *TempArgs =
-      new (SemaRef.Context) TemplateArgumentLoc[OccurringIndices.count()];
+      new (SemaRef.Context) TemplateArgumentLoc[Size];
   llvm::SmallVector<NamedDecl *> UsedParams;
   for (unsigned I = 0, J = 0, C = TemplateParams->size(); I != C; ++I) {
     SourceLocation Loc = ArgsAsWritten->NumTemplateArgs > I
@@ -2039,7 +2049,6 @@ void SubstituteParameterMappings::buildParameterMapping(
       TemplateParams->getLAngleLoc(), UsedParams,
       /*RAngleLoc=*/SourceLocation(),
       /*RequiresClause=*/nullptr);
-  unsigned Size = OccurringIndices.count();
   N.updateParameterMapping(
       std::move(OccurringIndices), std::move(OccurringIndicesForSubsumption),
       MutableArrayRef<TemplateArgumentLoc>{TempArgs, Size}, UsedList);
@@ -2050,6 +2059,11 @@ bool SubstituteParameterMappings::substitute(
   if (!N.hasParameterMapping())
     buildParameterMapping(N);
 
+  // Don't bother into substituting if we have determined the mapping maps to
+  // nothing.
+  if (N.getParameterMapping().empty())
+    return false;
+
   SourceLocation InstLocBegin, InstLocEnd;
   llvm::ArrayRef Arguments = ArgsAsWritten->arguments();
   if (Arguments.empty()) {