[LLVMdev] Changes in MachineInstruction/Peephole Optimizer?

[Alkis Evlogimenos]

Anshu Dasgupta wrote:
> Hi all,
> 
> The register allocator that I implemented is failing in the LLVM cvs 
> version, but not in LLVM 1.1. The generated code fails a check in the 
> x86 peephole optimizer:
> 
> llc: PeepholeOptimizer.cpp:128: bool 
> <unnamed>::PH::PeepholeOptimize(llvm::Machi
> neBasicBlock&, llvm::ilist_iterator<llvm::MachineInstr>&): Assertion 
> `MI->getNum
> Operands() == 2 && "These should all have 2 operands!"' failed.
> 
> 
> I've tracked it down to a difference between LLVM cvs and LLVM 1.1 in 
> PeepholeOptimizer.cpp:
> 
> In LLVM 1.1, PeepholeOptimizer.cpp: line 70:
> 
>  case X86::ADDri16:  case X86::ADDri32:
>   case X86::SUBri16:  case X86::SUBri32:
>   case X86::IMULri16: case X86::IMULri32:
>   case X86::ANDri16:  case X86::ANDri32:
>   case X86::ORri16:   case X86::ORri32:
>   case X86::XORri16:  case X86::XORri32:
>     assert(MI->getNumOperands() == 3 && "These should all have 3 
> operands!");
> 
> 
> While in the LLVM cvs version, PeepholeOptimizer, line 123:
> 
>  case X86::ADDri16:  case X86::ADDri32:
>   case X86::SUBri16:  case X86::SUBri32:
>   case X86::ANDri16:  case X86::ANDri32:
>   case X86::ORri16:   case X86::ORri32:
>   case X86::XORri16:  case X86::XORri32:
>     assert(MI->getNumOperands() == 2 && "These should all have 2 
> operands!");
> 
> 
> So, 1.1 and cvs expect different number of operands for the same machine 
> instruction. Do I have to change something in the register allocator to 
> account for this? Any idea why its working in 1.1 but not in the CVS 
> version?

This is due to a change in the requirements on the register allocators. 
In cvs register allocators need to make sure two-address instructions 
are correctly setup *and* remove the extra operand. This is very useful 
when debugging passes after the register allocator as they no longer 
need to check for the validity of two-adress instructions.

As an example in LLVM v1.1 you had:

A = B + C

and for the x86 you had to make sure that A and B are the same so you 
ended up with an instruction like:

D = D + C

with some compensation code added before it.

In cvs world you simply need to remove the first or the second operand 
and correctly mark D as a def and use. So you will end up with:

D += C

You may want to look at the TwoAddressInstructionPass which does that 
already for you as it transforms:

     A = B op C

to:

     A = B
     A op= C

Care must be taken if your allocator expects SSA form because the 
TwoAddressInstructionPass breaks it.

-- 

Alkis