[LLVMdev] Work in progress patch to bug 2606

Garrison Venn gvenn.cfe.dev at gmail.com
Tue Feb 16 05:10:40 PST 2010

The patch I recently attached to bug 2606, reproduced here, is my first attempt to solve this issue
as represented by the attached test cases. To solve the foreign Module GlobalVariable problem,
I modified JIT::getOrEmitGlobalVariable(...) to directly attempt to map a found "external" GlobalVariable.
To solve the foreign Module Function problem, I modified both JIT.{h,cpp} and JITEmitter.cpp to emit
a stub for a found "external" Function instance, when that foreign Module defined Function has not
yet been compiled.

All areas of interest are marked with // GMV Mod:

As I've taken liberties with existing code, and thereby may have touched test cases that will fail, I would like
to have those members of the list that have time, review this proposal for comments/changes.

Major mods:


1)	I included Module.h.

2)	JIT::getPointerToFunction(...) was modified to search for a non-mapped function in all "other" modules.
	If found a stub is emitted, which will lazily emit the real function at runtime.

	One issue here is that functions that are external to all modules, have their mappings delayed by the
	above loop. I also kept the hasAvailableExternallyLinkage() the same as before, as I'm not sure if this
	should also go through the same module procedure. The stub emission will only take place for function
	definitions that are marked with external linkage. Should other cases be allowed? Should visibility also 
	be considered?

3)	JIT::getOrEmitGlobalVariable(...) was modified to search for non-mapped global variables in all "other"
	modules. If found, the address of the foreign global variable is requested, and mapped. There is no
	delay here.

	An attempt is first made to find the global outside of all modules, before attempting to find the function
	in the known modules. This is the reverse logic used in #2. On searching, unlike for functions, a linkage 
	check was not made. Also the global variable declaration and definition must have the exact same type. 
	Is this the correct approach?

4)    The declaration of void* forceEmitFunctionStub(Function *F) was added to the class JIT. This is 
	implemented in JITEmitter.cpp.


1)	JIT::forceEmitFunctionStub(...) was added. It turns around and calls JITResolver::forceEmitFunctionStub(...).

	Is the JIT:getPointerToFunction(...) code path sensitive to such call overhead?

2)	JITResolver::forceEmitFunctionStub(Function *F) was added. It is a simplified version of JITResolver::getLazyFunctionStub(...),
	which does not add to pending. It uses the same JITResolver::JITCompilerFn(...) for runtime emission that 
	the lazy compilation system uses.

	Outside of the goal, I'm not sure what code is unnecessary here. Notice that the hasAvailableExternallyLinkage
	case is dealt with here even though forceEmitFunctionStub(...) would not be entered for this case. I'm forced to 
	use JITResolver::JITCompilerFn(...) as there is a static relationship between JITResolver and say X86JITInfo. See
	implementation of X86JITInfo::getLazyResolverFunction(...) (X86JITInfo.cpp). Should a new JITCompilerFn like 
	function be created for the purposes of resolving this issue? This would of course require modifications to the 
	subclasses of TargetJITInfo.

3)	JITResolver::JITCompilerFn(...) was modified to no longer exit on being used for non-lazy compilation scenarios.

	This means of course that the error test path behavior for this function has been modified. See #2 above.

Beyond any issues with the patch, there is a question, in my mind, as to whether 2606 is really a bug. Sure its resolution makes 
using the JIT simpler for cross module behavior, but current manual solutions may be more fined grained in their approach. 
If so a fix to 2606 would circumvent such handling.

Thanks for any time spent on this



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