forcing weak purity

[Artur Skawina]

On 05/23/12 13:45, Steven Schveighoffer wrote:
> On Tue, 22 May 2012 23:31:59 -0400, Alex Rønne Petersen <alex at lycus.org> wrote:
> 
>> On 23-05-2012 05:22, Steven Schveighoffer wrote:
>>
>>> This solution looks crappy to me:
>>>
>>> void gc_collect(void *unused = null);
>>
>> BTW, any compiler with alias analysis and LTO might even decide to remove the call even with the unused parameter, since it, well, isn't used. I think we need a language-level solution here.
> 
> The LTO would have to be able to make decisions at link time based on purity, because the call *does* do things, it just doesn't use the parameter.  I have no idea, maybe you are right, it would be a hard problem to fix if this happened.
>

A compiler can, once it notices the unused argument(s), (more or less) easily rewrite
(clone) the function and modify the callers to use the new version. And when the LTO
pass (re)compiles the program (from the intermediate representation in the object files)
it could already see the cloned version, and optimize based on that. So it is possible.

artur