Add a key method to Sema to optimize debug info size
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
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index bedea21..c3c6cad 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -189,6 +189,9 @@
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())