AST: Initialization with dllimport functions in C

The C++ language requires that the address of a function be the same
across all translation units.  To make __declspec(dllimport) useful,
this means that a dllimported function must also obey this rule.  MSVC
implements this by dynamically querying the import address table located
in the linked executable.  This means that the address of such a
function in C++ is not constant (which violates other rules).

However, the C language has no notion of ODR nor does it permit dynamic
initialization whatsoever.  This requires implementations to _not_
dynamically query the import address table and instead utilize a wrapper
function that will be synthesized by the linker which will eventually
query the import address table.  The effect this has is, to say the
least, perplexing.

Consider the following C program:
__declspec(dllimport) void f(void);

typedef void (*fp)(void);

static const fp var = &f;

const fp fun() { return &f; }

int main() { return fun() == var; }

MSVC will statically initialize "var" with the address of the wrapper
function and "fun" returns the address of the actual imported function.
This means that "main" will return false!

Note that LLVM's optimizers are strong enough to figure out that "main"
should return true.  However, this result is dependent on having
optimizations enabled!

N.B.  This change also permits the usage of dllimport declarators inside
of template arguments; they are sufficiently constant for such a
purpose.  Add tests to make sure we don't regress here.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211677 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 27 insertions, 8 deletions

diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index c57a802512..5897f2c485 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -1269,11 +1269,36 @@ static bool CheckLValueConstantExpression(EvalInfo &Info, SourceLocation Loc,
           LVal.getLValueCallIndex() == 0) &&
          "have call index for global lvalue");
 
-  // Check if this is a thread-local variable.
   if (const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>()) {
     if (const VarDecl *Var = dyn_cast<const VarDecl>(VD)) {
+      // Check if this is a thread-local variable.
       if (Var->getTLSKind())
         return false;
+
+      // Check if this is a dllimport variable.  Fail evaluation if we care
+      // about side effects; a dllimport variable rarely acts like a constant
+      // except in places like template arguments.  It never acts like a
+      // constant in C.
+      if ((!Info.getLangOpts().CPlusPlus ||
+           !Info.keepEvaluatingAfterSideEffect()) &&
+          Var->hasAttr<DLLImportAttr>())
+        return false;
+    }
+    if (const auto *FD = dyn_cast<const FunctionDecl>(VD)) {
+      // __declspec(dllimport) must be handled very carefully:
+      // We must never initialize an expression with the thunk in C++.
+      // Doing otherwise would allow the same id-expression to yield
+      // different addresses for the same function in different translation
+      // units.  However, this means that we must dynamically initialize the
+      // expression with the contents of the import address table at runtime.
+      //
+      // The C language has no notion of ODR; furthermore, it has no notion of
+      // dynamic initialization.  This means that we are permitted to
+      // perform initialization with the address of the thunk.
+      if (Info.getLangOpts().CPlusPlus &&
+          !Info.keepEvaluatingAfterSideEffect() &&
+          FD->hasAttr<DLLImportAttr>())
+        return false;
     }
   }
 
@@ -4373,11 +4398,8 @@ static bool EvaluateLValue(const Expr *E, LValue &Result, EvalInfo &Info) {
 }
 
 bool LValueExprEvaluator::VisitDeclRefExpr(const DeclRefExpr *E) {
-  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(E->getDecl())) {
-    if (FD->hasAttr<DLLImportAttr>())
-      return ZeroInitialization(E);
+  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(E->getDecl()))
     return Success(FD);
-  }
   if (const VarDecl *VD = dyn_cast<VarDecl>(E->getDecl()))
     return VisitVarDecl(E, VD);
   return Error(E);
@@ -4393,9 +4415,6 @@ bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
       Result.set(VD, Frame->Index);
       return true;
     }
-    // The address of __declspec(dllimport) variables aren't constant.
-    if (VD->hasAttr<DLLImportAttr>())
-      return ZeroInitialization(E);
     return Success(VD);
   }