Re: better error messages through assertions

[Andy Wingo <wingo@igalia.com>]

On Wed 30 Mar 2022 11:37, Ludovic Courtès <ludo@gnu.org> writes:

>> scheme@(guile-user)> (container-contents '())
>> ice-9/boot-9.scm:1685:16: In procedure raise-exception:
>> In procedure struct-vtable: Wrong type argument in position 1
> scheme@(guile-user)> ,use(srfi srfi-9)
> scheme@(guile-user)> (define-record-type <foo>
> 		       (make-foo x)
> 		       foo?
> 		       (x foo-x))
> scheme@(guile-user)> ,optimize (foo-x '())
> $9 = (if (eq? (struct-vtable '()) <foo>)
>   (struct-ref '() 0)
>   (throw 'wrong-type-arg
>          'foo-x
>          "Wrong type argument: ~S"
>          (list '())
>          (list '())))
>
> With Guile 3, it might be that adding an extra ‘struct?’ test would have
> little effect on performance; we’d need to check.

Would have no effect.

Incidentally, you might want to use ,optimize-cps;

  scheme@(guile-user)> ,optimize (foo-x '())
  $9 = (if (eq? (struct-vtable '()) <foo>)
    (struct-ref '() 0)
    (throw 'wrong-type-arg
           'foo-x
           "Wrong type argument: ~S"
           (list '())
           (list '())))
  scheme@(guile-user)> ,optimize-cps (foo-x '())
  L0:                                           ;               at <unknown>:15:14
    v0 := self
    L1(...)
  L1:
    receive()
    v1 := const ()                              ; arg           at <unknown>:15:21
    throw throw/value+data[#(wrong-type-arg "struct-vtable" "Wrong type argument in position 1 (expecting struct): ~S")](v1) ;  at <unknown>:15:14

L1(...) means, pass all values to L1.  In this case because there are
varargs on the stack from the procedure call.  L1 parses them with the
receive().  Anyway, here we see that with respect to the immediate '(),
that all the tests folded.  If we instead lift to a procedure:

  scheme@(guile-user)> ,optimize-cps (lambda (x) (foo-x x))
  L0:                                           ;               at <unknown>:16:14
    v0 := self
    L1(...)
  L1:
    receive()
    v1 := current-module()                      ; module        at <unknown>:16:14
    cache-set![0](v1)                           ;               at <unknown>:16:14
    v2 := const-fun L7                          ; _ 
    return v2                                   ;               at <unknown>:16:14

  L7:                                           ;               at <unknown>:16:14
    v3 := self
    L8(...)
  L8:
    v4 := receive(x)                            ; x 
    heap-object?(v4) ? L10() : L38()            ;               at <unknown>:16:26
  L10():
    struct?(v4) ? L11() : L38()                 ;               at <unknown>:16:26
  L38():
    throw throw/value+data[#(wrong-type-arg "struct-vtable" "Wrong type argument in position 1 (expecting struct): ~S")](v4) ;  at <unknown>:16:26
  L11():
    v5 := scm-ref/tag[struct](v4)               ; vtable        at <unknown>:16:26
    v6 := cache-ref[(0 . <foo>)]()              ; cached        at <unknown>:10:20
    heap-object?(v6) ? L19() : L14()            ;               at <unknown>:10:20
  L19():
    L20(v6)                                     ;               at <unknown>:10:20
  L14():
    v7 := cache-ref[0]()                        ; mod           at <unknown>:10:20
    v8 := const <foo>                           ; name          at <unknown>:10:20
    v9 := lookup-bound(v7, v8)                  ; var           at <unknown>:10:20
    cache-set![(0 . <foo>)](v9)                 ;               at <unknown>:10:20
    L20(v9)                                     ;               at <unknown>:10:20
  L20(v10):                                     ; box 
    v11 := scm-ref/immediate[(box . 1)](v10)    ; arg           at <unknown>:10:20
    eq?(v5, v11) ? L22() : L37()                ;               at <unknown>:16:26
  L37():
    throw throw/value+data[#(wrong-type-arg foo-x "Wrong type argument: ~S")](v4) ;  at <unknown>:16:26
  L22():
    v12 := word-ref/immediate[(struct . 6)](v5) ; rfields       at <unknown>:16:26
    v13 := v12                                  ; nfields       at <unknown>:16:26
    imm-u64-<[0](v13) ? L25() : L35()           ;               at <unknown>:16:26
  L35():
    v21 := const 0                              ; _             at <unknown>:16:26
    throw throw/value+data[#(out-of-range "struct-ref/immediate" "Argument 2 out of range: ~S")](v21) ;  at <unknown>:16:26
  L25():
    v14 := pointer-ref/immediate[(struct . 7)](v5) ; ptr        at <unknown>:16:26
    v15 := load-u64[0]()                        ; word          at <unknown>:16:26
    v16 := u32-ref[bitmask](v5, v14, v15)       ; bits          at <unknown>:16:26
    v17 := load-u64[1]()                        ; mask          at <unknown>:16:26
    v18 := ulogand(v17, v16)                    ; res           at <unknown>:16:26
    u64-imm-=[0](v18) ? L31() : L33()           ;               at <unknown>:16:26
  L33():
    v20 := const 0                              ; _             at <unknown>:16:26
    throw throw/value+data[#(wrong-type-arg "struct-ref/immediate" "Wrong type argument in position 2 (expecting boxed field): ~S")](v20) ;  at <unknown>:16:26
  L31():
    v19 := scm-ref/immediate[(struct . 1)](v4)  ; val           at <unknown>:16:26
    return v19                                  ;               at <unknown>:16:26

Here we see the first procedure which is the thunk that wraps the
expression.  Then in the beginning of the procedure at L7 you can see
there is a check for struct?, which has to be dominated by a true
heap-object? check.  Duplicate checks are elided.  So if SRFI-9 added a
`struct?` check it wouldn't be more code; rather it would be less,
actually, because instead of branching to L38, you'd branch to L37.

Too bad about all that other crap about checking whether the index is in
range and the field is boxed or not, though :-/  Probably there is a
better design...

Andy