llvm dev - Jun 2019 - 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
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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.


-Eli


________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Joan
Lluch via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Saturday, June 1, 2019 10:09 AM
To: llvm-dev
Subject: [EXT] [llvm-dev] 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
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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?)

Thanks

Joan Lluch

Tel: 620 28 45 13
> On 2 Jun 2019, at 03:23, Eli Friedman <efriedma at quicinc.com>
wrote:
> 
> 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.
> 
> -Eli
> 
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Joan
Lluch via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: Saturday, June 1, 2019 10:09 AM
> To: llvm-dev
> Subject: [EXT] [llvm-dev] 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
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