[clang] 586f65d - Add a key method to Sema to optimize debug info size

[Reid Kleckner via cfe-commits]

Author: Reid Kleckner
Date: 2019-11-19T12:42:33-08:00
New Revision: 586f65d31f32ca6bc8cfdb8a4f61bee5057bf6c8

URL: https://github.com/llvm/llvm-project/commit/586f65d31f32ca6bc8cfdb8a4f61bee5057bf6c8
DIFF: https://github.com/llvm/llvm-project/commit/586f65d31f32ca6bc8cfdb8a4f61bee5057bf6c8.diff

LOG: 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

Added: 
    

Modified: 
    clang/include/clang/Sema/Sema.h
    clang/lib/Sema/Sema.cpp

Removed: 
    


################################################################################
diff  --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 220b6c17835d..7d8ab59fe900 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -328,10 +328,13 @@ class PreferredTypeBuilder {
 };
 
 /// 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;
 

diff  --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index bedea2167950..c3c6cad277ff 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -189,6 +189,9 @@ Sema::Sema(Preprocessor &pp, ASTContext &ctxt, ASTConsumer &consumer,
   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())