[llvm-dev] What pattern string corresponds to CopyToReg?

Sun Nov 5 17:23:17 PST 2017

Hmm, well that seems to have worked, but I'm a little suspicious. This is
my instruction def:

// Load Immediate (LI)
def LI : IImmReg<0x010, (outs REG16:$dst), (ins i16imm:$src),
    [(set REG16:$dst, imm:$src)], "LI\t$dst, $src">;

The rest of the debug output:

Selecting: t3: ch,glue = CopyToReg t0, Register:i16 %R5, Constant:i16<127>
Selecting: t2: i16 = Register %R5
Selecting: t1: i16 = Constant<127>

ISEL: Starting pattern match on root node: t1: i16 = Constant<127>
  Initial Opcode index to 16
Creating constant: t5: i16 = TargetConstant<127>
  Morphed node: t1: i16 = LI TargetConstant:i16<127>
ISEL: Match complete!

Selected selection DAG: BB#0 'my_func:entry'
SelectionDAG has 6 nodes:
      t0: ch = EntryToken
      t1: i16 = LI TargetConstant:i16<127>
    t3: ch,glue = CopyToReg t0, Register:i16 %R5, t1
  t4: ch,glue = RTWP Register:i16 %R5, t3

So at this point, t3 is still a CopyToReg with R5 and another node that is
an LI with the constant. That doesn't make sense to me. Shouldn't the LI
node end up being a parent of R5 and the constant?

The phases proceed, and this makes even less sense:

# Machine code for function my_func: IsSSA, TracksLiveness

BB#0: derived from LLVM BB %entry
%vreg0<def> = LI 127; REG16:%vreg0
%R5<def> = COPY %vreg0; REG16:%vreg0
RTWP %R5

# End machine code for function my_func.

I'm not complaining too hard, though, because in the end I get the assembly
output I expect:

# BB#0:                                 # %entry
LI $r5, 127
RTWP

What I'm complaining about is that I still don't know how the DAG nodes
work. Here's the -view-sched-dags output:

[image: dags.jpg]

I just don't understand the LI node. From a dataflow perspective, LI does
depend on the constant, and not on the register R5. Is this just how LLVM
represents data stores? With a CopyToReg node?

There's something that I'm missing, but I don't know what it is :(

--Rob

On Sun, Nov 5, 2017 at 1:22 AM 陳韋任 <chenwj.cs97g at g2.nctu.edu.tw> wrote:

> Try match your instruction with (set REG16:$dst, i16imm:$src). Just give
> it a shot. :)
>
> 2017-11-05 12:56 GMT+08:00 Robert Baruch via llvm-dev <
> llvm-dev at lists.llvm.org>:
>
>> Well, that's the thing: I thought that was CopyToReg. I don't know what
>> the name of the node is to load one value into a register, so I don't know
>> how to construct such a pattern.
>>
>> On Sat, Nov 4, 2017 at 9:23 PM Craig Topper <craig.topper at gmail.com>
>> wrote:
>>
>>> Do you have a pattern for loading an i16 immediate into a 16-bit
>>> register?
>>>
>>> ~Craig
>>>
>>> On Sat, Nov 4, 2017 at 8:00 PM, Robert Baruch <robert.c.baruch at gmail.com
>>> > wrote:
>>>
>>>> Hmm, okay. Then what's the problem being reported here? I'm not sure
>>>> what I'm supposed to do with "LLVM ERROR: Cannot select: t1: i16 =
>>>> Constant<127>".BTW, the function is:
>>>>
>>>> ; ModuleID = 'return.c'
>>>> source_filename = "return.c"
>>>> target datalayout =
>>>> "E-m:e-p:16:16:16-i1:16:16-i8:16:16-i16:16:16-i32:16:16-i64:16:16-S16-n16"
>>>> target triple = "tms9900"
>>>>
>>>> ; Function Attrs: noinline nounwind optnone
>>>> define signext i16 @my_func() #0 {
>>>> entry:
>>>>   ret i16 127
>>>> }
>>>>
>>>> ------- debug output --------
>>>>
>>>> Optimized legalized selection DAG: BB#0 'my_func:entry'
>>>> SelectionDAG has 5 nodes:
>>>>     t0: ch = EntryToken
>>>>   t3: ch,glue = CopyToReg t0, Register:i16 %R5, Constant:i16<127>
>>>>   t4: ch = TMS9900ISD::Ret t3, Register:i16 %R5, t3:1
>>>>
>>>> ===== Instruction selection begins: BB#0 'entry'
>>>> Selecting: t4: ch = TMS9900ISD::Ret t3, Register:i16 %R5, t3:1
>>>>
>>>> ISEL: Starting pattern match on root node: t4: ch = TMS9900ISD::Ret t3,
>>>> Register:i16 %R5, t3:1
>>>>
>>>>   Morphed node: t4: ch,glue = Retr Register:i16 %R5, t3
>>>>
>>>> ISEL: Match complete!
>>>> Selecting: t3: ch,glue = CopyToReg t0, Register:i16 %R5,
>>>> Constant:i16<127>
>>>>
>>>> Selecting: t2: i16 = Register %R5
>>>>
>>>> Selecting: t1: i16 = Constant<127>
>>>>
>>>> ISEL: Starting pattern match on root node: t1: i16 = Constant<127>
>>>>
>>>>   Initial Opcode index to 0
>>>>   Match failed at index 0
>>>> LLVM ERROR: Cannot select: t1: i16 = Constant<127>
>>>> In function: my_func
>>>>
>>>>
>>>> On Sat, Nov 4, 2017 at 7:22 PM Craig Topper <craig.topper at gmail.com>
>>>> wrote:
>>>>
>>>>> CopyToReg is not handle by patterns. It should be passed through isel
>>>>> unchanged. It’s part of a special list of ISD opcodes that don’t change in
>>>>> SelectioDAGISel::SelectCodeCommon
>>>>>
>>>>> It will then be turned into a TargetOpcode::COPY in
>>>>> InstrEmitter::EmitSpecialNode when the DAG is turned into MachineInstrs.
>>>>>
>>>>> On Sat, Nov 4, 2017 at 7:02 PM Robert Baruch via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> So there's a DAG that looks like this in the debug output:
>>>>>>
>>>>>>   Selecting: t3: ch,glue = CopyToReg t0, Register:i16 %R5,
>>>>>> Constant:i16<127>
>>>>>>
>>>>>> In the instruction selection phase, what pattern would that match?
>>>>>> I've constructed this so far:
>>>>>>
>>>>>>   (??? REG16:$dst, i16imm:$src)
>>>>>>
>>>>>> but the problem is, I can't determine what to use as ???. There is
>>>>>> an ISD::CopyToReg enum value, but I don't know how that translates
>>>>>> to the string to use in the pattern.
>>>>>>
>>>>>> And more generally, how do I find out from a DAG diagram like the
>>>>>> ones output by -view-isel-dags which node type corresponds to which
>>>>>> pattern string?
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> --Rob
>>>>>>
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>> --
>>>>> ~Craig
>>>>>
>>>>
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
>
> --
> Wei-Ren Chen (陳韋任)
> Homepage: https://people.cs.nctu.edu.tw/~chenwj
>
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