On Thu, May 19, 2016 at 3:19 PM, Jason Merrill <ja...@redhat.com> wrote: > On Thu, May 19, 2016 at 12:30 AM, Basile Starynkevitch > <bas...@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.
That's nice. Can they apply to things like loops? [[gnu::no_unroll]] for (int i=0; i<4; ++i) a[i] = 0; Richard. > Jason
