llvm dev - Sep 2009 - [LLVMdev] [PATCH] & Question: Preserving ProfileInfo for backend.

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Hi,

the second part of my work is to preserve the profiling information
through all the transformation passes and make it available to the
backend machinery.

Attached is an example patch on how I plan to preserve the information
for a given transformation pass.

And now comes the question into place: whats the best way to attach the
profile info also the MachineBlocks and MachineFunctions? I was thinking
of converting the ProfileInfo into a template and using it for both
BasicBlocks and MachineBasicBlocks.

And where is the best point to transfer this information from the
bytecode CFG to the machinecode CFG?

Thanks, Andi

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On Sep 8, 2009, at 1:27 AM, Andreas Neustifter wrote:
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> Hash: SHA1
>
> Hi,
>
> the second part of my work is to preserve the profiling information
> through all the transformation passes and make it available to the
> backend machinery.
>
> Attached is an example patch on how I plan to preserve the information
> for a given transformation pass.
At a brief glance, this looks good.  It would be helpful to override the
verifyAnalysis method from the Pass class to verify that the profiling
information has been kept current.
>
> And now comes the question into place: whats the best way to attach  
> the
> profile info also the MachineBlocks and MachineFunctions? I was  
> thinking
> of converting the ProfileInfo into a template and using it for both
> BasicBlocks and MachineBasicBlocks.
>
> And where is the best point to transfer this information from the
> bytecode CFG to the machinecode CFG?
The SelectionDAGBuild phase is the only place where the precise  
relationship
between the BasicBlock CFG and the MachineBasicBlock CFG is
known.  It would also be possible to do the transfer in a later phase,  
though that
would require a fair amount of guesswork to determine how the two CFGs
correspond.

This does point out a limitation of doing instrumentation at the LLVM  
IR level;
it won't be able to cover branches inserted during CodeGen.

Dan

Hi,

Does the current LLVM backend support reading in profile information
(without preserving across transformations)? An earlier poster

http://groups.google.com/group/llvm-dev/browse_thread/thread/4bd65dbe84394bb7

noted that accessing execution counts in a MachineFunction pass (using
the BasicBlock* corresponding to the respective MachineBasicBlock)
returned 0 for all blocks.  Running llc with --debug-pass=Structure I
noticed that the NoProfileInfo pass was being executed.  I tried
adding a ProfileLoaderPass in the addPreRegAlloc function of the X86
target machine to load the profile information but receive the
following runtime error when the pass manager attempts to add the
ProfileLoader pass:

llc: <path to llvm>/llvm/lib/VMCore/PassManager.cpp:1597: virtual void
llvm::ModulePass::assignPassManager(llvm::PMStack&,
llvm::PassManagerType): Assertion `!PMS.empty() && "Unable to find
appropriate Pass Manager"' failed.

I'm not very familiar with the inner workings of the pass manager
framework.  Is there a simple fix that can allow existing profile
information to be loaded by backend passes?  I realize that the
profile data would not be completely accurate, but as a first order
approximation it could be useful until the proper framework is
implemented?

Thanks!