For example, I can easily define entry points for arbitrary functions. At run time I can choose which entry point to call and benchmark that entry point using existing performance benchmarking tools like the very powerful and comprehensive "perf" tool on linux. This provides a very powerful way to analyze the performance for free and in a convenient manner with little instrumentation. I do not want the main entry point to be clobbered for such things. Ideally I would just use a simple annotation for any arbitrary function to convert it into an entry point that can be chosen at runtime. I am just giving a general idea here without going into specifics. -harendra On 12 November 2017 at 21:58, Shea Levy wrote:

What would you be able to achieve with this that you couldn't achieve with branching in a fixed custom main function?

Thanks, Shea

Harendra Kumar writes:

...

Hi,

GHC allows choosing a main function at link time using the "-main-is" option. I was wondering if there is a possibility to choose the main function at runtime. Or even better, if something equivalent to "ghc -e" is somehow possible in a linked binary executable. If not, are there any plans to achieve something like that in future? Are there any theoretical, practical obstacles to that?

Thanks, Harendra _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

On 12 November 2017 at 23:18, Ben Gamari wrote:

Harendra Kumar writes:

...

Hi,

GHC allows choosing a main function at link time using the "-main-is" option. I was wondering if there is a possibility to choose the main function at runtime. Or even better, if something equivalent to "ghc -e" is somehow possible in a linked binary executable. If not, are there any plans to achieve something like that in future? Are there any theoretical, practical obstacles to that?

In general it's not easy to get a reference to a an arbitrary function in an object file by it's source name. Unless the symbol is exported the compiler is free to optimize the function or even drop it altogether.

I understand that. I was also wondering if there is way (some sort of annotation, pragma or any other workaround) to keep the symbols around without actually exporting them. Exporting either clobbers the module interface from user point of view or the modules need a wrapper to avoid exporting such symbols in user exposed modules. I would also expect an option to remove the effect of any such annotation so that the production build is not under optimized because of this. This is in fact one of the problems that I have been facing in general. I do not know of a good way to hide symbols from users but use them in tests and benchmarks during dev. What is the GHC recommended way to achieve this?

If you can guarantee that the functions you are about are exported then in principle you could probably do what you ask from a library. Just `dlopen` the executable (which would need to be compiled as a position independent exectuable), and dlsym the appropriately mangled name.

Yeah that is a good option for dynamically linked executables. In fact I have been using dlopen to implement Haskell plugins but did not think of using it for this use case. Thanks for reminding me. In a static executable, does it make sense to make the RTS choose an exported symbol as entry point using an RTS option? -harendra

On 13 November 2017 at 00:29, Saurabh Nanda wrote:

...

In fact I have been using dlopen to implement Haskell plugins but did not think of using it for this use case.

Absolutely OT. Is this technique different from what facebook/simonmar have recently open-sourced? Please blog about it.

Take a look at this: http://hackage.haskell.org/package/plugins -harendra

On 13 November 2017 at 03:47, Ben Gamari wrote:

Harendra Kumar writes:

...

On 12 November 2017 at 23:18, Ben Gamari wrote:

...

In general it's not easy to get a reference to a an arbitrary function in an object file by it's source name. Unless the symbol is exported the compiler is free to optimize the function or even drop it altogether.

I understand that. I was also wondering if there is way (some sort of annotation, pragma or any other workaround) to keep the symbols around without actually exporting them. Exporting either clobbers the module interface from user point of view or the modules need a wrapper to avoid exporting such symbols in user exposed modules. I would also expect an option to remove the effect of any such annotation so that the production build is not under optimized because of this. This is in fact one of the problems that I have been facing in general. I do not know of a good way to hide symbols from users but use them in tests and benchmarks during dev. What is the GHC recommended way to achieve this?

The usual way of dealing with this is to expose the symbols from a `.Internal` module.

That's what I meant when I referred to a wrapper module above. However, it requires you to create two modules even if one extra symbol is needed for dev purposes, a lot of boilerplate for a simple thing. Also, the symbols are anyway exposed to the users, we just ask the users to not look at those. I am wondering if using a preprocessor (CPP) conditional around the optional exports is a better way then, you will have to build differently but you won't have to create another module and expose extra symbols? -harendra