[LLVMdev] FP Constants spilling to memory in x86 code generation

[Morten Ofstad]

Chris Lattner wrote:
> On Mon, 13 Dec 2004, Morten Ofstad wrote:
> 
>> I have made a patch along these lines. Although I reused the 
>> JITMemoryManager object, I am allocating constant pools from another 
>> block of memory. This fixes my remaining leaks. It would be nice if 
>> also the global variables were allocated in this way, but it's not 
>> needed for my application since I'm managing that memory myself and 
>> using ExecutionEngine::addGlobalMapping.
> 
> Ok, sounds good.  Here are some comments:
> 
> 3. This will fail if the JIT wants to allocate more than 512K of
>    constants.  Can you just have it allocate another block of memory if it
>    runs out of space?  Also, it might be useful to start the initial block
>    much smaller, say 4K of memory, and double it when space is
>    exhausted, as most programs don't use 1/2 meg of constant pools :)

I just wanted to keep it simple - the allocation of memory for functions 
is done in the same way, grab a huge block and hope it's enough. I think 
the whole JITMemoryManager needs to be improved, this is just a 
temporary solution to the memory leak problem. My philosophy is that if 
you can't do it properly, at least change as little as possible...

I attached the updated patch which fit in 80 columns.

>> Later on I'm going to need either a way of freeing memory for 
>> functions/constant pools or a way of recovering from out of memory, as 
>> our application is going to run as a server and hopefully be happily 
>> JIT'ing away for days on end.
> 
> There is a (currently unimplemented) method for doing this:
> ExecutionEngine::freeMachineCodeForFunction.
> 
> It should be straight-forward to free the memory for a function, though 
> it will make the JITEmitter a bit more complex (it will have to track 
> regions of freed memory) to reallocate them.

I think the reason why it's still unimplemented is because it's not at 
all straight-forward. The problem is that the amount of memory needed to 
compile a function is only known _after_ the function is compiled. The 
current system just writes the functions one after another into a large 
block of memory, but if you want to re-use free'd space you need to know 
in advance that it's large enough to hold your function.

One possible solution is to do some low level code to manage memory 
pages. The idea is to count how much live code is on a page and if it 
reaches 0 you return the page to the OS thus creating a gap in the 
address space. This way you don't have to move anything and you can keep 
writing new functions at the end.

Another possible solution is to compile the functions to a buffer and 
then move them to the smallest free block which is big enough to contain 
the function when the compilation is finished and you know the size of 
the function. This approach require relocation information to be 
generated as part of the compilation process.

m.

PS. Happy new year everyone!
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