Pipe operator can cause ICE during SOAC fusion

[Programmerino]

I tested this bug on versions 0.20.5 and the latest git master 8067ecf65d5eca0721d9fd808f8eb1c5153c8464 on a Linux x86-64 machine and reproduced it both times. The error occurs on all backends. I'm sorry the title isn't that descriptive, but I don't really know what's going on here...

Failing code:

entry ice =
    let go _ = loop _ = [] for _ in [0,1] do [0i32]
    in
    (go 0) ++ (0 |> go) -- If the pipe operator is not used, it works

Compiler error:

[  +0.000023] Reading and type-checking source program
[  +0.227272] Defunctorising
[  +0.000731] Monomorphising
[  +0.031572] Lifting lambdas
[  +0.000227] Defunctionalising
[  +0.000449] Converting to core IR
[  +0.000703] Type-checking internalised program
[  +0.000771] Running pass simplify
[  +0.001644] Running pass Inline conservatively
[  +0.002796] Running pass simplify
[  +0.000752] Running pass Inline aggressively
[  +0.000764] Running pass simplify
[  +0.000851] Running pass CSE
[  +0.000432] Running pass simplify
[  +0.000707] Running pass Fuse SOACs
Internal compiler error.  Please report this:
  https://github.com/diku-dk/futhark/issues
Type error after pass 'Fuse SOACs':
In expression of statement
  {defunc_0_f_res_4726 : ({}, i64),
   defunc_0_f_res_4727 : ({}, [loop₂_4659]i32),
   discard_4728 : ({}, i64),
   lifted_go_res_4729 : ({}, i64),
   lifted_go_res_4730 : ({}, [loop₂_4631]i32),
   discard_4731 : ({}, i64)}
Type error:
Stream with inconsistent accumulator type in lambda.
let {loop_init_4717 : ({}, [0i64]i32)} =
  [] : []i32
let {for_in_arr_4718 : ({}, [2i64]i32)} =
  [0i32, 1i32] : []i32
let {loopres_4719 : ({}, [1i64]i32)} =
  replicate([1i64], 0i32)
let {defunc_0_f_res_4726 : ({}, i64),
     defunc_0_f_res_4727 : ({}, [loop₂_4659]i32),
     discard_4728 : ({}, i64),
     lifted_go_res_4729 : ({}, i64),
     lifted_go_res_4730 : ({}, [loop₂_4631]i32),
     discard_4731 : ({}, i64)} =
  streamSeq(2i64,
            {for_in_arr_4718},
            {0i64, loop_init_4717, 0i64, 0i64, loop_init_4717, 0i64},
            \ {chunk_size_4732 : i64, loop₂_outer_4733 : i64,
               nameless_outer_4734 : [loop₂_4659]i32, offset_4735 : i64,
               loop₂_outer_4736 : i64, nameless_outer_4737 : [loop₂_4631]i32,
               offset_4738 : i64, for_in_arr_chunk_4739 : [chunk_size_4732]i32}
              : {i64,
                 [loop₂_4659]i32,
                 i64,
                 i64,
                 [loop₂_4631]i32,
                 i64} ->
              let {x_4740 : ({}, i64),
                   x_4741 : ({loopres_4719, nameless_outer_4734},
                             [x_4740]i32)} =
                -- Result for nameless_4744 aliases loopres_4719
                loop {loop₂_4743 : i64,
                      nameless_4744 : [loop₂_4743]i32} = {loop₂_outer_4733,
                                                          nameless_outer_4734}
                for j_4742:i64 < chunk_size_4732 do {
                  {1i64, loopres_4719}
                }
              let {x_4745 : ({}, i64)} = add_nw64(chunk_size_4732, offset_4735)
              let {x_4746 : ({}, i64),
                   x_4747 : ({loopres_4719, nameless_outer_4737},
                             [x_4746]i32)} =
                -- Result for nameless_4750 aliases loopres_4719
                loop {loop₂_4749 : i64,
                      nameless_4750 : [loop₂_4749]i32} = {loop₂_outer_4736,
                                                          nameless_outer_4737}
                for j_4748:i64 < chunk_size_4732 do {
                  {1i64, loopres_4719}
                }
              let {x_4751 : ({}, i64)} = add_nw64(chunk_size_4732, offset_4738)
              in {x_4740, x_4741, x_4745, x_4746, x_4747, x_4751})
let {conc_tmp_4758 : ({}, i64)} =
  add_nw64(defunc_0_f_res_4726, lifted_go_res_4729)
let {++_res_4759 : ({}, [conc_tmp_4758]i32)} =
  concat@0(conc_tmp_4758, lifted_go_res_4730, defunc_0_f_res_4727)

entry("ice",
      {},
      {direct})
  entry_ice ()
  : {i64, [?0]i32} = {
  {conc_tmp_4758, ++_res_4759}
}

[athas]

I can reproduce. Thanks for the concise test program.

[athas]

The pipe is a red herring; it's just that the two applications get CSE'd down to one if it isn't there. It's a straightforward bug in horizontal stream fusion, and present since at least 0.20.6.