Optimizing Compare instruction selection

I attempt to optimize the use of the ‘CMP’ instruction on my architecture by removing the instruction instances where the Status Register already had the correct status flags.

The cmp instruction in my architecture is the typical one that compares two registers, or a register with an immediate, and sets the Status Flags accordingly. I implemented my ‘cmp’ instruction in LLVM by custom lowering the SETCC, SELECT_CC and BR_CC in the usual way, mostly following the ARM implementation. In particular the target cmp instruction is created and glued to a target conditional instruction, such as a conditional branch, or a conditional select.

The generated code may look like this:

add R0, R1, R2
cmp R0, #0
breq Label

This works fine, but now I would want to go one step further:

As part of the SETCC and SELECT_CC lowering, I can determine if the conditions are met in my architecture to avoid generating a ‘cmp’ instruction at all. For example, if the left operand of the SETCC is an ‘addition’, the right operand is constant zero, and the condition is 'ISD::CondCode::SETEQ, I can safely assume that the Status Register in my architecture already has the right condition just before the ‘cmp’, which has been set by the target ‘add’ instruction. So at this point I want to avoid generating the ‘cmp’ because it is redundant.

My goal is to replace the assembly code shown above, by just this:

add R0, R1, R2
breq Label

The ‘cmp’ was redundant in this case because the add instruction already set the Z (zero) flag

I tried several things, but I got stuck in some way in all of them. For example, an approach that I tried is to have a ‘dummy_cmp’ instruction, that I generate instead of the regular ‘cmp’ when the comparison is redundant. The ‘dummy_cmp’ must be removed at a latter stage, ideally during the selDAGToDAG phase, but I failed to do so.

  • So my first question is. How do I remove the ‘dummy_cmp’ node in the MyTargetISelDAGtoDAG::Select() function?

Another approach that I tried, which I regard as preferable, is to directly avoid the creation of a ‘cmp’ instruction during ISelLowering. Instead of ‘cmp’, use the LHS operand of the SETCC operand if it mets the conditions described above. The problem that I found with this approach is that the ‘cmp’ is just a ‘glue’ to the target branch or select instruction, however, the LHS is an actual operand (for example an ‘add’ node), so I need to create a ‘glue’ for it, in order to be attached to the target branch instruction.

  • My second question is therefore, how do I create a ‘glue’ for an existing ‘add’ node that I can attach to the target conditional instruction as a replacement of the ‘cmp’ instruction?

Joan Lluch

There are basically two possible approaches here.

One approach is to just wait until after isel to handle this: check for an addition instruction immediately followed by an cmp; then erase the cmp. See, for example, ARMBaseInstrInfo::optimizeCompareInstr.

Another, you make a dedicated target-specific ISel node that produces two results: essentially, one is the result of the arithmetic, and the other is the status register. See, for example, X86TargetLowering::LowerBRCOND.

I don’t think you’d want to try to do anything in Select(); I mean, you could try to pattern-match (cmp x, (add x, y)) or whatever, but you’d probably run into trouble if the add has multiple uses.


Hi Eli,

Thank you very much for your response.

In fact, I had already tried the X86 approach before, i.e explicitly using the status register. This is the approach that appeals more to me. I left it parked because it also produced some problems (but I left it commented out). So I have now re-lived the code, and it works fine in most cases, but there’s a particular case that causes LLVM to stop with an assertion. Apparently LLVM tries to use the SR (Status Register) register as a general purpose one, which is not allowed in my architecture. The problem exists even before I added the referred optimisations. The code that causes the problem is this:

int doSmth( int y );

int test( int y )
int neg = y < 0;
if ( neg )
y = - y;

int rv = doSmth( y );

return neg ? - rv : rv;

Apparently, LLVM attempts to physically use the result of a CMP instruction through a function call by storing it on a temporary register. This is found before the doSmth function call,

t30: i16 = CMPkr16 t4, TargetConstant:i16<0>
t36: ch,glue = CopyToReg t0, Register:i16 $sr, t30
t32: i16 = NEGSETCC TargetConstant:i16<4>, t36:1

And this is generated after the call

t35: ch,glue = CopyToReg t0, Register:i16 $sr, t30
t31: i16 = SELCC t19, t18, TargetConstant:i16<4>, t35:1
t21: ch,glue = CopyToReg t18:1, Register:i16 $r0, t31

NEGSETCC and SELCC are genuine instructions of my target architecture, they use the SR along with operands to produce a result.

