[LLVMdev] code-altering Passes for llc

[Dan Gohman]

On Aug 2, 2009, at 7:09 AM, Artjom Kochtchi wrote:

>
> Greetinigs,
>
> I am extending llc to include runtime checks for calls (in X86). So  
> a call
> 'call target' is altered to look like this:
>
>  [some check]
>  jne error_function
>  call target
>
> I've done this by implementing a MachineFunctionPass that is  
> instantiated
> and added to the PassManager in X86TargetMachine::addPreRegAlloc.
>
> In order to create the jne-instruction I need some BasicBlock that  
> contains
> the error routine. So I tried to create a ModulePass to insert some  
> basic
> block e. g. in the beginning of the program, to use it for the
> jne-instruction. Unfortunately, llc then produces the error
>
>  llc: PassManager.cpp:1597:
>    virtual void llvm::ModulePass::assignPassManager(llvm::PMStack&,
> llvm::PassManagerType):
>    Assertion `!PMS.empty() && "Unable to find appropriate Pass  
> Manager"'
> failed.
>
> Adding a MachineFunctionPass the same way works fine. I've read here  
> on the
> mailing list that some Pass dependencies cause this error, but I  
> have no
> dependencies specified.
>
> How do I get the ModulePass to run? Is a ModulePass the right way to
> accomplish my aim, altogether?

You don't need a ModulePass to modify a CFG. A MachineFunctionPass
is fine for this.

The actual problem may be that your pass doesn't preserve some
Analysis that CodeGen is using. Since CodeGen is function-oriented,
ModulePasses cannot be freely scheduled.

>
> One more question concerning passes:
> Clearly, inserting a jne-instruction changes control flow, so the  
> CFG is not
> up to date after the pass. Do I set AnalysisUsage in  
> getAnalysisUsage to
> preserve nothing? In that case, is the CFG updated automatically by  
> LLVM or
> do I need to apply the changes during the Pass not only the the
> MachineFunction, but also the the CFG?

The default if you don't override getAnalysisUsage is that everything is
considered clobbered.  With a MachineFunctionPass, you can modify
the MachineFunction (including the MachineBasicBlock CFG), but
you shouldn't modify the LLVM IR Function.

Dan