From: Richard Smith <richard-llvm@metafoo.co.uk>
Date: Fri, 2 Mar 2012 03:16:32 +0000 (+0000)
Subject: PR12145: Avoid emitting loads of constexpr variables in contexts where there
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=a3cac5b76a5243d8fca812d3a276950b0cfc56b6;p=clang

PR12145: Avoid emitting loads of constexpr variables in contexts where there
is no odr-use of the variable. Go slightly beyond what the standard requires
for variables of reference type.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151879 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/CodeGen/CGExprScalar.cpp b/lib/CodeGen/CGExprScalar.cpp
index 125e431bff..f7ab880e6c 100644
--- a/lib/CodeGen/CGExprScalar.cpp
+++ b/lib/CodeGen/CGExprScalar.cpp
@@ -212,18 +212,21 @@ public:
   // l-values.
   Value *VisitDeclRefExpr(DeclRefExpr *E) {
     Expr::EvalResult Result;
-    if (!E->EvaluateAsRValue(Result, CGF.getContext()))
-      return EmitLoadOfLValue(E);
+    bool IsReferenceConstant = false;
+    QualType EvalTy = E->getType();
+    if (!E->EvaluateAsRValue(Result, CGF.getContext())) {
+      // If this is a reference, try to determine what it is bound to.
+      if (!E->getDecl()->getType()->isReferenceType() ||
+          !E->EvaluateAsLValue(Result, CGF.getContext()))
+        return EmitLoadOfLValue(E);
+
+      IsReferenceConstant = true;
+      EvalTy = E->getDecl()->getType();
+    }
 
     assert(!Result.HasSideEffects && "Constant declref with side-effect?!");
 
-    llvm::Constant *C;
-    if (Result.Val.isInt())
-      C = Builder.getInt(Result.Val.getInt());
-    else if (Result.Val.isFloat())
-      C = llvm::ConstantFP::get(VMContext, Result.Val.getFloat());
-    else
-      return EmitLoadOfLValue(E);
+    llvm::Constant *C = CGF.CGM.EmitConstantValue(Result.Val, EvalTy, &CGF);
 
     // Make sure we emit a debug reference to the global variable.
     if (VarDecl *VD = dyn_cast<VarDecl>(E->getDecl())) {
@@ -233,6 +236,9 @@ public:
       CGF.EmitDeclRefExprDbgValue(E, C);
     }
 
+    if (IsReferenceConstant)
+      return EmitLoadOfLValue(CGF.MakeNaturalAlignAddrLValue(C, E->getType()));
+
     return C;
   }
   Value *VisitObjCSelectorExpr(ObjCSelectorExpr *E) {
diff --git a/test/CodeGenCXX/const-init-cxx11.cpp b/test/CodeGenCXX/const-init-cxx11.cpp
index 5366d6d485..776a1e12dd 100644
--- a/test/CodeGenCXX/const-init-cxx11.cpp
+++ b/test/CodeGenCXX/const-init-cxx11.cpp
@@ -337,3 +337,30 @@ namespace VirtualBase {
   X<D> x;
   // CHECK: call {{.*}}@_ZN11VirtualBase1XINS_1DEEC1Ev
 }
+
+// PR12145
+namespace Unreferenced {
+  int n;
+  constexpr int *p = &n;
+  // We must not emit a load of 'p' here, since it's not odr-used.
+  int q = *p;
+  // CHECK-NOT: _ZN12Unreferenced1pE
+  // CHECK: %0 = load i32* @_ZN12Unreferenced1nE
+  // CHECK-NEXT: store i32 %0, i32* @_ZN12Unreferenced1qE
+  // CHECK-NOT: _ZN12Unreferenced1pE
+
+  // Technically, we are not required to substitute variables of reference types
+  // initialized by constant expressions, because the special case for odr-use
+  // of variables in [basic.def.odr]p2 only applies to objects. But we do so
+  // anyway.
+
+  constexpr int &r = n;
+  // CHECK-NOT: _ZN12Unreferenced1rE
+  int s = r;
+
+  const int t = 1;
+  const int &rt = t;
+  int f(int);
+  int u = f(rt);
+  // CHECK: call i32 @_ZN12Unreferenced1fEi(i32 1)
+}