ahatanak added inline comments.
================
Comment at: include/clang/Basic/Attr.td:894
@@ -893,1 +893,3 @@
+def DisableTailCalls : InheritableAttr {
+ let Spellings = [GNU<"disable_tail_calls">,
----------------
aaron.ballman wrote:
> Pardon me if this is obvious, but -- are there times when you would want to
> disable tail calls but leave other optimizations in place? I'm trying to
> understand why these attributes are orthogonal.
Yes, that's correct. The new function attribute we are adding shouldn't disable
other optimizations that are normally run at -O2 or -O3.
================
Comment at: include/clang/Basic/Attr.td:896
@@ +895,3 @@
+ let Spellings = [GNU<"disable_tail_calls">,
+ CXX11<"clang", "disable_tail_calls">];
+ let Subjects = SubjectList<[Function, ObjCMethod]>;
----------------
aaron.ballman wrote:
> Should this attribute be plural? Are there multiple tail calls within a
> single method that can be optimized away?
I named it after the existing IR function attribute and I'm guessing the plural
name was chosen because functions can have multiple tail call sites. I think
the singular name is better if we want this attribute to mean "disable tail
call optimization".
================
Comment at: include/clang/Basic/AttrDocs.td:1619
@@ +1618,3 @@
+ let Content = [{
+Use ``__attribute__((disable_tail_calls)))`` to instruct the backend not to do
tail call optimization.
+ }];
----------------
aaron.ballman wrote:
> I would avoid using the exact syntax here because this is a GNU and C++
> attribute. Could just say:
>
> The ``disable_tail_calls`` attribute instructs the backend to not perform
> tail call optimization.
OK, done.
================
Comment at: lib/Sema/SemaDeclAttr.cpp:4882
@@ +4881,3 @@
+ case AttributeList::AT_DisableTailCalls:
+ handleSimpleAttribute<DisableTailCallsAttr>(S, D, Attr);
+ break;
----------------
aaron.ballman wrote:
> Okay, that makes sense. I can contrive examples of noreturn where TCO could
> happen, it just took me a while to think of them. ;-)
>
> What about other semantic checks, like warning the user about disabling TCO
> when TCO could never be enabled in the first place? Can you disable TCO for
> functions that are marked __attribute__((naked))? What about returns_twice?
>
> Unfortunately, we have a lot of attributes for which we've yet to write
> documentation, so you may need to look through Attr.td.
Since "naked" allows only inline assembly statements, it should be an error to
have disable-tail-calls and naked on the same function. I made changes in Sema
to detect that case.
My understanding is that you can't do tail call optimization on call sites of a
function that calls a "return_twice" function. However, attaching
"return_twice" on the calling function doesn't block tail call optimizations on
the call sites inside the function.
I didn't find any other attributes that seemed incompatible with
disable-tail-calls.
http://reviews.llvm.org/D12547
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