bug#74357: c-mode: Some syntactic constructs cause unreasonable typing lag

[Björn Lindqvist]

> > From: Björn Lindqvist <bjourne@gmail.com>
> > Date: Thu, 14 Nov 2024 21:39:53 +0100
> >
> > This bug has been present in c-mode for at least a year, but I haven't
> > gotten around to report it. The reason is that bugs about latency are
> > erratic and tricky to triage. Typing when the cursor is on some
> > syntatic constructs in c-mode causes severe lag on the order of several
> > hundred milliseconds on my (admittedly slow) laptop. The lag
> > makes c-mode almost unusable. Here is an MWE:
> >
> >     void foo(uint dc_dim, uint sc_dim,
> >              uint fy_dim, uint fx_dim,
> >              __global const float * restrict F,
> >              uint sy_dim, uint sx_dim,
> >              __global const float * restrict S,
> >              uint padding,
> >              __global float * restrict D) {
> >         uint dy_dim = sy_dim + 2 * padding - fy_dim + 1;
> >         uint dx_dim = sx_dim + 2 * padding - fx_dim + 1;
> >
> >         // Place cursor at "y" in "dy_dim". Hold "y" and observe lag.
> >         uint dn = dc_dim * dy_dim * dx_dim;
> >         uint sn = sc_dim * sy_dim * sx_dim;
> >     }
> >
> > I can make typing even laggier by wrapping foo in foo, like this:
> >
> >     void foo(...) {
> >     ...
> >     void foo(uint dc_dim, uint sc_dim,
> >              uint fy_dim, uint fx_dim,
> >              __global float * restrict F,
> >              uint sy_dim, uint sx_dim,
> >              __global float * restrict S,
> >              uint padding,
> >              __global float * restrict D) {
> >         uint dy_dim = sy_dim + 2 * padding - fy_dim + 1;
> >         uint dx_dim = sx_dim + 2 * padding - fx_dim + 1;
> >
> >         // Place cursor at d[y]_dim. Hold "y". Observe lag in c-mode.
> >         uint dn = dc_dim * dy_dim * dx_dim;
> >         uint sn = sc_dim * sy_dim * sx_dim;
> >     }
> >     }
> >
> > The more times I wrap foo the slower c-mode gets. So if you have a
> > fast computer try nesting foo in foo 50 times... I have observed the
> > same annoying input lag on multiple computers and I don't use any
> > special c-mode configuration.
>
> I've wrapped the snippet with 50 foo, and I still don't see any
> significant lags.
>
> Does it matter where you type, for reproducing the lag?
>
> Also, can you run this under a profiler (M-x profiler-start) and then
> show the full expanded profile produced by "M-x profiler-report" after
> several tens of seconds of typing with the lag?
>
> Adding Alan to the discussion.

Hello Eli, thanks for the swift reply.


I've created a much larger example so you can see what I mean with
"wrapping foo in foo":

https://gist.github.com/bjourne/8f705c5879aa966accf354008623f6bb

Open file in c-mode, Go to line 328, place the cursor on "y" in "dy_dim"
and press and hold "y". Unless you have a supercomputer the lag will be
really apparent. Disable Global Font-lock mode and repeat the exercise.
Lag will be gone. Lag is worse if I use non-jitted Emacs, but really
apparent in jitted Emacs too. Lag goes away if I use c-ts-mode instead
of c-mode.

I want to emphasize that lag is present in normal code too, but easier
to detect in these specially crafted samples.

My system-configuration-features:

"ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ
JPEG JSON LCMS2 LIBOTF LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP
NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM GTK3
ZLIB"

Profiler report:

https://gist.github.com/bjourne/842695100e99c8fd6ef87fcdd0a6ed0b


-- 
mvh/best regards Björn Lindqvist