It turns out that the debug info describing the Sema class is an
appreciable percentage of the total object file size of objects in Sema.
By adding a key function, clang is able to optimize the debug info size
by emitting a forward declaration in TUs that do not define the key
function.
On Windows, with clang-cl, these are the total sizes of object files in
Sema before and after this change, compiling with optimizations and
debug info:
before: 335,012 KB
after: 278,116 KB
delta: -56,896 KB
percent: -17.0%
The effect on link time was negligible, despite having ~56MB less input.
On Linux, with clang, these are the same sizes using DWARF -g and
optimizations:
before: 603,756 KB
after: 515,340 KB
delta: -88,416 KB
percent: -14.6%
I didn't use type units, DWARF-5, fission, or any other special flags.
Reviewed By: thakis
Differential Revision: https://reviews.llvm.org/D70340
};
/// Sema - This implements semantic analysis and AST building for C.
-class Sema {
+class Sema final {
Sema(const Sema &) = delete;
void operator=(const Sema &) = delete;
+ /// A key method to reduce duplicate debug info from Sema.
+ virtual void anchor();
+
///Source of additional semantic information.
ExternalSemaSource *ExternalSource;
SemaPPCallbackHandler->set(*this);
}
+// Anchor Sema's type info to this TU.
+void Sema::anchor() {}
+
void Sema::addImplicitTypedef(StringRef Name, QualType T) {
DeclarationName DN = &Context.Idents.get(Name);
if (IdResolver.begin(DN) == IdResolver.end())