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How does nativecomp compile circular lists?

From: Jimmy Yuen Ho Wong
Subject: How does nativecomp compile circular lists?
Date: Tue, 27 Jul 2021 08:21:51 +0100

I'm not exactly sure how to even ask this question, as I've seen various packages fail to natively compile circular lists or optimize it wrongly. So I've come up with a simple test case for me to be able to disassemble the bytecode and the native code, and see if I could spot the problem, but disassembling natively compiled code doesn't seem to work.

Test case:

(defun test-circular-list ()
  (let ((l (list 1 2 3)))
    (setcdr (last l) l)

(print (test-circular-list))

(provide 'test-circular-list)

M-x emacs-lisp-byte-compile-and-load
M-x disassemble RET test-circular-list (works)

M-x emacs-lisp-native-compile-and-load
M-x disassemble RET test-circular-list (doesn't work)

Debugger entered--Lisp error: (search-failed "^.*<F746573742d63697263756c61722d6c697374_test_cir...")
  disassemble-internal(test-circular-list 0 nil)
  disassemble(test-circular-list nil 0 t)
  funcall-interactively(disassemble test-circular-list nil 0 t)
  command-execute(disassemble record)
  execute-extended-command(nil "disassemble" nil)
  funcall-interactively(execute-extended-command nil "disassemble" nil)

Furthermore, this test case, though superficially similar, doesn't seem to be able to reproduce the same failure to natively compile issue as seen in powerline.

There seems to be more than one thing wrong with this journey into my deep dive into how native compiling circular lists work.

Jimmy Yuen Ho Wong

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