[llvm-dev] Question about 'DAGTypeLegalizer::SplitVecOp_EXTRACT_VECTOR_ELT'

[Demikhovsky, Elena via llvm-dev]

Please open a bugzilla ticket and attach your testcase. It will allow us to debug and fix the problem.
Thanks

-           Elena

From: JinGu [mailto:jingu at codeplay.com]
Sent: Saturday, September 16, 2017 00:38
To: Demikhovsky, Elena <elena.demikhovsky at intel.com>; daniel_l_sanders at apple.com <daniel_l_sanders at apple.com>; Jon Chesterfield <jonathanchesterfield at gmail.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: Question about 'DAGTypeLegalizer::SplitVecOp_EXTRACT_VECTOR_ELT'


Hi Elena,
Thanks for your response.

The store is ok but the extending load generates assertion after the store because MemVT is i8 and VT is i1 on following line.

assert(MemVT.getScalarType().bitsLT(VT.getScalarType()) && "Should only be an extending load, not truncating!")

so I think we need to use non-extending load for element size less than 8bit on "DAGTypeLegalizer::SplitVecOp_EXTRACT_VECTOR_ELT" like this roughly.
if (N->getOperand(0).getValueType().getVectorElementType().getSizeInBits() < 8) {
  return DAG.getLoad(N->getValueType(0), dl, Store, StackPtr, MachinePointerInfo());
} else {
   return DAG.getExtLoad(ISD::EXTLOAD, dl, N->getValueType(0), Store, StackPtr, MachinePointerInfo(), EltVT);
}

How do you think about it?

Thanks,
JinGu Kang

On 15/09/2017 18:42, Demikhovsky, Elena wrote:

extends the elements to 8bit and stores them on stack.

Store is responsible for zero-extend. This is the policy...



-  Elena





-----Original Message-----

From: jingu at codeplay.com<mailto:jingu at codeplay.com> [mailto:jingu at codeplay.com]

Sent: Friday, September 15, 2017 17:45

To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>; Demikhovsky, Elena <elena.demikhovsky at intel.com><mailto:elena.demikhovsky at intel.com>; daniel_l_sanders at apple.com<mailto:daniel_l_sanders at apple.com>

Subject: Re: Question about 'DAGTypeLegalizer::SplitVecOp_EXTRACT_VECTOR_ELT'



Can someone give the comment about it please?



Thanks,



JinGu Kang





On 14/09/17 12:05, jingu at codeplay.com<mailto:jingu at codeplay.com> wrote:

Hi All,



I have a question about splitting 'EXTRACT_VECTOR_ELT' with 'v2i1'. I

have a llvm IR code snippet as following:



llvm IR code snippet:



for.body:                                         ; preds = %entry,

%for.cond

  %i.022 = phi i32 [ 0, %entry ], [ %inc, %for.cond ]

  %0 = icmp ne <2 x i32> %vecinit1, <i32 0, i32 -23>

  %1 = extractelement <2 x i1> %0, i32 %i.022

  %vecext4 = extractelement <2 x i32> %vecinit1, i32 %i.022

  %vecext5 = extractelement <2 x i32> <i32 0, i32 -23>, i32 %i.022

  %cmp6 = icmp ne i32 %vecext4, %vecext5

  %cmp7 = xor i1 %1, %cmp6



...



and the SelectionDAG before TypeLegalizer is like this.



  t0: ch = EntryToken

  t2: i32,ch = CopyFromReg t0, Register:i32 %vreg0

  t3: ch = ValueType:i32

      t5: i32,ch = CopyFromReg t2:1, Register:i32 %vreg1

    t7: i32 = AssertZext t5, ValueType:ch:i1

  t8: v2i32 = BUILD_VECTOR t2, t7

  t11: v2i32 = BUILD_VECTOR Constant:i32<0>, Constant:i32<-23>

  t15: i32,ch = CopyFromReg t0, Register:i32 %vreg2

          t22: i32 = add t15, Constant:i32<1>

        t24: ch = CopyToReg t0, Register:i32 %vreg3, t22

        t27: ch = CopyToReg t0, Register:i32 %vreg8, Constant:i32<-1>

      t31: ch = TokenFactor t24, t27

            t13: v2i1 = setcc t8, t11, setne:ch

          t16: i1 = extract_vector_elt t13, t15

            t17: i32 = extract_vector_elt t8, t15

            t18: i32 = extract_vector_elt t11, t15

          t19: i1 = setcc t17, t18, setne:ch

        t20: i1 = xor t16, t19



...



I have not added any vector register class so 'DAGTypeLegalizer' tries

to split the "t16: i1 = extract_vector_elt t13, t15" because  t13's

result type is 'v2i1'. If the size of vector element is less than

8bit, 'DAGTypeLegalizer::SplitVecOp_EXTRACT_VECTOR_ELT()' function

extends the elements to 8bit and stores them on stack. Finally, the

function generates 'ExtLoad' to load specific element. But if the

element's size is less than 8bit, I think it could be wrong. It looks

it needs just 'Load' or "Load and Truncate" to match the result type

of 'EXTRACT_VECTOR_ELT'. How do you think about it? If I missed

something, please let me know.



Thanks,



JinGu Kang





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