[LLVMdev] problem trying to write an LLVM register-allocation pass

Lang Hames lhames at gmail.com
Wed Oct 31 14:55:50 PDT 2012

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.


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
> _______________________________________________
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> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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