Morten Ofstad
2004-Dec-06 14:45 UTC
[LLVMdev] FP Constants spilling to memory in x86 code generation
Hello, I've tracked down a new memory leak which started happening when I enabled constant propagation (I created my own passmanager which I run before calling getPointerToGlobal to JIT the function I have generated - is this OK?). The reason is that FP constants are being spilled to memory as there is no immediate fp load available. In my case this is a bit unnecessary since the constant is in fact a (constant) global variable which I have already added a memory mapping for. But the constant folding can of course end up making new constants which _have_ to be spilled to memory. I guess what I'd like to know is if the process of spilling constants to memory could be a bit more controlled, maybe using the JIT memory manager and putting it in with the function stubs? If not, I have to add tracking to the allocations here so they can be freed when the function is deleted. This is not so easy since there is no reference to the function which we are generating machine code for inside the copyConstantToRegister function. Any ideas? m. PS. Regarding my earlier post about function stubs -- The stub generation is working fine for the x86 target, it was Visual Studio generating the stubs I was seing because I have Fix&Continue turned on :-P
Chris Lattner
2004-Dec-08 02:30 UTC
[LLVMdev] FP Constants spilling to memory in x86 code generation
On Mon, 6 Dec 2004, Morten Ofstad wrote:> Hello, > > I've tracked down a new memory leak which started happening when I enabled > constant propagation (I created my own passmanager which I run before calling > getPointerToGlobal to JIT the function I have generated - is this OK?).Yes, that's perfectly ok! The interface to do this could probably be improved, but I definitely intended for JIT applications like yours to add standard (and even custom) passes to the pass manager to be run automatically before the JIT runs.> The reason is that FP constants are being spilled to memory as there is > no immediate fp load available. In my case this is a bit unnecessary > since the constant is in fact a (constant) global variable which I have > already added a memory mapping for. But the constant folding can of > course end up making new constants which _have_ to be spilled to memory.Okay, if you're using X86 FP math, there is really little choice except to spill the FP constant to the constant pool for a function. This should be completely transparent to you.> I guess what I'd like to know is if the process of spilling constants to > memory could be a bit more controlled, maybe using the JIT memory manager and > putting it in with the function stubs?Yes, this can and should definitely be improved. If you look at ExecutionEngine/JIT/JITEmitter.cpp:emitConstantPool, you can see that the JIT is just new'ing a block of memory for every constant pool that is needed. This is, admittedly, antisocial for your application, so if you'd like to make a memory manager for it, feel free.> If not, I have to add tracking to the allocations here so they can be > freed when the function is deleted. This is not so easy since there is > no reference to the function which we are generating machine code for > inside the copyConstantToRegister function. Any ideas?I think that adding something like the JITMemoryManager for the constant pools would make sense. I'm not sure that reusing the JITMemoryManager is a great idea, though it could be done. In particular, some architectures have cache problems when data and code live too close to each other. I'm not familiar with the details, but it seems safe to put constants somewhere that is not intentionally close to the code. Perhaps others have a more informed opinion about this than I do.> PS. Regarding my earlier post about function stubs -- The stub generation is > working fine for the x86 target, it was Visual Studio generating the stubs I > was seing because I have Fix&Continue turned on :-PAh, ok, good deal! :) -Chris -- http://nondot.org/sabre/ http://llvm.cs.uiuc.edu/
Morten Ofstad
2004-Dec-13 19:25 UTC
[LLVMdev] FP Constants spilling to memory in x86 code generation
Chris Lattner wrote:> On Mon, 6 Dec 2004, Morten Ofstad wrote: >> I guess what I'd like to know is if the process of spilling constants >> to memory could be a bit more controlled, maybe using the JIT memory >> manager and putting it in with the function stubs? > > Yes, this can and should definitely be improved. If you look at > ExecutionEngine/JIT/JITEmitter.cpp:emitConstantPool, you can see that > the JIT is just new'ing a block of memory for every constant pool that > is needed. This is, admittedly, antisocial for your application, so if > you'd like to make a memory manager for it, feel free.>> I think that adding something like the JITMemoryManager for the constant > pools would make sense. I'm not sure that reusing the JITMemoryManager > is a great idea, though it could be done. In particular, some > architectures have cache problems when data and code live too close to > each other. I'm not familiar with the details, but it seems safe to put > constants somewhere that is not intentionally close to the code. > Perhaps others have a more informed opinion about this than I do.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. 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. For the moment I will just delete the whole ExecutionEngine object and recompile everything every now and again. Although not a perfect solution, at least it works. m. -------------- next part -------------- An embedded and charset-unspecified text was scrubbed... Name: diff.txt URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20041213/636701ff/attachment.txt>
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