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Re: [PATCH 0/3] Support for mandatory tail calls
- From: Basile Starynkevitch <basile at starynkevitch dot net>
- To: Jeff Law <law at redhat dot com>, David Malcolm <dmalcolm at redhat dot com>, gcc-patches at gcc dot gnu dot org, jit at gcc dot gnu dot org
- Date: Thu, 19 May 2016 06:30:58 +0200
- Subject: Re: [PATCH 0/3] Support for mandatory tail calls
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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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