1 files changed, 87 insertions, 22 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index d5ce4870f18..f0d8619faa3 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -1422,10 +1422,11 @@ static ConstantEmissionKind checkVarTypeForConstantEmission(QualType type) {
 }
 
 /// Try to emit a reference to the given value without producing it as
-/// an l-value.  This is actually more than an optimization: we can't
-/// produce an l-value for variables that we never actually captured
-/// in a block or lambda, which means const int variables or constexpr
-/// literals or similar.
+/// an l-value.  This is just an optimization, but it avoids us needing
+/// to emit global copies of variables if they're named without triggering
+/// a formal use in a context where we can't emit a direct reference to them,
+/// for instance if a block or lambda or a member of a local class uses a
+/// const int variable or constexpr variable from an enclosing function.
 CodeGenFunction::ConstantEmission
 CodeGenFunction::tryEmitAsConstant(DeclRefExpr *refExpr) {
   ValueDecl *value = refExpr->getDecl();
@@ -2450,33 +2451,97 @@ static LValue EmitGlobalNamedRegister(const VarDecl *VD, CodeGenModule &CGM) {
   return LValue::MakeGlobalReg(Address(Ptr, Alignment), VD->getType());
 }
 
+/// Determine whether we can emit a reference to \p VD from the current
+/// context, despite not necessarily having seen an odr-use of the variable in
+/// this context.
+static bool canEmitSpuriousReferenceToVariable(CodeGenFunction &CGF,
+                                               const DeclRefExpr *E,
+                                               const VarDecl *VD,
+                                               bool IsConstant) {
+  // For a variable declared in an enclosing scope, do not emit a spurious
+  // reference even if we have a capture, as that will emit an unwarranted
+  // reference to our capture state, and will likely generate worse code than
+  // emitting a local copy.
+  if (E->refersToEnclosingVariableOrCapture())
+    return false;
+
+  // For a local declaration declared in this function, we can always reference
+  // it even if we don't have an odr-use.
+  if (VD->hasLocalStorage()) {
+    return VD->getDeclContext() ==
+           dyn_cast_or_null<DeclContext>(CGF.CurCodeDecl);
+  }
+
+  // For a global declaration, we can emit a reference to it if we know
+  // for sure that we are able to emit a definition of it.
+  VD = VD->getDefinition(CGF.getContext());
+  if (!VD)
+    return false;
+
+  // Don't emit a spurious reference if it might be to a variable that only
+  // exists on a different device / target.
+  // FIXME: This is unnecessarily broad. Check whether this would actually be a
+  // cross-target reference.
+  if (CGF.getLangOpts().OpenMP || CGF.getLangOpts().CUDA ||
+      CGF.getLangOpts().OpenCL) {
+    return false;
+  }
+
+  // We can emit a spurious reference only if the linkage implies that we'll
+  // be emitting a non-interposable symbol that will be retained until link
+  // time.
+  switch (CGF.CGM.getLLVMLinkageVarDefinition(VD, IsConstant)) {
+  case llvm::GlobalValue::ExternalLinkage:
+  case llvm::GlobalValue::LinkOnceODRLinkage:
+  case llvm::GlobalValue::WeakODRLinkage:
+  case llvm::GlobalValue::InternalLinkage:
+  case llvm::GlobalValue::PrivateLinkage:
+    return true;
+  default:
+    return false;
+  }
+}
+
 LValue CodeGenFunction::EmitDeclRefLValue(const DeclRefExpr *E) {
   const NamedDecl *ND = E->getDecl();
   QualType T = E->getType();
 
+  assert(E->isNonOdrUse() != NOUR_Unevaluated &&
+         "should not emit an unevaluated operand");
+
   if (const auto *VD = dyn_cast<VarDecl>(ND)) {
     // Global Named registers access via intrinsics only
     if (VD->getStorageClass() == SC_Register &&
         VD->hasAttr<AsmLabelAttr>() && !VD->isLocalVarDecl())
       return EmitGlobalNamedRegister(VD, CGM);
 
-    // A DeclRefExpr for a reference initialized by a constant expression can
-    // appear without being odr-used. Directly emit the constant initializer.
-    VD->getAnyInitializer(VD);
-    if (E->isNonOdrUse() == NOUR_Constant && VD->getType()->isReferenceType()) {
-      llvm::Constant *Val =
-        ConstantEmitter(*this).emitAbstract(E->getLocation(),
-                                            *VD->evaluateValue(),
-                                            VD->getType());
-      assert(Val && "failed to emit reference constant expression");
-      // FIXME: Eventually we will want to emit vector element references.
-
-      // Should we be using the alignment of the constant pointer we emitted?
-      CharUnits Alignment = getNaturalTypeAlignment(E->getType(),
-                                                    /* BaseInfo= */ nullptr,
-                                                    /* TBAAInfo= */ nullptr,
-                                                    /* forPointeeType= */ true);
-      return MakeAddrLValue(Address(Val, Alignment), T, AlignmentSource::Decl);
+    // If this DeclRefExpr does not constitute an odr-use of the variable,
+    // we're not permitted to emit a reference to it in general, and it might
+    // not be captured if capture would be necessary for a use. Emit the
+    // constant value directly instead.
+    if (E->isNonOdrUse() == NOUR_Constant &&
+        (VD->getType()->isReferenceType() ||
+         !canEmitSpuriousReferenceToVariable(*this, E, VD, true))) {
+      VD->getAnyInitializer(VD);
+      llvm::Constant *Val = ConstantEmitter(*this).emitAbstract(
+          E->getLocation(), *VD->evaluateValue(), VD->getType());
+      assert(Val && "failed to emit constant expression");
+
+      Address Addr = Address::invalid();
+      if (!VD->getType()->isReferenceType()) {
+        // Spill the constant value to a global.
+        Addr = CGM.createUnnamedGlobalFrom(*VD, Val,
+                                           getContext().getDeclAlign(VD));
+      } else {
+        // Should we be using the alignment of the constant pointer we emitted?
+        CharUnits Alignment =
+            getNaturalTypeAlignment(E->getType(),
+                                    /* BaseInfo= */ nullptr,
+                                    /* TBAAInfo= */ nullptr,
+                                    /* forPointeeType= */ true);
+        Addr = Address(Val, Alignment);
+      }
+      return MakeAddrLValue(Addr, T, AlignmentSource::Decl);
     }
 
     // FIXME: Handle other kinds of non-odr-use DeclRefExprs.
@@ -2512,7 +2577,7 @@ LValue CodeGenFunction::EmitDeclRefLValue(const DeclRefExpr *E) {
   // FIXME: We should be able to assert this for FunctionDecls as well!
   // FIXME: We should be able to assert this for all DeclRefExprs, not just
   // those with a valid source location.
-  assert((ND->isUsed(false) || !isa<VarDecl>(ND) ||
+  assert((ND->isUsed(false) || !isa<VarDecl>(ND) || E->isNonOdrUse() ||
           !E->getLocation().isValid()) &&
          "Should not use decl without marking it used!");