Division operator is optimized away if function is defined by macro, despite div-by-zero still being possible

[Patrick Poitras]

```
(defvar *zero* 0)
(defmacro bug-maker (name &body body)
  `(defun ,name () ,@body))
(bug-maker doesnt-error (/ 3 *zero*) nil)
(bug-maker errors (/ 3 *zero*))
(doesnt-error)
(errors)
```

Here, doesnt-error throws out the (/ 3 *zero*), likely because it's assuming that the result is pure and unused, therefore optimizes it away. This is despite division having a side-effect of throwing an error if division by zero occurs.

Bug reproduced on:

SBCL 2.3.3: 5.15.0-60-generic #66-Ubuntu SMP Fri Jan 20 14:29:49 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
SBCL 2.4.10-1.1-suse: 5.15.167.4-microsoft-standard-WSL2 #1 SMP Tue Nov 5 00:21:55 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

*features*
(:QUICKLISP :ASDF3.3 :ASDF3.2 :ASDF3.1 :ASDF3 :ASDF2 :ASDF :OS-UNIX
 :NON-BASE-CHARS-EXIST-P :ASDF-UNICODE :ARENA-ALLOCATOR :X86-64 :GENCGC :64-BIT
 :ANSI-CL :COMMON-LISP :ELF :IEEE-FLOATING-POINT :LINUX :LITTLE-ENDIAN
 :PACKAGE-LOCAL-NICKNAMES :SB-CORE-COMPRESSION :SB-LDB :SB-PACKAGE-LOCKS
 :SB-THREAD :SB-UNICODE :SBCL :UNIX)

[Stas Boukarev (stassats)]

Have you ensured that this is in safe code?

[Patrick Poitras (pfpoitras)]

This is in safe code, yes. If you mean like, there's no (safety 0) anywhere.

[Stas Boukarev (stassats)]

Here is the definition of safe code: https://www.lispworks.com/documentation/HyperSpec/Body/26_glo_s.htm#safe

[Patrick Poitras (pfpoitras)]

Turns out the macro is irrelevant.

(defvar *zero* 0)
(defun blarg ()
  (/ 3 *zero*)
  nil)

Also removes it. I guess that makes it consistent, although unexpected.

Weirdly enough,
(defun blarg ()
  (/ 3 0) nil)

still throws an error, as does

(defun blarg ()
  (let ((zero 0))
     (/ 3 zero))
   nil)

I was expecting the default safety profile to maybe forego some type-checking, but not to alter runtime behavior to the extent that those two functions have different behavior than the first.

Is there more information somewhere about what the SBCL policy is on what the default safety mode is permitted to ignore, in terms of the side-effects of numerical functions or just any explanation, really?