llvm dev - Oct 2012 - [LLVMdev] problem trying to write an LLVM register-allocation pass

I'm trying to write a MachineFunctionPass to do register allocation.  I 
have code that worked with an old version of LLVM.  It does not work 
with llvm-3.1. (or various other versions that I've tried).

The first problem is that including this line:

  AU.addRequiredID(TwoAddressInstructionPassID);

in method getAnalysisUsage causes a runtime error:

Unable to schedule 'Eliminate PHI nodes for register allocation' 
required by 'Unnamed pass: implement Pass::getPassName()'
Unable to schedule pass
UNREACHABLE executed at ...

I'm invoking the pass like this (given input file foo.c):

clang -emit-llvm -O0 -c foo.c -o foo.bc
opt -mem2reg foo.bc > foo.ssa
mv foo.ssa foo.bc
llc -load Debug/lib/P4.so -regalloc=gc foo.bc


I've attached my entire file (it's very short).  Any help would be much 
appreciated!

Susan Horwitz
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Hi Susan,

The meaning of "addRequired(X)" is that your pass needs X to be run,
and
for X to be preserved by all passes that run after X and before your
pass. The PHIElemination and TwoAddressInstruction passes do not preserve
each other, hence there's no way for the pass manager to schedule them for
you if you addRequire(...) them.

The trick is that CodeGen will schedule both of these passes to be run
before _any_ register allocation pass (see Passes.cpp), so you needn't
require them explicitly - you can just assume they have been run. If you
just remove those lines from your getAnalysisUsage method your pass should
now run as you expect.

Cheers,
Lang.

On Wed, Oct 31, 2012 at 1:46 PM, Susan Horwitz <horwitz at cs.wisc.edu>
wrote:
> I'm trying to write a MachineFunctionPass to do register allocation.  I
> have code that worked with an old version of LLVM.  It does not work with
> llvm-3.1. (or various other versions that I've tried).
>
> The first problem is that including this line:
>
>  AU.addRequiredID(**TwoAddressInstructionPassID);
>
> in method getAnalysisUsage causes a runtime error:
>
> Unable to schedule 'Eliminate PHI nodes for register allocation'
required
> by 'Unnamed pass: implement Pass::getPassName()'
> Unable to schedule pass
> UNREACHABLE executed at ...
>
> I'm invoking the pass like this (given input file foo.c):
>
> clang -emit-llvm -O0 -c foo.c -o foo.bc
> opt -mem2reg foo.bc > foo.ssa
> mv foo.ssa foo.bc
> llc -load Debug/lib/P4.so -regalloc=gc foo.bc
>
>
> I've attached my entire file (it's very short).  Any help would be
much
> appreciated!
>
> Susan Horwitz
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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Thanks Lang!

Here's another question: I'm trying to process this input:

int main() {
   return 0;
}

but I'm getting an error
  Assertion `!Fn.getRegInfo().getNumVirtRegs() && "Regalloc must
assign
all vregs"' failed.

At the start of runOnMachineFunction I call 
Fn.getRegInfo().getNumVirtRegs();
and find that there is 1 virtual register.  However,  MRI->reg_empty(vreg)
tells me that it is not used or defined.  So my register-allocation code 
never sees it, and thus can't allocate a preg for it.  I tried using 
MRI->replaceRegWith(vreg, preg);
(where preg is available to vreg's register class) but that didn't work.
  When I look, the number of vregs in the function is still 1.

Can you help with this?

Thanks again!

Susan

On 10/31/2012 04:55 PM, Lang Hames wrote:
> Hi Susan,
>
> The meaning of "addRequired(X)" is that your pass needs X to be
run, and
> for X to be preserved by all passes that run after X and before your
> pass. The PHIElemination and TwoAddressInstruction passes do not
> preserve each other, hence there's no way for the pass manager to
> schedule them for you if you addRequire(...) them.
>
> The trick is that CodeGen will schedule both of these passes to be run
> before _any_ register allocation pass (see Passes.cpp), so you needn't
> require them explicitly - you can just assume they have been run. If you
> just remove those lines from your getAnalysisUsage method your pass
> should now run as you expect.
>
> Cheers,
> Lang.
>
> On Wed, Oct 31, 2012 at 1:46 PM, Susan Horwitz <horwitz at cs.wisc.edu
> <mailto:horwitz at cs.wisc.edu>> wrote:
>
>     I'm trying to write a MachineFunctionPass to do register
allocation.
>       I have code that worked with an old version of LLVM.  It does not
>     work with llvm-3.1. (or various other versions that I've tried).
>
>     The first problem is that including this line:
>
>       AU.addRequiredID(__TwoAddressInstructionPassID);
>
>     in method getAnalysisUsage causes a runtime error:
>
>     Unable to schedule 'Eliminate PHI nodes for register
allocation'
>     required by 'Unnamed pass: implement Pass::getPassName()'
>     Unable to schedule pass
>     UNREACHABLE executed at ...
>
>     I'm invoking the pass like this (given input file foo.c):
>
>     clang -emit-llvm -O0 -c foo.c -o foo.bc
>     opt -mem2reg foo.bc > foo.ssa
>     mv foo.ssa foo.bc
>     llc -load Debug/lib/P4.so -regalloc=gc foo.bc
>
>
>     I've attached my entire file (it's very short).  Any help would
be
>     much appreciated!
>
>     Susan Horwitz
>
>     _______________________________________________
>     LLVM Developers mailing list
>     LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
http://llvm.cs.uiuc.edu
>     http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>