[LLVMdev] FP Constants spilling to memory in x86 code generation
sabre at nondot.org
Tue Dec 7 18:30:00 PST 2004
On Mon, 6 Dec 2004, Morten Ofstad wrote:
> 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 :-P
Ah, ok, good deal! :)
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