llvm dev - Jan 2011 - [LLVMdev] Marking a function prototype as being "persistent"

Hello and happy new year,


I'm using LLVM to JIT-compile the XL programming language. I've recently
added a whole-program optimization phase that gives excellent results, but I
noticed that the StripDeadPrototypesPass was removing all references to my
runtime support functions.

Apparently, this depends on the oddly-named UnitAtATime parameter. Set it to
false. Try again. Suggestion: what about a comment explaining what
"UnitAtATime" means :-)

Even then, my runtime function prorotypes are still being stripped away by the
GlobalDCE added by createStandardLTOPasses.

So my question is: what is the correct, officially recommended way to reference
runtime functions so that global DCE won't get rid of them? Or am I supposed
to re-create the corresponding Function instances for every translation?


Thank you in advance for enlightening me
Christophe

Hi Christophe,
> I'm using LLVM to JIT-compile the XL programming language. I've
recently added a whole-program optimization phase that gives excellent results,
but I noticed that the StripDeadPrototypesPass was removing all references to my
runtime support functions.
why is that a problem?
> Apparently, this depends on the oddly-named UnitAtATime parameter. Set it
to false. Try again. Suggestion: what about a comment explaining what
"UnitAtATime" means :-)
If UnitAtATime is false this means that you are optimizing functions as you are
generating them, rather than first generating all functions and other globals
and only then optimizing.  It's basically a historical anachronism coming
from
the way GCC used to work.
> Even then, my runtime function prorotypes are still being stripped away by
the GlobalDCE added by createStandardLTOPasses.
Sure, and why not?  Unused prototypes are not used for anything, they won't
turn up in the generated code for example.
> So my question is: what is the correct, officially recommended way to
reference runtime functions so that global DCE won't get rid of them? Or am
I supposed to re-create the corresponding Function instances for every
translation?
There's some kind of disconnect here, so you need to explain more :)

Ciao, Duncan.

On Thu, Jan 6, 2011 at 10:06 AM, Duncan Sands <baldrick at free.fr>
wrote:
>> So my question is: what is the correct, officially recommended way to
reference runtime functions so that global DCE won't get rid of them? Or am
I supposed to re-create the corresponding Function instances for every
translation?
>
> There's some kind of disconnect here, so you need to explain more :)
Presumably since he's using the JIT he's generating more code later
that uses the runtime functions, which he wants to be marked as
available.

The disconnect is wanting to apply whole-program/LTO type
optimizations before you have the whole program.

I think the answer is to not use the standard LTO passes and to create
your own versions that are careful not to do things like delete types
and prototypes that you might need later.

Reid