[LLVMdev] Interprocedural Register Allocation

[Madhusudan C.S]

Hi Jakob,

I have been trying to learn how the CodeGen passes work and I am playing
around with the -debug-pass option. I tried implementing a bare
CallGraphSCCPass based Pass in the CodeGen which basically does nothing for
now. I mostly tried to replicate what RegAlloc passes do. I did this
instead of modifying the existing RegAlloc passes to use CallGraphSCCPass
because that was becoming way too complicated for simple experiments I was
trying when I tried to do that. I am attaching the diffs of what I actually
tried to do for reference here. It is not really well documented and there
is a lot of copy/paste code there, but well, this is not really the final
code to be submitted or anything, just to play around, so I have not really
put efforts into comments etc. yet.

So when I build LLVM and try to run the pass with the following command:
$ llc --debug -cgregalloc=cg <$HOME/cpptry/manyfuncs.bc

I get the following error.

Args: llc --debug -cgregalloc=cg
Subtarget features: SSELevel 6, 3DNowLevel 0, 64bit 1
Pass ID not registered
UNREACHABLE executed at /media/python/workspace/llvm/
lib/CodeGen/Passes.cpp:324!
0  llc             0x0000000001453dfe
1  llc             0x00000000014542fa
2  libpthread.so.0 0x00007f020b750cb0
3  libc.so.6       0x00007f020a99f425 gsignal + 53
4  libc.so.6       0x00007f020a9a2b8b abort + 379
5  llc             0x000000000143b1a6
6  llc             0x0000000000f3cb02 llvm::TargetPassConfig::addPass(void
const*) + 146
7  llc             0x0000000000f3ddef
llvm::TargetPassConfig::addCGRegAlloc(llvm::CallGraphSCCPass*) + 47
8  llc             0x0000000000f3d7df
llvm::TargetPassConfig::addMachinePasses() + 719
9  llc             0x0000000000e5a3af
10 llc             0x0000000000e5994c
llvm::LLVMTargetMachine::addPassesToEmitFile(llvm::PassManagerBase&,
llvm::formatted_raw_ostream&, llvm::TargetMachine::CodeGenFileType, bool,
void const*, void const*) + 92
11 llc             0x00000000005f6002 main + 4946
12 libc.so.6       0x00007f020a98a76d __libc_start_main + 237
13 llc             0x00000000005f4bd1
Stack dump:
0.    Program arguments: llc --debug -cgregalloc=cg
Aborted (core dumped)

I am not really sure, why it says "Pass ID is not registered". I did quite
a lot of internet searching about it and whatever I try to look for I end
up on the exact LLVM line of code which prints this error line. So, I am
not sure what is going on. Is this because CallGraphSCCPass does not
register anything with the MachineFunctionPass-es? Or is this because
CallGraphSCCPass does not do some magic which is specific to CodeGen
passes? Can you please help me with this a bit? May be some pointers?


On Fri, Nov 2, 2012 at 2:03 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:

>
> On Oct 31, 2012, at 1:41 PM, Madhusudan C.S <madhusudancs at gmail.com>
> wrote:
>
> >   I have spent last 4 weeks trying to figure out how to implement
> > Interprocedural Register Allocation. I must admit that I was really
> > overwhelmed with LLVM's codebase while trying to figure this out :)
> > There is so much to know! I think I have reached a point where I
> > have some sort of basic understanding of what needs to be done,
> > but I need some help from here on. So here is the summary of
> > what I know and for what I need more help.
> >
> > I see that lib/CodeGen/Passes.cpp adds PrologEpilogInserter (PEI
> > henceforth) pass after the RegAlloc pass. Do I understand correctly
> > that LLVM first performs a given pass on all the functions before
> > moving to the next pass?
>
> No, it's the other way around.
>
> You may want to read up on the pass manager and play with the -debug-pass
> option.
>
> /jakob
>
>


-- 
Thanks and regards,
  Madhusudan.C.S
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