PNaCl: Add NoAlias attributes in ExpandByVal and ExpandVarArgs passes

lib/Transforms/NaCl/ExpandByVal.cpp

 //===- ExpandByVal.cpp - Expand out use of "byval" and "sret" attributes---===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This pass expands out by-value passing of structs as arguments and
@@ skipping 64 matching lines @@
   for (unsigned Slot = 0; Slot < Attrs.getNumSlots(); ++Slot) {
     unsigned Index = Attrs.getSlotIndex(Slot);
     AttrBuilder AB;
     for (AttributeSet::iterator Attr = Attrs.begin(Slot), E = Attrs.end(Slot);
          Attr != E; ++Attr) {
       if (!Attr->isAlignAttribute() &&
           Attr->getKindAsEnum() != Attribute::ByVal &&
           Attr->getKindAsEnum() != Attribute::StructRet) {
         AB.addAttribute(*Attr);
       }
+      // IR semantics require that ByVal implies NoAlias.  However, IR
+      // semantics do not require StructRet to imply NoAlias.  For
+      // example, a global variable address can be passed as a
+      // StructRet argument, although Clang does not do so and Clang
+      // explicitly adds NoAlias to StructRet arguments.
+      if (Attr->getKindAsEnum() == Attribute::ByVal) {
+        AB.addAttribute(Attribute::get(Context, Attribute::NoAlias));
+      }
     }
     AttrList.push_back(AttributeSet::get(Context, Index, AB));
   }
   return AttributeSet::get(Context, AttrList);
 }
 
 // ExpandCall() can take a CallInst or an InvokeInst.  It returns
 // whether the instruction was modified.
 template <class InstType>
 static bool ExpandCall(DataLayout *DL, InstType *Call) {
@@ skipping 87 matching lines @@
       }
     }
   }
 
   return Modified;
 }
 
 ModulePass *llvm::createExpandByValPass() {
   return new ExpandByVal();
 }