[Mesa-dev] [PATCH] radeon/llvm: Use LLVM C API for compiling LLVM IR to ISA.

[Mathias Fröhlich]

Tom,

> -class LLVMEnsureMultithreaded {
> -public:
> -   LLVMEnsureMultithreaded()
> -   {
> -      llvm_start_multithreaded();
> -   }
> -};
> -
> -static LLVMEnsureMultithreaded lLVMEnsureMultithreaded;

Removing this leads to crashes in llvm with applications that concurrently 
work on different gl contexts.

When I did this change I also provided a testcase that never made it into 
piglit. See:
http://lists.freedesktop.org/archives/piglit/2012-August/003100.html
I had addresssed the concerns at that time with a v2 patch but got lost 
without being checked in. May be somebody can push this now?

Regarding the point where this funciton is called I had choosen static 
initialization time since llvm requires this function to be called single 
threaded which we cannot guarantee in any case. Keep in mind that you need to 
ensure this function called non concurrently even against applications that 
itself already use the llvm libs in some way while the driver is loaded.
But the best bet is to do this in the dynamic loder which is itself 
serialized, so I could avoid calling this function concurrently by 
initialization of different contexts. That should at least shield against 
applications that itself do the same trick by calling this funtion in the 
dlopen phase in some static initializer ...
We may get around part of this problem with dlopening the driver with better 
isolation but up to now the problem can get that far.

You can find more thoughts about that problem regarding llvmpipe which still 
has this problem is written down here:
http://lists.freedesktop.org/archives/mesa-dev/2013-January/033015.html

So, a better place where you can switch llvm to be multithreaded is 
apprecheated. But please make sure it happens at some point, the earlier, the 
better.

If you want to have plain c files that behaves the same like what we have today 
you can use a gcc extension (untested):

static void r600_llvm_init() __attribute__ ((__constructor__));
void r600_llvm_init()
{
  llvm_start_multithreaded();
}

Greetings

Mathias