As you can see ‘t30’ is used both before and after the function call, which is what I think that causes trouble. In particular, the assertion that I get is this:

Assertion failed: (RegClass->isAllocatable() && “Virtual register RegClass must be allocatable.”), function createVirtualRegister, file /Users/joan/LLVM+CLANG/llvm-7.0.1.src/lib/CodeGen/MachineRegisterInfo.cpp, line 170.

If I remove the ‘isAllocatable=0’ setting on the SR register, then LLVM will try to spill the SR by calling ‘storeRegToStackSlot’, which then I stop with my own assertion because the SR can’t be spilled.

If I then remove also the ‘let CopCost = -1’ for the SR register, then LLVM attempts to move it to a general purpose register by calling ‘copyPhysRegs’, which again I stop with an assertion because it’s not legal in my architecture.

So, I’m stuck as well with this approach.

Any ideas, on what I can try?

I think, that at this point I need to stop LLVM from attempting to reuse the result of the CMP instruction before and after the function call, and instead make it generate a new cmp instruction after the call. But how do I achieve that?

(maybe a difficulty is that I am Custom lowering ’’SELECT_CC’ and ‘BR_CC’, and Expanding ’SELECT’ and ‘BRCOND’, instead of the opposite? Does this make a difference?)


Joan Lluch

Tel: 620 28 45 13

Have you implemented TargetRegisterInfo::getCrossCopyRegClass?


Hi Eli,

Thanks again for your reply.

I am unsure about implementing the getCrossCopyRegClass for my target. My target does not support or allow moves to and from the SR. The SR exists because it has implicit involvement in some instructions, but it is opaque to the assembler and to the user as a register. I mean, there are no instructions to directly move or read it, or even access it directly. So I think that implementing the getCrossCopyRegClass -to tell LLVM that it can be copied to another register class- is against the actual target features (unless I am missing the actual purpose of the function)

I ended implementing target Branch and Select instructions that have a glue input, and another pair that have a GR register as input. Both map to the same machine instructions through Pattern matching in TargetInstrInfo.td. This is what I mean, for the case of the Conditional Branch instruction :

let Uses = [SR] in {
  def BRCC : T2ccbr< "br", "br"> ;

def : Pat<(CPU74brcc_g bb:$a, imm:$cc), (BRCC jmptarget:$a, imm:$cc, 0)>;
def : Pat<(CPU74brcc_i bb:$a, imm:$cc, GR16:$ins), (BRCC jmptarget:$a, imm:$cc, GR16:$ins)>;
Then, in MyTargetISelDAGToDAG I create ‘CPU74brcc_g' instructions if they are glued to a ‘compare’ instruction, or ‘CPU74brcc_i' instructions if they link to the result of an arithmetic instruction. During instruction selection these are replaced by actual, physical, BRCC instructions. This solution has proved to work fine without having to explicitly involve the SR. Of course, the SR is still implicitly involved by the correct definition of ‘Uses’ and ‘Defs’ on the corresponding instructions.

Joan Lluch

You’re allowed to return “nullptr” from getCrossCopyRegClass, and the scheduler will try to do the right thing, possibly cloning the instruction that produces the flag.


Hi Eli,

I already got it working in the way I described, but thanks for the suggestion. I will try this as well and if it works I may chose it as it appears to require less custom code.

Joan Lluch