Internal Compiler Error: unknown variable after Fuse SOACs

[poeface]

I found a new compiler bug today. The start of the complaint was:

Internal compiler error.  Please report this:                                                                                                                                                                      
  https://github.com/diku-dk/futhark/issues                                                                                                                                                                        
Type error after pass 'Fuse SOACs':                                                                                                                                                                                
In function entry_main                                                                                                                                                                                             
When checking function body                                                                                                                                                                                        
In expression of statement                                                                                                                                                                                         
  {replicate_14296 : ({}, [MN2_13061][MN2_13061]f64)}                                                                                                                                                              
In subexp defunc_1_map_res_13270                                                                                                                                                                                   
Use of unknown variable defunc_1_map_res_13270.

I'm just putting the start because it then proceeded to print out a bunch of IR information (I think) which would be too long for this format.

Here is the futhark commit I am using: git: 3f2a2ef (Sat Jan 21 22:23:55 2023 +0100).

Here is a test program that produces the bug: SOAC_bug.zip

Apologies that it's so long! I tried to shorten it as much as possible, but since this bug is happening comparatively late in the compilation process, it seems to depend on a lot of overlapping pieces of code.

[athas]

Reduced example.

def matmul_diag_full [n][m] (ds: [n]f64) (A: [n][m]f64): [n][m]f64 =
  map2 (\d as -> map (*d) as) ds A

def matmul_full_diag [n][m] (A: [n][m]f64) (ds: [m]f64): [n][m]f64 =
  transpose (map2 (\d as -> map (*d) as) ds (transpose A))

entry calcS (k0: f64) (D: []f64) (W: [][]f64)  =
  let X = map (\d -> f64.exp (f64.neg d * k0 * 1)) D
  let temp = X `matmul_diag_full` W `matmul_full_diag` X
  in temp

[athas]

Introduced with the new fusion engine in 0.21.13. Probably not a very deep bug. I might not have time to fix it this evening, though.

[athas]

It is almost certainly related to the optimisation that allows fusion through transpositions.