Subject: "Wrong number of arguments to 1"

From: Taylan Kammer <[email-masked]>
Date: Mon, 24 May 2021 00:11:10 +0200

Note: merged with https://bugs.gnu.org/42757 as it's the same bug.

Andy is in CC since we almost certainly need his input. :-)

I've been working on this since a few days (had to learn much about the VM)
and here's an explanation of what's going on.

Firstly here's a minimal procedure that exhibits the bug:

  (define (test x)
    (define (inner-proc a) #f)
    (inner-proc 0 x))

Calling this procedure will always result in the error message:

  Wrong number of arguments to 0

Because the first argument to inner-proc is 0.  Were the first argument x,
then the argument we pass to 'test' would appear in the error message.

That's our bug at a high level: the first argument to inner-proc being
reported as the procedure that raised the error.

The disassembly is quite slim:

Disassembly of #<procedure test (x)> at #x559c98764348:

   0    (instrument-entry 82)
   2    (assert-nargs-ee/locals 2 0)    ;; 2 slots (1 arg)
   3    (make-immediate 1 2)            ;; 0
   4    (handle-interrupts)
   5    (tail-call-label 2)             ;; inner-proc at #x559c98764364

----------------------------------------
Disassembly of inner-proc at #x559c98764364:

   0    (instrument-entry 81)
   2    (assert-nargs-ee/locals 1 0)    ;; 1 slot (0 args)
   3    (make-immediate 0 4)            ;; #f
   4    (handle-interrupts)             
   5    (return-values)                 

Here's some explanations for those who aren't savvy with the VM, starting
from the top.  (The instructions, like assert-nargs-ee/locals, are defined
in libguile/vm-engine.c.)

Ignore the instrument-entry and handle-interrupts instructions.

- (assert-nargs-ee/locals 2 0) checks the number of arguments, ensuring
  that there's a total of 2 slots on the stack. [1]

- (make-immediate 1 2) puts the Scheme number 0 in slot 0. [2]

- (tail-call-label 2) jumps to the beginning of inner-proc. [3]

- (assert-nargs-ee/locals 1 0) checks the number of arguments, ensuring
  that there's a total of 1 slot on the stack. [4]

That's where the error is raised, by calling error_wrong_num_args from
libguile/intrinsics.c, because there's 2 slots on the stack, not 1.
Error_wrong_num_args tries to get the currently executed procedure from
slot 0, and finds the number 0 there, wrongly using that instead.

[1] Slot 0 is usually filled with the procedure being executed itself,
    before it's called, so the number of slots is usually the number of
    arguments plus one.

[2] Slot references in the VM are referenced backwards from N-1 to 0,
    where N is the number of slots.  In our case, as we have 2 slots,
    the number 1 refers to slot 0.  The reason has to do with the Guile
    stack growing downwards.  The constant 0 is represented by 2 because
    of its type tag as an "immediate" int, see scm.h for details.

[3] The 2 represents the relative position of the first instruction of
    inner-proc, in 4-byte units.  It's pretty close, as it's compiled
    right aside our top-level procedure 'test'.  Since we're currently
    on instruction 5 of 'test', and instructions are 4 bytes long, and
    'test' begins at 0x559c98764348, this means we're jumping to:

      0x559c98764348 + 5*4 + 2*4 = 0x559c98764364

    Which happens to be the address of inner-proc, see? :-)

[4] I don't know why only one slot even though it has one argument; it
    should be two slots.  Maybe an optimization, as the compiler decides
    that the procedure doesn't ever need a reference to itself?

To summarize: for some reason the compiler decides to *not* use an extra
slot for the currently-executed procedure when calling inner-proc, which
leads to the first argument to inner-proc (in this case 0) being used as
the "procedure being executed" during error reporting.

Perhaps that optimization (assuming it is one and not simply a bug in
the compiler) should be disabled, or maybe the code shouldn't even
compile since it can be statically proven that a procedure is being
called with the wrong number of arguments.  This is where I defer to
the Guile compiler experts.

-- 
Taylan