llvm dev - Oct 2008 - [LLVMdev] Getting target machine specific information at run-time

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
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu        http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On Wednesday 08 October 2008 04:34, Roman Levenstein wrote:
> 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?
I don't think it's written anywhere but I asked on the list and Devang 
indicated that this is the case.

                                            -Dave