[Marc Feeley here, the author of the Gambit Scheme compiler that generates the code which would benefit from tail-calls]
What is needed (for Gambit) is a way for the programmer to express in the source code that the tail-call is known to be optimizable, i.e. it does not violate any of the tail-calling constraints, such as all local variables are dead (including those whose address was taken with “&var”) at the moment of the call.
The code generated by the Gambit compiler is sufficiently complex that it would be impossible (in the halting problem sense) to check that the tail-calling constraints are not violated. However, I know the constraints are not violated because the code was designed that way. And if I’m mistaken in this belief, then I am the one that will bare the blame of the ensuing problems.
So having a way to check during or after the compilation that tail-calls were optimized is not sufficient. A Scheme compiler that sometimes fails to compile is not very useful. What is needed is a guarantee that specific C calls are optimized.
FYI the code generated by Gambit is put in “host” functions that look like this:
typedef struct ___processor_state_struct { … };
typedef struct ___processor_state_struct *___processor_state;
typedef void (*___host)(___processor_state ___ps);
static void host1(___processor_state ___ps) {
…
___host h = …;
h(___ps);
}
So the last statement in the host function is a call to the next host function. The parameter is the same type, and indeed the same value (a pointer to a structure containing the state of the virtual machine such as registers), and there is no result.
I would think this is the easiest situation to handle for a tail-call optimization.
Marc
> On Dec 9, 2021, at 5:57 AM, Florian Weimer <fweimer@xxxxxxxxxx> wrote:
>
> * Segher Boessenkool:
>
>> On Wed, Dec 08, 2021 at 04:17:33PM -0700, Jeff Law via Gcc-help wrote:
>>> On 12/7/2021 3:38 PM, Bradley Lucier via Gcc-help wrote:
>>>> So I've been investigating whether gcc's -foptimize-sibling-calls
>>>> option (for which I've found very little documentation) might produce
>>>> similar results.
>>> The option has been around for probably 20+ years at this point, you
>>> just might not have been aware of it. They try, but do not guarantee
>>> tail call optimization. They have all kinds of checks that might cause
>>> any particular call site to be rejected for tail call elimination. It's
>>> enabled by default at O2 or higher.
>>
>> (Including -Os.)
>>
>> GCC will never not do a tail call if it knows any way to do it.
>
> GCC will never do a tail call if the target function is known not to
> return normally, to preserve the stack backtrace.
>
> Thanks,
> Florian
