[MS Compat] Add support for __declspec(noalias)

The attribute '__declspec(noalias)' communicates that the function only
accesses memory pointed to by its pointer-typed arguments.

llvm-svn: 242728

clang/include/clang/Basic/Attr.td

@@ -867,6 +867,12 @@ def ReturnsTwice : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
+def NoAlias : InheritableAttr {
+  let Spellings = [Declspec<"noalias">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [NoAliasDocs];
+}
+
 def NoCommon : InheritableAttr {
   let Spellings = [GCC<"nocommon">];
   let Subjects = SubjectList<[Var]>;

clang/include/clang/Basic/AttrDocs.td

@@ -1583,3 +1583,13 @@ The ``returns_nonnull`` attribute indicates that a particular function (or Objec
 The ``returns_nonnull`` attribute implies that returning a null pointer is undefined behavior, which the optimizer may take advantage of. The ``_Nonnull`` type qualifier indicates that a pointer cannot be null in a more general manner (because it is part of the type system) and does not imply undefined behavior, making it more widely applicable
 }];
 }
+
+def NoAliasDocs : Documentation {
+  let Category = DocCatFunction;
+  let Heading = "noalias";
+  let Content = [{
+The ``noalias`` attribute indicates that the only memory accesses inside
+function are loads and stores from objects pointed to by its pointer-typed
+arguments, with arbitrary offsets.
+  }];
+}

clang/lib/CodeGen/CGCall.cpp

@@ -1421,13 +1421,16 @@ void CodeGenModule::ConstructAttributeList(const CGFunctionInfo &FI,
         FuncAttrs.addAttribute(llvm::Attribute::NoReturn);
     }
 
-    // 'const' and 'pure' attribute functions are also nounwind.
+    // 'const', 'pure' and 'noalias' attributed functions are also nounwind.
     if (TargetDecl->hasAttr<ConstAttr>()) {
       FuncAttrs.addAttribute(llvm::Attribute::ReadNone);
       FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
     } else if (TargetDecl->hasAttr<PureAttr>()) {
       FuncAttrs.addAttribute(llvm::Attribute::ReadOnly);
       FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
+    } else if (TargetDecl->hasAttr<NoAliasAttr>()) {
+      FuncAttrs.addAttribute(llvm::Attribute::ArgMemOnly);
+      FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
     }
     if (TargetDecl->hasAttr<RestrictAttr>())
       RetAttrs.addAttribute(llvm::Attribute::NoAlias);

clang/lib/Sema/SemaDeclAttr.cpp

@@ -4723,6 +4723,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_Mode:
     handleModeAttr(S, D, Attr);
     break;
+  case AttributeList::AT_NoAlias:
+    handleSimpleAttribute<NoAliasAttr>(S, D, Attr);
+    break;
   case AttributeList::AT_NoCommon:
     handleSimpleAttribute<NoCommonAttr>(S, D, Attr);
     break;

clang/test/CodeGen/ms-declspecs.c

@@ -33,7 +33,12 @@ __declspec(noinline) void t2() {}
 __declspec(noreturn) void f20_t(void);
 void f20(void) { f20_t(); }
 
+__declspec(noalias) void noalias_callee(int *x);
+// CHECK: call void @noalias_callee({{.*}}) [[NA:#[0-9]+]]
+void noalias_caller(int *x) { noalias_callee(x); }
+
 // CHECK: attributes [[NAKED]] = { naked noinline nounwind{{.*}} }
 // CHECK: attributes [[NUW]] = { nounwind{{.*}} }
 // CHECK: attributes [[NI]] = { noinline nounwind{{.*}} }
 // CHECK: attributes [[NR]] = { noreturn }
+// CHECK: attributes [[NA]] = { nounwind argmemonly{{.*}} }

clang/test/Parser/MicrosoftExtensions.c

@@ -5,7 +5,7 @@ typedef int (__cdecl *tptr)(void);
 void (*__fastcall fastpfunc)(void);
 extern __declspec(dllimport) void __stdcall VarR4FromDec(void);
 __declspec(deprecated) __declspec(deprecated) char * __cdecl ltoa( long _Val, char * _DstBuf, int _Radix);
-__declspec(safebuffers) __declspec(noalias) __declspec(restrict) void * __cdecl xxx(void *_Memory); /* expected-warning{{__declspec attribute 'safebuffers' is not supported}} expected-warning{{__declspec attribute 'noalias' is not supported}} */
+__declspec(safebuffers) __declspec(noalias) __declspec(restrict) void * __cdecl xxx(void *_Memory); /* expected-warning{{__declspec attribute 'safebuffers' is not supported}} */
 typedef __w64 unsigned long ULONG_PTR, *PULONG_PTR;
 
 void * __ptr64 PtrToPtr64(const void *p) {