[PATCH] D12922: Add support for function attribute "notail"

[Duncan P. N. Exon Smith via cfe-commits]

I wonder if 'notailcall' be better, since it contains a verb (like 'noinline').  I don't have a strong opinion; just putting the alternative out there.

> On 2015-Sep-16, at 19:21, Akira Hatanaka via cfe-commits <cfe-commits at lists.llvm.org> wrote:
> 
> ahatanak created this revision.
> ahatanak added a reviewer: aaron.ballman.
> ahatanak added a subscriber: cfe-commits.
> 
> This patch adds support for a new function attribute "notail". The attribute is used to disable tail call optimization on calls to functions marked with the attribute.
> 
> This is different from the attribute proposed in D12547, which disables tail call optimizations on call sites within the marked function.
> 
> http://reviews.llvm.org/D12547
> 
> http://reviews.llvm.org/D12922
> 
> Files:
>  include/clang/Basic/Attr.td
>  include/clang/Basic/AttrDocs.td
>  lib/CodeGen/CGCall.cpp
>  lib/Sema/SemaDeclAttr.cpp
>  test/CodeGen/attr-no-tail.c
> 
> Index: test/CodeGen/attr-no-tail.c
> ===================================================================
> --- /dev/null
> +++ test/CodeGen/attr-no-tail.c
> @@ -0,0 +1,25 @@
> +// RUN: %clang_cc1 -triple x86_64-apple-macosx10.7.0 %s -emit-llvm -o - | FileCheck %s
> +
> +// CHECK: define i32 @callee0(i32 %a) [[ATTRNOTAIL0:#[0-9]+]] {
> +// CHECK: declare i32 @callee1(i32) [[ATTRNOTAIL1:#[0-9]+]]
> +// CHECK: declare i32 @callee2(i32) [[ATTRTAIL0:#[0-9]+]]
> +
> +int callee0(int a) __attribute__((notail)) {
> +  return a + 1;
> +}
> +
> +int callee1(int) __attribute__((notail));
> +
> +int callee2(int);
> +
> +int caller0(int a) {
> +  if (a > 0)
> +    return callee0(a);
> +  if (a < 0)
> +    return callee1(a);
> +  return callee2(a);
> +}
> +
> +// CHECK: attributes [[ATTRNOTAIL0]] = { {{.*}}notail{{.*}} }
> +// CHECK: attributes [[ATTRNOTAIL1]] = { {{.*}}notail{{.*}} }
> +// CHECK-NOT: attributes [[ATTRTAIL0]] = { {{.*}}notail{{.*}} }
> Index: lib/Sema/SemaDeclAttr.cpp
> ===================================================================
> --- lib/Sema/SemaDeclAttr.cpp
> +++ lib/Sema/SemaDeclAttr.cpp
> @@ -4878,6 +4878,9 @@
>   case AttributeList::AT_ReturnsTwice:
>     handleSimpleAttribute<ReturnsTwiceAttr>(S, D, Attr);
>     break;
> +  case AttributeList::AT_NoTail:
> +    handleSimpleAttribute<NoTailAttr>(S, D, Attr);
> +    break;
>   case AttributeList::AT_Used:
>     handleUsedAttr(S, D, Attr);
>     break;
> Index: lib/CodeGen/CGCall.cpp
> ===================================================================
> --- lib/CodeGen/CGCall.cpp
> +++ lib/CodeGen/CGCall.cpp
> @@ -1414,6 +1414,8 @@
>       FuncAttrs.addAttribute(llvm::Attribute::NoReturn);
>     if (TargetDecl->hasAttr<NoDuplicateAttr>())
>       FuncAttrs.addAttribute(llvm::Attribute::NoDuplicate);
> +    if (TargetDecl->hasAttr<NoTailAttr>())
> +      FuncAttrs.addAttribute(llvm::Attribute::NoTail);
> 
>     if (const FunctionDecl *Fn = dyn_cast<FunctionDecl>(TargetDecl)) {
>       const FunctionProtoType *FPT = Fn->getType()->getAs<FunctionProtoType>();
> Index: include/clang/Basic/AttrDocs.td
> ===================================================================
> --- include/clang/Basic/AttrDocs.td
> +++ include/clang/Basic/AttrDocs.td
> @@ -1612,3 +1612,10 @@
> arguments, with arbitrary offsets.
>   }];
> }
> +
> +def NoTailDocs : Documentation {
> +  let Category = DocCatFunction;
> +  let Content = [{
> +Tail call optimization is not performed on calls to a function marked ``notail``.
> +  }];
> +}
> Index: include/clang/Basic/Attr.td
> ===================================================================
> --- include/clang/Basic/Attr.td
> +++ include/clang/Basic/Attr.td
> @@ -1020,6 +1020,12 @@
>   let Documentation = [Undocumented];
> }
> 
> +def NoTail : InheritableAttr {
> +  let Spellings = [GNU<"notail">, CXX11<"clang", "notail">];
> +  let Subjects = SubjectList<[Function, ObjCMethod]>;
> +  let Documentation = [NoTailDocs];
> +}
> +
> def NoThrow : InheritableAttr {
>   let Spellings = [GCC<"nothrow">, Declspec<"nothrow">];
>   let Documentation = [Undocumented];
> 
> 
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