[LLVMdev] DebugInfo: LTO Metadata Size reduction by removing some cycles

[David Blaikie]

>From a thread with Adrian on llvm-commits I looked a little at cases where
DwarfCompileUnit's getOrCreateContextDIE's "fallback to return the CU"
didn't fire when a CU DIE was required (ie: when the "getDIE" call actually
found the CU before the fallback happened)

This seems unnecessary, and any case where we do this (where a metadata
node has a non-null context DIE pointing to the CU) is just wasted space.
By ensuring we always use null to refer to the CU we'll reduce cycles and
metadata nodes in LTO situations allowing the nodes to be re-used across
CUs in LTO cases.

I've attached the CLang patch to do this, but I don't want to commit this
until the LLVM test cases have been cleaned up (the easiest way to find
these test cases would be to add an assert in
CompileUnit::getORCreateContextDIE. Changing this:

  if (DIE *ContextDIE = getDIE(Context))
    return ContextDIE;

to this:

  if (DIE *ContextDIE = getDIE(Context)) {
    assert(ContextDIE != CUDie);
    return ContextDIE;
  }

Then finding all the test cases that fail and null out any context fields
that refer to the CU, thus relying on the fallback instead.

Since we're not working on LTO right now, cleaning up those test cases
(20-30, it's not a lot but more than I have time for) isn't something I can
do right now but seemed like it might be interesting to you. And I figured
you'd be able to see if this helps size much, whereas I've not been looking
at/analyzing LTO size costs.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20131115/d922ab4c/attachment.html>
-------------- next part --------------
diff --git lib/CodeGen/CGDebugInfo.cpp lib/CodeGen/CGDebugInfo.cpp
index 9b131fd..c75fd05 100644
--- lib/CodeGen/CGDebugInfo.cpp
+++ lib/CodeGen/CGDebugInfo.cpp
@@ -138,7 +138,7 @@ void CGDebugInfo::setLocation(SourceLocation Loc) {
 /// getContextDescriptor - Get context info for the decl.
 llvm::DIScope CGDebugInfo::getContextDescriptor(const Decl *Context) {
   if (!Context)
-    return TheCU;
+    return llvm::DIScope();
 
   llvm::DenseMap<const Decl *, llvm::WeakVH>::iterator
     I = RegionMap.find(Context);
@@ -155,7 +155,7 @@ llvm::DIScope CGDebugInfo::getContextDescriptor(const Decl *Context) {
     if (!RDecl->isDependentType())
       return getOrCreateType(CGM.getContext().getTypeDeclType(RDecl),
                                         getOrCreateMainFile());
-  return TheCU;
+  return llvm::DIScope();
 }
 
 /// getFunctionName - Get function name for the given FunctionDecl. If the
diff --git test/CodeGenCXX/debug-info-namespace.cpp test/CodeGenCXX/debug-info-namespace.cpp
index a2d7ede..03e70c7 100644
--- test/CodeGenCXX/debug-info-namespace.cpp
+++ test/CodeGenCXX/debug-info-namespace.cpp
@@ -52,8 +52,8 @@ int func(bool b) {
 // CHECK: [[I:![0-9]*]] = {{.*}}, metadata [[NS]], metadata !"i", {{.*}} ; [ DW_TAG_variable ] [i]
 // CHECK: [[MODULES]] = metadata !{metadata [[M1:![0-9]*]], metadata [[M2:![0-9]*]], metadata [[M3:![0-9]*]], metadata [[M4:![0-9]*]], metadata [[M5:![0-9]*]], metadata [[M6:![0-9]*]], metadata [[M7:![0-9]*]], metadata [[M8:![0-9]*]], metadata [[M9:![0-9]*]], metadata [[M10:![0-9]*]], metadata [[M11:![0-9]*]], metadata [[M12:![0-9]*]]}
 // CHECK: [[M1]] = metadata !{i32 {{[0-9]*}}, metadata [[CTXT]], metadata [[NS]], i32 11} ; [ DW_TAG_imported_module ]
-// CHECK: [[M2]] = metadata !{i32 {{[0-9]*}}, metadata [[CU]], metadata [[CTXT]], i32 {{[0-9]*}}} ; [ DW_TAG_imported_module ]
-// CHECK: [[M3]] = metadata !{i32 {{[0-9]*}}, metadata [[CU]], metadata [[CTXT]], i32 15, metadata !"E"} ; [ DW_TAG_imported_module ]
+// CHECK: [[M2]] = metadata !{i32 {{[0-9]*}}, null, metadata [[CTXT]], i32 {{[0-9]*}}} ; [ DW_TAG_imported_module ]
+// CHECK: [[M3]] = metadata !{i32 {{[0-9]*}}, null, metadata [[CTXT]], i32 15, metadata !"E"} ; [ DW_TAG_imported_module ]
 // CHECK: [[M4]] = metadata !{i32 {{[0-9]*}}, metadata [[LEX2:![0-9]*]], metadata [[NS]], i32 19} ; [ DW_TAG_imported_module ]
 // CHECK: [[LEX2]] = metadata !{i32 {{[0-9]*}}, metadata [[FILE2]], metadata [[LEX1:![0-9]+]], i32 {{[0-9]*}}, i32 0, i32 {{.*}}} ; [ DW_TAG_lexical_block ]
 // CHECK: [[LEX1]] = metadata !{i32 {{[0-9]*}}, metadata [[FILE2]], metadata [[FUNC]], i32 {{[0-9]*}}, i32 0, i32 {{.*}}} ; [ DW_TAG_lexical_block ]