XCore target: symbols in the .cp.rodata section must be 4 byte aligned

[robertlytton]

Summary

For the xcore target, the abi requires us to place global constant data with c linkage in the .cp,rodata section.
All symbols in this section require 4 byte alignment.

• Please review the following from a clang perspective *

Please note the patch to EmitGlobalVarDefinition which now only calls setAlignment if it has not already been set.

[• friedgold]

I think this should set the align to the maximum of the decl alignment and 4 so that, for example, if the user requests a minimum alignment using the aligned attribute it will be respected.

lib/CodeGen/CodeGenModule.cpp

if (D) {
     if (getContext().isMSStaticDataMemberInlineDefinition(D)) {
       EmitGlobalVarDefinition(D);
     }
 
     // Handle XCore specific ABI requirements.
     if (getTarget().getTriple().getArch() == llvm::Triple::xcore &&
         D->getLanguageLinkage() == CLanguageLinkage &&
         D->getType().isConstant(Context) &&
-        isExternallyVisible(D->getLinkageAndVisibility().getLinkage()))
+        isExternallyVisible(D->getLinkageAndVisibility().getLinkage())) {
       GV->setSection(".cp.rodata");
+      GV->setAlignment(4);
+    }
   }
 
   if (AddrSpace != Ty->getAddressSpace())
     return llvm::ConstantExpr::getAddrSpaceCast(GV, Ty);
 
   return GV;
 }
 
if (D->hasAttr<AnnotateAttr>())
     AddGlobalAnnotations(D, GV);
 
   GV->setInitializer(Init);
 
   // If it is safe to mark the global 'constant', do so now.
   GV->setConstant(!NeedsGlobalCtor && !NeedsGlobalDtor &&
                   isTypeConstant(D->getType(), true));
 
+  // Set the alignment if it has not already been set.
+  if (GV->getAlignment() == 0)
     GV->setAlignment(getContext().getDeclAlign(D).getQuantity());
 
   // Set the llvm linkage type as appropriate.
   llvm::GlobalValue::LinkageTypes Linkage =
       getLLVMLinkageVarDefinition(D, GV->isConstant());
 
   // On Darwin, the backing variable for a C++11 thread_local variable always
   // has internal linkage; all accesses should just be calls to the
   // Itanium-specified entry point, which has the normal linkage of the

test/CodeGen/xcore-abi.c

 
 // CHECK: @cgx = external constant i32, section ".cp.rodata"
 extern const int cgx;
 int fcgx() { return cgx;}
 // CHECK: @g1 = global i32 0, align 4
 int g1;
 // CHECK: @cg1 = constant i32 0, section ".cp.rodata", align 4
 const int cg1;
+// CHECK: @cg2 = constant i8 0, section ".cp.rodata", align 4
+const char cg2;
+
 
 #include <stdarg.h>
 struct x { int a[5]; };
 void f(void*);
 void testva (int n, ...) {
   // CHECK-LABEL: testva
   va_list ap;
   va_start(ap,n);