Produce errors when an assignment expression would use a common symbol.

An assignment will produce a symbol with a given section and offset. There is
no way to represent something like "1 byte after a common symbol".

This matches the behavior of GNU as.

Part of PR22217.

llvm-svn: 226470

1 files changed, 11 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index e3c2443f4a2..b5b72827299 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -201,7 +201,17 @@ const MCSymbol *MCAsmLayout::getBaseSymbol(const MCSymbol &Symbol) const {
   if (!A)
     return nullptr;
 
-  return &A->getSymbol();
+  const MCSymbol &ASym = A->getSymbol();
+  const MCAssembler &Asm = getAssembler();
+  const MCSymbolData &ASD = Asm.getSymbolData(ASym);
+  if (ASD.isCommon()) {
+    // FIXME: we should probably add a SMLoc to MCExpr.
+    Asm.getContext().FatalError(SMLoc(),
+                                "Common symbol " + ASym.getName() +
+                                    " cannot be used in assignment expr");
+  }
+
+  return &ASym;
 }
 
 uint64_t MCAsmLayout::getSectionAddressSize(const MCSectionData *SD) const {