[clang] [Clang] Give empty template parameter mapping an empty MLTAL (PR #164051)
Younan Zhang via cfe-commits
cfe-commits at lists.llvm.org
Fri Oct 17 22:55:33 PDT 2025
https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/164051
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.
>From 1c34050a8c047fa3ebe42f0b9a468b527fe90d3a Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sat, 18 Oct 2025 13:13:23 +0800
Subject: [PATCH 1/2] [Clang] Give empty template parameter mapping an empty
MLTAL
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.)
---
clang/lib/Sema/SemaConcept.cpp | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 04a73181831d8..950b14c60696b 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -570,6 +570,11 @@ ConstraintSatisfactionChecker::SubstitutionInTemplateArguments(
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(),
@@ -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()) {
>From 9beea6e3a66dce9ed2086d5fbc1fb74b69b56a09 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sat, 18 Oct 2025 13:53:44 +0800
Subject: [PATCH 2/2] NFC: Fix spelling: OuterMost -> Outermost
---
clang/lib/Sema/SemaConcept.cpp | 34 +++++++++++++++++-----------------
1 file changed, 17 insertions(+), 17 deletions(-)
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 950b14c60696b..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,
@@ -564,8 +564,8 @@ 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);
@@ -612,7 +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++) {
@@ -620,19 +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);
}
@@ -641,9 +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();
@@ -791,13 +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();
@@ -885,9 +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;
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