[llvm] r235079 - TRUNCATE constant folding - minor fix for rL233224
James Molloy
james at jamesmolloy.co.uk
Thu Apr 16 12:41:27 PDT 2015
Hi Mehdi,
It's not altogether surprising- after type legalisation BUILD_VECTOR can't
have any illegal types feeding it, and i16 is illegal. I32 is the smallest
legal type.
Therefore it must be able deal with truncation!
Cheers,
James
On Thu, 16 Apr 2015 at 20:24, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
> > 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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