[LLVMdev] Getting target machine specific information at run-time

[Roman Levenstein]

Hi David,


----- Ursprüngliche Mail ----
> Von: David Greene <dag at cray.com>
> An: llvmdev at cs.uiuc.edu
> Gesendet: Dienstag, den 7. Oktober 2008, 18:14:45 Uhr
> Betreff: Re: [LLVMdev] Getting target machine specific information at run-time
> 
> On Tuesday 07 October 2008 06:57, Roman Levenstein wrote:
> > Hi,
> >
> > I'm playing with some experimental register allocators for LLVM. One of
> > them needs to build a so-called register class tree for representing the
> > aliasing information among register classes. This tree is not function or
> 
> Hmm, it seems I've been here before...  :)


Nice to know ;)
 
> > I'd like to be able to compute this information only once during the
> > run-time, e.g. when target machine information is being created. But I
> > couldn't find out how it can be done. Then I thought that it should be
> > possible to do it in the doInitialization() function of my
> > MachineFunctionPass. Then it would be done once per LLVM module, which is
> > still acceptable. I almost implemented this solution, but I have problems
> > accessing the architecture dependent bits of the Module, i.e. TargetMachine
> > objects specific for the current architecture, as selected by the --march
> > option of the compiler. I tried using the getClosestTargetForJIT and
> > getClosestStaticTargetForModule functions of the TargetMachineRegistry
> > class. But all that was without success. I always get back only X86-based
> > targets (which is the target architecture of my development machine) even
> > though I explicitly select another target for cross-compilation by doing
> > --march=sparc or --march=alpha.
> 
> I did the once-per-module solution to this exact problem.  Here's what it 
> looks like:
> 
> class RA {
>     const TargetMachine* tm_;
>     const TargetRegisterInfo* tri_;
>     const TargetInstrInfo * tii_;
> [...]
> bool RA::runOnMachineFunction(MachineFunction &fn) {
>   mf_ = &fn;
>   tm_ = &fn.getTarget();
>   tri_ = tm_->getRegisterInfo();
>   tii_ = tm_->getInstrInfo();
> 
>   if (!initialized) {
>     build_class_tree();
>     initialized = true;
> 
> The "initialized" flag is there to make sure it only happens once per module.  

This solved my problem! Thanks! 
It was not obvious that the MachineFunctionPass objects are created only once per module.
Therefore didn't use this trick right from the beginning. BTW, is it described anywhere in the docs that MachineFunctionPass 
objects are created only once per module and not for each function? 
Is it also guaranteed or a property of the current PassManager implementation?

> You need a MachineFunction to get at the target info so I don't think it can 
> be done in doInitialization.


True.

Thanks, 
-Roman

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