[clang-tools-extra] [clang-tidy] Add performance-expensive-value-or check (PR #200166)

[Endre Fülöp via cfe-commits]

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+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ExpensiveValueOrCheck.h"
+#include "../utils/OptionsUtils.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::performance {
+
+ExpensiveValueOrCheck::ExpensiveValueOrCheck(StringRef Name,
+                                             ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      SizeThreshold(Options.get("SizeThreshold", 8U)),
+      WarnOnRvalueOptional(Options.get("WarnOnRvalueOptional", false)),
+      OptionalTypes(utils::options::parseStringList(
+          Options.get("OptionalTypes", "::std::optional"))) {}
+
+void ExpensiveValueOrCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "SizeThreshold", SizeThreshold);
+  Options.store(Opts, "WarnOnRvalueOptional", WarnOnRvalueOptional);
+  Options.store(Opts, "OptionalTypes",
+                utils::options::serializeStringList(OptionalTypes));
+}
+
+void ExpensiveValueOrCheck::registerMatchers(MatchFinder *Finder) {
+  auto OptionalTypesMatcher = hasAnyName(OptionalTypes);
+
+  Finder->addMatcher(
+      cxxMemberCallExpr(callee(cxxMethodDecl(hasName("value_or"),
+                                             ofClass(OptionalTypesMatcher))))
+          .bind("call"),
+      this);
+}
+
+void ExpensiveValueOrCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *Call = Result.Nodes.getNodeAs<CXXMemberCallExpr>("call");
+  if (!Call)
+    return;
+
+  const Expr *ObjExpr = Call->getImplicitObjectArgument();
+  if (!WarnOnRvalueOptional && ObjExpr && !ObjExpr->isLValue())
+    return;
+
+  const QualType ValueType = Call->getType().getCanonicalType();
+  if (ValueType->isDependentType() || ValueType->isIncompleteType())
+    return;
+
+  const ASTContext &Ctx = *Result.Context;
+  const int64_t ValueSize = Ctx.getTypeSizeInChars(ValueType).getQuantity();
+  const bool IsExpensive = !ValueType.isTriviallyCopyableType(Ctx) ||
+                           ValueSize > static_cast<int64_t>(SizeThreshold);
+
+  if (!IsExpensive)
+    return;
+
+  diag(Call->getExprLoc(),
+       "'value_or' copies expensive type %0; consider using 'operator*' or "
+       "'value()' with a separate fallback")
+      << ValueType;
----------------
gamesh411 wrote:

`FormatASTNodeDiagnosticArgument` seems to be the way Clang Tidy uses `ConvertTypeToDiagnosticString`. One complication, though: Call->getType() returns the canonical type after template substitution, so the sugar is lost.
Getting the sugared form would require extracting the template argument from the `TemplateSpecializationType` of the optional object.
Supporting different template cases (specialization, partial specialization) makes me think that it may be better not to burden this PR with this; let's consider this in a follow-up.


https://github.com/llvm/llvm-project/pull/200166