Linker: Disallow linking appending globals with different addrspaces

[arsenm]

Summary

The current appending linkage handling implicitly assumes this by
using a basic ConstantExpr::getBitCast to resolve type
mismatches. Avoid this edge case so we don't need to keep the type
mismatch replacement code around after opaque pointers.

[nikic]

LGTM, this just makes the existing limitation explicit, and I don't think there's a point in supporting this without some use-case.

[arsenm]

2600a58931d8d49f21a32fd69a844b76fc5cb0b2

llvm/lib/Linker/IRMover.cpp

if (DstGV && !DstGV->isDeclaration() && !SrcGV->isDeclaration()) {
 
     if (DstGV->hasGlobalUnnamedAddr() != SrcGV->hasGlobalUnnamedAddr())
       return stringErr(
           "Appending variables with different unnamed_addr need to be linked!");
 
     if (DstGV->getSection() != SrcGV->getSection())
       return stringErr(
           "Appending variables with different section name need to be linked!");
+
+    if (DstGV->getAddressSpace() != SrcGV->getAddressSpace())
+      return stringErr("Appending variables with different address spaces need "
+                       "to be linked!");
   }
 
   // Do not need to do anything if source is a declaration.
   if (SrcGV->isDeclaration())
     return DstGV;
 
   Type *EltTy = cast<ArrayType>(TypeMap.get(SrcGV->getValueType()))
                     ->getElementType();

llvm/test/Linker/appending-global-err6.ll

+; RUN: not llvm-link %s %p/Inputs/appending-global.ll -S -o - 2>&1 | FileCheck %s
+; RUN: not llvm-link %p/Inputs/appending-global.ll %s -S -o - 2>&1 | FileCheck %s
+
+; Negative test to check that global variables with appending linkage
+; and different address spaces cannot be linked.
+
+; CHECK: error: Appending variables with different address spaces need to be linked!
+
+@var = appending addrspace(1) global [ 1 x ptr ] undef