Teach sema and codegen about the difference between address of labels,

which is a common idiom to improve PIC'ness of code using the addr of
label extension.  This implementation is a gross hack, but the only other
alternative would be to teach evalutate about this horrid combination.
While GCC allows things like "&&foo - &&bar + 1", people don't use this
in practice.  This implements PR5131.

llvm-svn: 83957

1 files changed, 22 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index fc24144f442..9621e2a21c0 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -402,7 +402,7 @@ public:
   llvm::Constant *VisitCompoundLiteralExpr(CompoundLiteralExpr *E) {
     return Visit(E->getInitializer());
   }
-  
+    
   llvm::Constant *EmitMemberFunctionPointer(CXXMethodDecl *MD) {
     assert(MD->isInstance() && "Member function must not be static!");
     
@@ -446,6 +446,27 @@ public:
     return 0;
   }
     
+  llvm::Constant *VisitBinSub(BinaryOperator *E) {
+    // This must be a pointer/pointer subtraction.  This only happens for
+    // address of label.
+    if (!isa<AddrLabelExpr>(E->getLHS()->IgnoreParenNoopCasts(CGM.getContext())) ||
+       !isa<AddrLabelExpr>(E->getRHS()->IgnoreParenNoopCasts(CGM.getContext())))
+      return 0;
+    
+    llvm::Constant *LHS = CGM.EmitConstantExpr(E->getLHS(),
+                                               E->getLHS()->getType(), CGF);
+    llvm::Constant *RHS = CGM.EmitConstantExpr(E->getRHS(),
+                                               E->getRHS()->getType(), CGF);
+
+    const llvm::Type *ResultType = ConvertType(E->getType());
+    LHS = llvm::ConstantExpr::getPtrToInt(LHS, ResultType);
+    RHS = llvm::ConstantExpr::getPtrToInt(RHS, ResultType);
+        
+    // No need to divide by element size, since addr of label is always void*,
+    // which has size 1 in GNUish.
+    return llvm::ConstantExpr::getSub(LHS, RHS);
+  }
+    
   llvm::Constant *VisitCastExpr(CastExpr* E) {
     switch (E->getCastKind()) {
     case CastExpr::CK_ToUnion: {