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

[Mehdi Amini]

> On Apr 16, 2015, at 10:52 AM, Ahmed Bougacha <ahmed.bougacha at gmail.com> wrote:
> 
> 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?

I didn’t know BUILD_VECTOR had an implicit TRUNCATE.  Does it carry only implicit truncate or other conversion as well (EXTEND for instance)?

So we are not allowed to fold naively an EXTRACT_VECTOR_ELEMENT that is fed with a BUILD_VECTOR?
This does not seem in-line with usual behavior in the DAG and it makes combining more complex.


Thanks,

Mehdi

> 
> -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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