backquote of constants should be constant

[James Y Knight]

That is,
(defconstant +foo+ 1)
(defun foo () `(,+foo+))
should return a constant list, not cons up a new one every time.

Here's a patch. Seems to work.

I wonder if any of the complication in backq.lisp can be removed if it can assume the compiler does this.

[Stas Boukarev (stassats)]

"Should" as in "the standard prescribes it" or in "it would be nice"?

[James Y Knight (foom)]

Should as in "this is a significant missed optimization".

[James Y Knight (foom)]

BTW, here's some discussions about whether the result of backquote is considered mutable or not. The spec is not very clear, but it seems like the consensus is that backquote can return literal data whenever it feels like it. Which is good, because SBCL already does that in many cases. This just adds a few more cases. :)

https://groups.google.com/group/comp.lang.lisp/browse_thread/thread/74c41c41aeb80528
https://groups.google.com/group/comp.lang.lisp/browse_thread/thread/43e14ad1ba72dd0d
https://groups.google.com/group/comp.lang.lisp/browse_thread/thread/385f2adf7a3175fd

[Paul Khuong (pvk)]

Committed in f250391 (Constant-fold backquote of constant expressions)