[IRGen] Evaluate constant static variables referenced through member

expressions

C++ allows us to reference static variables through member expressions. Prior to
this commit, non-integer static variables that were referenced using a member
expression were always emitted using lvalue loads. The old behaviour introduced
an inconsistency between regular uses of static variables and member expressions
uses. For example, the following program compiled and linked successfully:

struct Foo {
   constexpr static const char *name = "foo";
};
int main() {
  return Foo::name[0] == 'f';
}

but this program failed to link because "Foo::name" wasn't found:

struct Foo {
   constexpr static const char *name = "foo";
};
int main() {
  Foo f;
  return f.name[0] == 'f';
}

This commit ensures that constant static variables referenced through member
expressions are emitted in the same way as ordinary static variable references.

rdar://33942261

Differential Revision: https://reviews.llvm.org/D36876

llvm-svn: 311772

1 files changed, 24 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 93ca6011e16..5f8f34e9bcc 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -1339,6 +1339,25 @@ CodeGenFunction::tryEmitAsConstant(DeclRefExpr *refExpr) {
   return ConstantEmission::forValue(C);
 }
 
+static DeclRefExpr *tryToConvertMemberExprToDeclRefExpr(CodeGenFunction &CGF,
+                                                        const MemberExpr *ME) {
+  if (auto *VD = dyn_cast<VarDecl>(ME->getMemberDecl())) {
+    // Try to emit static variable member expressions as DREs.
+    return DeclRefExpr::Create(
+        CGF.getContext(), NestedNameSpecifierLoc(), SourceLocation(), VD,
+        /*RefersToEnclosingVariableOrCapture=*/false, ME->getExprLoc(),
+        ME->getType(), ME->getValueKind());
+  }
+  return nullptr;
+}
+
+CodeGenFunction::ConstantEmission
+CodeGenFunction::tryEmitAsConstant(const MemberExpr *ME) {
+  if (DeclRefExpr *DRE = tryToConvertMemberExprToDeclRefExpr(*this, ME))
+    return tryEmitAsConstant(DRE);
+  return ConstantEmission();
+}
+
 llvm::Value *CodeGenFunction::EmitLoadOfScalar(LValue lvalue,
                                                SourceLocation Loc) {
   return EmitLoadOfScalar(lvalue.getAddress(), lvalue.isVolatile(),
@@ -3540,6 +3559,11 @@ EmitExtVectorElementExpr(const ExtVectorElementExpr *E) {
 }
 
 LValue CodeGenFunction::EmitMemberExpr(const MemberExpr *E) {
+  if (DeclRefExpr *DRE = tryToConvertMemberExprToDeclRefExpr(*this, E)) {
+    EmitIgnoredExpr(E->getBase());
+    return EmitDeclRefLValue(DRE);
+  }
+
   Expr *BaseExpr = E->getBase();
   // If this is s.x, emit s as an lvalue.  If it is s->x, emit s as a scalar.
   LValue BaseLV;
@@ -3566,9 +3590,6 @@ LValue CodeGenFunction::EmitMemberExpr(const MemberExpr *E) {
     return LV;
   }
 
-  if (auto *VD = dyn_cast<VarDecl>(ND))
-    return EmitGlobalVarDeclLValue(*this, E, VD);
-
   if (const auto *FD = dyn_cast<FunctionDecl>(ND))
     return EmitFunctionDeclLValue(*this, E, FD);