Assertion failure in AArch64 backend: "Detected cycle in SelectionDAG"

[tstellar]

Bugzilla Link	43056
Resolution	FIXED
Resolved on	Aug 27, 2019 07:31
Version	trunk
OS	Linux
Blocks	#41819
Attachments	Original Test Case, Reduced test case
CC	@Arnaud-de-Grandmaison-ARM,@zmodem,@smithp35,@TNorthover

Extended Description

Reproduce with:

llc -march=aarch64 -mcpu=thunderxt88 < aarch64-sdag-cycle-reduced.ll

Full error message:

Detected cycle in SelectionDAG
Offending node:
t659: v4f32,i64,ch = AArch64ISD::LD1DUPpost<(load 4 from %ir.96)> t489, t460, t120
t460: i64,ch = load<(load 8 from %ir."&context.constants_ptr[]7")> t456, t2, undef:i64
t2: i64,ch = CopyFromReg t0, Register:i64 %7
t1: i64 = Register %7
t11: i64 = undef
t120: i64 = shl t118, Constant:i64<2>
t118: i64 = sign_extend t117
t117: i32 = extract_vector_elt t589, Constant:i64<0>
t589: v4i32 = and t584, t587
t584: v4i32 = AArch64ISD::VSHL t407, Constant:i32<2>
t407: v4i32 = add t582, t655
t582: v4i32 = AArch64ISD::VSHL t96, Constant:i32<1>
t96: v4i32 = bitcast t659
t659: v4f32,i64,ch = AArch64ISD::LD1DUPpost<(load 4 from %ir.96)> t489, t460, t120

            t124: i32 = Constant<1>
          t655: v4i32 = AArch64ISD::MOVIshift Constant:i32<3>, Constant:i32<0>
            t106: i32 = Constant<3>
            t19: i32 = Constant<0>
        t97: i32 = Constant<2>
      t587: v4i32 = xor t583, t586
        t583: v4i32 = AArch64ISD::CMHS t407, t647
          t407: v4i32 = add t582, t655
            t582: v4i32 = AArch64ISD::VSHL t96, Constant:i32<1>
              t96: v4i32 = bitcast t659

              t124: i32 = Constant<1>
            t655: v4i32 = AArch64ISD::MOVIshift Constant:i32<3>, Constant:i32<0>
              t106: i32 = Constant<3>
              t19: i32 = Constant<0>
          t647: v4i32 = AArch64ISD::DUP t457
            t457: i32,ch = load<(load 4 from %ir.95)> t456, t73, undef:i64
              t73: i64 = add t2, Constant:i64<128>


              t11: i64 = undef
        t586: v4i32 = BUILD_VECTOR Constant:i32<-1>, Constant:i32<-1>, Constant:i32<-1>, Constant:i32<-1>
          t585: i32 = Constant<-1>
          t585: i32 = Constant<-1>
          t585: i32 = Constant<-1>
          t585: i32 = Constant<-1>
    t10: i64 = Constant<0>
t119: i64 = Constant<2>

[tstellar]

assigned to @TNorthover

[zmodem]

Tim, could you help triage this?

[TNorthover]

Yep, looking into it now.

[TNorthover]

Trunk's affected too so I'll fix there first.

I think the issue is that when searching for cycles we pre-insert the vector-building node into Visited (AArch64ISelLowering.cpp:10695). This has the effect of terminating the search if that node is seen, which isn't right because it can be part of a cycle.

The corresponding code in DAGCombiner.cpp inserts the base address, which I think is a performance optimization: since it's known to be a cycle-free predecessor of everything we care about we can stop looking if we see it. I think that's what we should be doing here too.

So: https://reviews.llvm.org/D66639

[zmodem]

So: https://reviews.llvm.org/D66639

Merged that to release_90 in r370063.