defmacro compiler notes for List* and Append

[Robert Brown]

Somewhere between version 2.3.1 and 2.3.4 I think ... Backquote expressions in macro definitions started producing compiler notes when the macro function is compiled with higher than normal speed. For instance, this form:

(defmacro define-foobar (finalization-rounds double)
  (declare (optimize (debug 2) (safety 0) (speed 3)))
  (let ((finalize (make-list finalization-rounds :initial-element '(sip-round v0 v1 v2 v3))))
    `(defun foobar ()
       (let ()
         ,@finalize
         ,(if (not double)
              '(logxor v0 v1 v2 v3)
              `(let ()
                 ,@finalize
                 (values h0 h1)))))))

Generates notes when compiled with SBCL 2.3.11.167-1ba132815

bob> (compile-file "bug.lisp")
; compiling file "/home/brown/bug.lisp" (written 26 JAN 2024 06:16:13 PM):

; file: /home/brown/bug.lisp
; in: defmacro define-foobar => defun foobar
; `(LET ()
; ,@BOB::FINALIZE
; (VALUES BOB::H0 BOB::H1))
; --> SB-IMPL::|Append|
; ==>
; 1
;
; note: unable to optimize due to type uncertainty: The first argument is a list, not a null.

; in: defmacro define-foobar
; `(DEFUN BOB::FOOBAR ()
; (LET ()
; ,@BOB::FINALIZE
; ,(IF (NOT BOB::DOUBLE)
; '(LOGXOR BOB::V0 BOB::V1 BOB::V2 BOB::V3)
; `(LET ()
; ,@BOB::FINALIZE
; (VALUES BOB::H0 BOB::H1)))))
; --> SB-IMPL::|List*| SB-IMPL::|Append|
; ==>
; 1
;
; note: unable to optimize due to type uncertainty: The first argument is a list, not a null.
;
; compilation unit finished
; printed 2 notes

; wrote /home/brown/bug.fasl
; compilation finished in 0:00:00.036

[Fedorov Alexander (gleefre)]

FWIW

This can be reproduced (at least partly) with a smaller example:

(defun foo (list)
  (declare (optimize speed))
  `(,@list tail))

Generated note:
; in: defun foo
; `(,@LIST TAIL)
; ==>
; 1
;
; note: unable to
; optimize
; due to type uncertainty:
; The first argument is a list, not a null.
;
; compilation unit finished
; printed 1 note

This behavior started in sbcl-2.3.7.

There is a possibly related section in NEWS:
> * optimization: the compiler removes known-NIL arguments from calls to
> APPEND and NCONC, and empty sequences from calls to CONCATENATE.

And it can be reproduced by using APPEND instead of the backquote:

(defun foo (list)
  (declare (optimize speed))
  (append list '(tail)))

Generated note:
; in: defun foo
; (APPEND LIST '(TAIL))
;
; note: unable to
; optimize
; due to type uncertainty:
; The first argument is a list, not a null.
;
; compilation unit finished
; printed 1 note