MS ABI: Implement the MSVC 2015 scheme for scope disambiguation

consider C++ that looks like:
  inline int &f(bool b) {
    if (b) {
      static int i;
      return i;
    }
    static int i;
    return i;
  }

Both 'i' variables must have distinct (and stable) names for linkage
purposes.  The MSVC 2013 ABI would number the variables using a count of
the number of scopes that have been created.  However, the final 'i'
returns to a scope that has already been created leading to a mangling
collision.

MSVC 2015 fixes this by giving the second 'i' the name it would have if
it were declared before the 'if'.  However, this results in ABI breakage
because the mangled name, in cases where there was no ambiguity, would
now be different.

We implement the new behavior and only enable it if we are targeting the
MSVC 2015 ABI, otherwise the old behavior will be used.

This fixes PR18131.

llvm-svn: 232766

1 files changed, 24 insertions, 11 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 2fd231b720b..1234afb8fb7 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -3416,6 +3416,17 @@ Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
   return ParsedFreeStandingDeclSpec(S, AS, DS, MultiTemplateParamsArg());
 }
 
+// The MS ABI changed between VS2013 and VS2015 with regard to numbers used to
+// disambiguate entities defined in different scopes.
+// While the VS2015 ABI fixes potential miscompiles, it is also breaks
+// compatibility.
+// We will pick our mangling number depending on which version of MSVC is being
+// targeted.
+static unsigned getMSManglingNumber(const LangOptions &LO, Scope *S) {
+  return LO.isCompatibleWithMSVC(19) ? S->getMSCurManglingNumber()
+                                     : S->getMSLastManglingNumber();
+}
+
 void Sema::handleTagNumbering(const TagDecl *Tag, Scope *TagScope) {
   if (!Context.getLangOpts().CPlusPlus)
     return;
@@ -3428,7 +3439,8 @@ void Sema::handleTagNumbering(const TagDecl *Tag, Scope *TagScope) {
     MangleNumberingContext &MCtx =
         Context.getManglingNumberContext(Tag->getParent());
     Context.setManglingNumber(
-        Tag, MCtx.getManglingNumber(Tag, TagScope->getMSLocalManglingNumber()));
+        Tag, MCtx.getManglingNumber(
+                 Tag, getMSManglingNumber(getLangOpts(), TagScope)));
     return;
   }
 
@@ -3437,8 +3449,8 @@ void Sema::handleTagNumbering(const TagDecl *Tag, Scope *TagScope) {
   if (MangleNumberingContext *MCtx = getCurrentMangleNumberContext(
           Tag->getDeclContext(), ManglingContextDecl)) {
     Context.setManglingNumber(
-        Tag,
-        MCtx->getManglingNumber(Tag, TagScope->getMSLocalManglingNumber()));
+        Tag, MCtx->getManglingNumber(
+                 Tag, getMSManglingNumber(getLangOpts(), TagScope)));
   }
 }
 
@@ -4135,10 +4147,11 @@ Decl *Sema::BuildAnonymousStructOrUnion(Scope *S, DeclSpec &DS,
   if (VarDecl *NewVD = dyn_cast<VarDecl>(Anon)) {
     if (getLangOpts().CPlusPlus && NewVD->isStaticLocal()) {
       Decl *ManglingContextDecl;
-      if (MangleNumberingContext *MCtx =
-              getCurrentMangleNumberContext(NewVD->getDeclContext(),
-                                            ManglingContextDecl)) {
-        Context.setManglingNumber(NewVD, MCtx->getManglingNumber(NewVD, S->getMSLocalManglingNumber()));
+      if (MangleNumberingContext *MCtx = getCurrentMangleNumberContext(
+              NewVD->getDeclContext(), ManglingContextDecl)) {
+        Context.setManglingNumber(
+            NewVD, MCtx->getManglingNumber(
+                       NewVD, getMSManglingNumber(getLangOpts(), S)));
         Context.setStaticLocalNumber(NewVD, MCtx->getStaticLocalNumber(NewVD));
       }
     }
@@ -5925,11 +5938,11 @@ Sema::ActOnVariableDeclarator(Scope *S, Declarator &D, DeclContext *DC,
 
   if (getLangOpts().CPlusPlus && NewVD->isStaticLocal()) {
     Decl *ManglingContextDecl;
-    if (MangleNumberingContext *MCtx =
-            getCurrentMangleNumberContext(NewVD->getDeclContext(),
-                                          ManglingContextDecl)) {
+    if (MangleNumberingContext *MCtx = getCurrentMangleNumberContext(
+            NewVD->getDeclContext(), ManglingContextDecl)) {
       Context.setManglingNumber(
-          NewVD, MCtx->getManglingNumber(NewVD, S->getMSLocalManglingNumber()));
+          NewVD, MCtx->getManglingNumber(
+                     NewVD, getMSManglingNumber(getLangOpts(), S)));
       Context.setStaticLocalNumber(NewVD, MCtx->getStaticLocalNumber(NewVD));
     }
   }