[PATCH 0/3] Support for mandatory tail calls

[Basile Starynkevitch]

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.

Cheers

-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
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