[clang-tidy] New option for misc-throw-by-value-catch-by-reference

Catching trivial objects by value is not dangerous but may be
inefficient if they are too large. This patch adds an option
`WarnOnLargeObject` to the checker to also warn if such an object
is caught by value. An object is considered as "large" if its
size is greater than `MaxSize` which is another option. Default
value is the machine word of the architecture (size of the type
`size_t`).

Differential Revision: https://reviews.llvm.org/D61851

llvm-svn: 361225

1 files changed, 16 insertions, 2 deletions

diff --git a/clang-tools-extra/clang-tidy/misc/ThrowByValueCatchByReferenceCheck.cpp b/clang-tools-extra/clang-tidy/misc/ThrowByValueCatchByReferenceCheck.cpp
index b7077a1225f..7b1f7eafe1e 100644
--- a/clang-tools-extra/clang-tidy/misc/ThrowByValueCatchByReferenceCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/ThrowByValueCatchByReferenceCheck.cpp
@@ -20,7 +20,10 @@ namespace misc {
 ThrowByValueCatchByReferenceCheck::ThrowByValueCatchByReferenceCheck(
     StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
-      CheckAnonymousTemporaries(Options.get("CheckThrowTemporaries", true)) {}
+      CheckAnonymousTemporaries(Options.get("CheckThrowTemporaries", true)),
+      WarnOnLargeObject(Options.get("WarnOnLargeObject", false)),
+      // Cannot access `ASTContext` from here so set it to an extremal value.
+      MaxSize(Options.get("MaxSize", std::numeric_limits<uint64_t>::max())) {}
 
 void ThrowByValueCatchByReferenceCheck::registerMatchers(MatchFinder *Finder) {
   // This is a C++ only check thus we register the matchers only for C++
@@ -150,8 +153,19 @@ void ThrowByValueCatchByReferenceCheck::diagnoseCatchLocations(
     // If it's not a pointer and not a reference then it must be caught "by
     // value". In this case we should emit a diagnosis message unless the type
     // is trivial.
-    if (!caughtType.isTrivialType(context))
+    if (!caughtType.isTrivialType(context)) {
       diag(varDecl->getBeginLoc(), diagMsgCatchReference);
+    } else if (WarnOnLargeObject) {
+      // If the type is trivial, then catching it by reference is not dangerous.
+      // However, catching large objects by value decreases the performance.
+
+      // We can now access `ASTContext` so if `MaxSize` is an extremal value
+      // then set it to the size of `size_t`.
+      if (MaxSize == std::numeric_limits<uint64_t>::max())
+        MaxSize = context.getTypeSize(context.getSizeType());
+      if (context.getTypeSize(caughtType) > MaxSize)
+        diag(varDecl->getBeginLoc(), diagMsgCatchReference);
+    }
   }
 }