Implement caching for the linkage and visibility calculations of

declarations.

The motivation for this patch is that linkage/visibility computations
are linear in the number of redeclarations of an entity, and we've run
into a case where a single translation unit has > 6500 redeclarations
of the same (unused!) external variable. Since each redeclaration
involves a linkage check, the resulting quadratic behavior makes Clang
slow to a crawl. With this change, a simple test with 512
redeclarations of a variable syntax-checks ~20x faster than
before.

That said, I hate this change, and will probably end up reverting it
in a few hours. Reasons to hate it:
  - It makes NamedDecl larger, since we don't have enough free bits in
  Decl to squeeze in the extra information about caching.
  - There are way too many places where we need to invalidate this
  cache, because the visibility of a declaration can change due to
  redeclarations (!). Despite self-hosting and passing the testsuite,
  I have no confidence that I've found all of places where this cache
  needs to be invalidated.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120808 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 32 insertions, 0 deletions

diff --git a/lib/AST/DeclBase.cpp b/lib/AST/DeclBase.cpp
index 843e907dea..3758ca1065 100644
--- a/lib/AST/DeclBase.cpp
+++ b/lib/AST/DeclBase.cpp
@@ -337,6 +337,38 @@ void Decl::dropAttrs() {
   getASTContext().eraseDeclAttrs(this);
 }
 
+void Decl::addAttr(Attr *A) {
+  if (NamedDecl *ND = dyn_cast<NamedDecl>(this))
+    if (VisibilityAttr *Visibility = dyn_cast<VisibilityAttr>(A)) {
+      bool ClearVisibility = true;
+      if (VarDecl *VD = dyn_cast<VarDecl>(this)) {
+        if (VD->getPreviousDeclaration()) {
+          VisibilityAttr *PrevVisibility 
+            = VD->getPreviousDeclaration()->getAttr<VisibilityAttr>();
+          if  (PrevVisibility &&
+               PrevVisibility->getVisibility() == Visibility->getVisibility())
+            ClearVisibility = false;
+        }
+      } else if (FunctionDecl *FD = dyn_cast<FunctionDecl>(this)) {
+        if (FD->getPreviousDeclaration()) {
+          VisibilityAttr *PrevVisibility 
+            = FD->getPreviousDeclaration()->getAttr<VisibilityAttr>();
+          if  (PrevVisibility &&
+               PrevVisibility->getVisibility() == Visibility->getVisibility())
+            ClearVisibility = false;
+        }
+      }
+      
+      if (ClearVisibility)
+        ND->ClearLinkageAndVisibilityCache();
+    }
+  
+  if (hasAttrs())
+    getAttrs().push_back(A);
+  else
+    setAttrs(AttrVec(1, A));
+}
+
 const AttrVec &Decl::getAttrs() const {
   assert(HasAttrs && "No attrs to get!");
   return getASTContext().getDeclAttrs(this);