[LLVMdev] [RFC] [PATCH] add tail call optimization to thumb1-only targets

[Bjoern Haase]

Am 13.01.2015 um 12:59 schrieb Bruce Hoult:
> That link says "currently supported on x86/x86-64 and PowerPC". The purpose
> of the current patch appears to be adding support for this flag for Thumb-1?
>
> (with the implication that it is already supported for Thumb-2 and ARM32
> and the documentation is out of date)
>
> I could, of course, be confused.
>
> On Tue, Jan 13, 2015 at 11:55 PM, John Brawn <john.brawn at arm.com> wrote:
>
>> Forcing tailcalls, even when it isn’t profitable in terms of performance
>> (or space in the case of
>>
>> -Os, though I can’t off the top of my head think of any case where a
>> faster tail call would also
>>
>> be larger), is what the -tailcallopt option is for: see
>>
>> http://llvm.org/docs/CodeGenerator.html#tail-call-optimization
>>
The patch is for enabling tail call and sibling call optimization in the
form that is currently also
supported for thumb2 and arm.

> Bruce Hoult wrote :
>
>TCO is to minimise the use of precious stack space, and in fact to allow
>certain algorithms and program transformations to run in constant stack
>space.
>
>
>If a programmer assumes that TCO is available and writes their program
>using continuation-passing style, and then TCO does not actually happen,
>that is a correctness issue and the program will overflow the stack and
>crash very quickly.

My motivation for this patch is specifically, that I am dealing with a
communication stack where
state machines use continuation-passing style coding and where tail call
optimization
is important. The present level of sibling/tail call optimization
supported by thumb2 will do.
With the current head version for thumb1 we have a problem.

Therefore I'd very much appreciate having tail-call optimization
available also for thumb1 in llvm
mainline. Personally, I would not mind requiring a special compile
switch for activating the support for thumb1.
I.e. not activating it as part of the standard optimization levels Os,
O2 or O3.

Yours,

Björn.