Don't indent already indented output when pretty-printing

[ak-coram]

Note: Currently I believe the issue is with Cider, but it also relates
to recent changes on the master branch of Emacs, so I'm not entirely
sure this is the right place to post this report. I think this is more
of a feature request anyway.

Emacs has introduced lisp-indentation changes which don't reparse
the code as often as before and rely on previous parse-state to
improve performance. I believe this change is causing performance
issues with cider-emit-into-popup-buffer, when pretty-printing
non-trivial amounts of data.

I profiled Emacs and found that it spends most of the time garbage
collecting and lisp-indent-calc-next during pretty-printing.

I've also found that if I comment out the indent-sexp call from the
cider-emit-into-popup-buffer function, the issue is gone.

I'm not really sure what other features cider-emit-into-popup-buffer
is used for (except pretty-printing), but don't pretty-print functions
already indent their output? Why is there a need for Emacs to indent
the output again?

If you agree, I can submit a pull request for
cider-emit-into-popup-buffer with an optional parameter to inhibit
indentation.

[bbatsov]

Emacs has introduced lisp-indentation changes which don't reparse
the code as often as before and rely on previous parse-state to
improve performance. I believe this change is causing performance
issues with cider-emit-into-popup-buffer, when pretty-printing
non-trivial amounts of data.

Hmm, I didn't notice this at all.

I profiled Emacs and found that it spends most of the time garbage
collecting and lisp-indent-calc-next during pretty-printing.

It's often a good idea to increase the GC limit (which is super conservative by default) - see http://bling.github.io/blog/2016/01/18/why-are-you-changing-gc-cons-threshold/

I'm not really sure what other features cider-emit-into-popup-buffer
is used for (except pretty-printing), but don't pretty-print functions
already indent their output? Why is there a need for Emacs to indent
the output again?

We have to see all usages in the codebase - I think it might be used somewhere for non-pretty printed results.

If you agree, I can submit a pull request for
cider-emit-into-popup-buffer with an optional parameter to inhibit
indentation.

Sure. At the very list this can be added just for the cases that really need it (if any).

[bbatsov]

I see there are just 5 usages of this function in the entire codebase, so it should be easy for you to find your way. Seems to me the only usage that really needs the indent-sexp is this one:

(defun cider--emit-interactive-eval-output (output repl-emit-function)
  "Emit output resulting from interactive code evaluation.

The OUTPUT can be sent to either a dedicated output buffer or the current
REPL buffer.  This is controlled by `cider-interactive-eval-output-destination'.
REPL-EMIT-FUNCTION emits the OUTPUT."
  (pcase cider-interactive-eval-output-destination
    (`output-buffer (let ((output-buffer (or (get-buffer cider-output-buffer)
                                             (cider-popup-buffer cider-output-buffer t))))
                      (cider-emit-into-popup-buffer output-buffer output)
                      (pop-to-buffer output-buffer)))
    (`repl-buffer (funcall repl-emit-function output))
    (_ (error "Unsupported value %s for `cider-interactive-eval-output-destination'"
              cider-interactive-eval-output-destination))))

Take a look yourself and make the changes accordingly. Some of this uses of this function don't even output sexps, so there this is completely worthless. An oversight on our part I guess.