[llvm] r235079 - TRUNCATE constant folding - minor fix for rL233224

[Ahmed Bougacha]

On Thu, Apr 16, 2015 at 10:44 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:
> Hi Simon,
>
> I have an out-of-tree target broken by this commits, and I have a feeling that the problem is in this change.
>
> I have this node:  v2i32  BUILD_VECTOR { i32 = Constant<0>,  i32 = undef  }
> That I try to truncate:
>  getNode(ISD::TRUNCATE, dl, v2i16, N)
>
> After your change, getNode() returns this node:
>
>  N = v2i16  BUILD_VECTOR { i32 = Constant<0>,  i32 = undef  }
>
> Note that the operands are not truncated, which breaks the DAG type system.

It does?  BUILD_VECTOR can truncate integers, and that's the point of
this fix, no?

-Ahmed

> I assume you are relying on FoldConstantArithmetic() to magically convert the constant to the truncated type, however it does not work with undef.
>
> I have the feeling that the proper way requires to wrap every operand with a TRUNCATE before calling getNode, but you may see an alternative?
>
> Thanks,
>
> Mehdi
>
>
>
>> On Apr 16, 2015, at 1:21 AM, Simon Pilgrim <llvm-dev at redking.me.uk> wrote:
>>
>> Author: rksimon
>> Date: Thu Apr 16 03:21:09 2015
>> New Revision: 235079
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=235079&view=rev
>> Log:
>> TRUNCATE constant folding - minor fix for rL233224
>>
>> Fix for test case found by James Molloy - TRUNCATE of constant build vectors can be more simply achieved by simply replacing with a new build vector node with the truncated value type - no need to touch the scalar operands at all.
>>
>> Added:
>>    llvm/trunk/test/CodeGen/AArch64/fold-constants.ll
>> Modified:
>>    llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
>>
>> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp?rev=235079&r1=235078&r2=235079&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp Thu Apr 16 03:21:09 2015
>> @@ -2851,13 +2851,16 @@ SDValue SelectionDAG::getNode(unsigned O
>>         // FIXME: Entirely reasonable to perform folding of other unary
>>         // operations here as the need arises.
>>         break;
>> +      case ISD::TRUNCATE:
>> +        // Constant build vector truncation can be done with the original scalar
>> +        // operands but with a new build vector with the truncated value type.
>> +        return getNode(ISD::BUILD_VECTOR, DL, VT, BV->ops());
>>       case ISD::FNEG:
>>       case ISD::FABS:
>>       case ISD::FCEIL:
>>       case ISD::FTRUNC:
>>       case ISD::FFLOOR:
>>       case ISD::FP_EXTEND:
>> -      case ISD::TRUNCATE:
>>       case ISD::UINT_TO_FP:
>>       case ISD::SINT_TO_FP: {
>>         // Let the above scalar folding handle the folding of each element.
>>
>> Added: llvm/trunk/test/CodeGen/AArch64/fold-constants.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/fold-constants.ll?rev=235079&view=auto
>> ==============================================================================
>> --- llvm/trunk/test/CodeGen/AArch64/fold-constants.ll (added)
>> +++ llvm/trunk/test/CodeGen/AArch64/fold-constants.ll Thu Apr 16 03:21:09 2015
>> @@ -0,0 +1,21 @@
>> +; RUN: llc -mtriple=aarch64-linux-gnu -o - %s | FileCheck %s
>> +
>> +define i64 @dotests_616() {
>> +; CHECK-LABEL: dotests_616
>> +; CHECK:       movi d0, #0000000000000000
>> +; CHECK-NEXT:  umov w8, v0.b[2]
>> +; CHECK-NEXT:  sbfx w8, w8, #0, #1
>> +; CHECK-NEXT:  fmov s0, w8
>> +; CHECK-NEXT:  fmov x0, d0
>> +; CHECK-NEXT:  ret
>> +entry:
>> +  %0 = bitcast <2 x i64> zeroinitializer to <8 x i16>
>> +  %1 = and <8 x i16> zeroinitializer, %0
>> +  %2 = icmp ne <8 x i16> %1, zeroinitializer
>> +  %3 = extractelement <8 x i1> %2, i32 2
>> +  %vgetq_lane285 = sext i1 %3 to i16
>> +  %vset_lane = insertelement <4 x i16> undef, i16 %vgetq_lane285, i32 0
>> +  %4 = bitcast <4 x i16> %vset_lane to <1 x i64>
>> +  %vget_lane = extractelement <1 x i64> %4, i32 0
>> +  ret i64 %vget_lane
>> +}
>>
>>
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