Re: [PATCH 0/3] Support for mandatory tail calls

[Jason Merrill <jason at redhat dot com>]

On Thu, May 19, 2016 at 12:30 AM, Basile Starynkevitch
<basile@starynkevitch.net> wrote:
> On 05/19/2016 12:12 AM, Jeff Law wrote:
>>
>> On 05/17/2016 04:01 PM, David Malcolm wrote:
>>>
>>> There have been requests [1] for libgccjit to better support
>>> functional programming by supporting the contination-passing style,
>>> in which every function "returns" by calling a "continuation"
>>> function pointer.
>>>
>>> These calls must be guaranteed to be implemented as a jump,
>>> otherwise the program could consume an arbitrary amount of stack
>>> space as it executed.
>>>
>>> This patch kit implements this.
>>>
>>> Patch 1 is a preliminary tweak to calls.c
>>>
>>> Patch 2 implements a new flag in tree.h: CALL_EXPR_MUST_TAIL_CALL,
>>> which makes calls.c try harder to implement a flagged call as a
>>> tail-call/sibling call, and makes it issue an error if
>>> the optimization is impossible.  It doesn't implement any
>>> frontend support for setting the flag (instead using a plugin
>>> to test it).  We had some discussion on the jit list about possibly
>>> introducing a new builtin for this, but the patch punts on this
>>> issue.
>>
>> I wonder if we should have an attribute so that the flag can be set for
>> C/C++ code.  I've seen requests for forcing tail calls in C/C++ code several
>> times in the past, precisely to support continuations.
>
> Why an attribute? Attributes are on declarations. I think it should better
> be some pragma like _Pragma(GCC tail cail, foo(x,y)) or some builtin (or
> else some syntax extension like goto return foo(x,y); ...) because what we
> really want is to annotate a particular call to be tail-recursive.

C++11 attributes can apply to expression-statements as well, e.g.

[[gnu::tail_call]] fn();

though not to sub-expressions.

Jason